Get ready to dive into the world of the Protein Sparing Modified Fast (PSMF) diet—a lightning-fast fat loss method that’s earned its stripes as a muscle-preserving powerhouse.
But, like any potent tool, the PSMF can be a double-edged sword if wielded carelessly.
If you’re in a hurry to shed those extra pounds, it’s crucial to tread cautiously. You’ll need to ensure your body gets precisely what it craves to avoid:
- Cravings that threaten to sabotage your progress,
- The dreaded rebound bingeing that derails your efforts, and
- Potential risks to your health and well-being (as you’re about to discover).
In this article, we’re your trusty guides, here to school you on the essential principles of the protein-sparing modified fast. We’ll show you how to lose fat safely and sustainably while keeping your precious muscle mass intact.
Now, here’s the kicker: After this deep dive, you might conclude that the extreme version of PSMF isn’t your cup of tea.
But fear not, because you’ll also glean the fundamental principles to maximize satiety for weight loss. These principles have propelled thousands of individuals towards their goals in our Macros Masterclass and Data-Driven Fasting Challenges.
Are you ready to embark on this enlightening journey towards your fitness aspirations?
- What Is a Protein Sparing Modified Fast?
- How Does a Protein Sparing Modified Fast Work?
- The Key Principles of the Protein Sparing Modified Fast
- Why Is It Called Protein Sparing Modified Fast?
- How Can I Measure the Protein in My Body?
- Why Would I Want to ‘Spare Protein’?
- When Would a PSMF Be Better Than a Water Fast?
- Why Would I Want to Do A PSMF?
- Who Shouldn’t Do A PSMF?
- How Long Can I Do a Protein Sparing Modified Fast?
- Signs It Might Be Time to Switch to ‘Maintenance’
- Is Protein Sparing Modified Fasting Safe?
- History of the Protein Sparing Modified Fast
- How Quickly Can I Lose Weight on a Protein-Sparing Modified Fast?
- How Quickly Can I Lose Fat?
- Does the PSMF Diet Lower Your Metabolism?
- Can I Lose Fat and Gain Muscle at the Same Time?
- Why Does a Protein Sparing Modified Fast Work?
- Why is a PSMF Diet So Hard to Maintain?
- How Many Calories Are in a PSMF?
- How Much Protein Do I Need on a PSMF Diet?
- More Protein is Ideal, Especially if You’re Active
- Could I Eat ‘Too Much Protein’ on a Protein-Sparing Modified Fast?
- Do I Need to Follow PSMF Macros Every Day?
- Do I Need to Track My Calories on a Protein Sparing Modified Fast?
- What is a Sustainable Rate of Weight Loss?
- How Can I Work Towards a PSMF Diet Approach?
- Is A Protein Sparing Modified Fast Ketogenic?
- Can I Use Protein Powders, Bars, or Other Products on a PSMF?
- How Much Fat Should I Consume on a PSMF?
- How Many Carbohydrates Should I Consume on a PSMF?
- You May Need Some Carbs to Support Intense Activity
- Protein % Has the Biggest Impact on Satiety!
- Why Micronutrients Are SO Important!
- What’s the Relationship Between Protein % and Nutrient Density?
- What Should I Eat on a PSMF Diet?
- Nutrient-Dense Protein Sparing Modified Fast Recipes
- Key Takeaways for a Successful PSMF
What Is a Protein Sparing Modified Fast?
A protein-sparing modified fast (PSMF):
- limits energy from carbs and dietary fat while
- prioritises dietary protein to ‘spare’ your body’s precious stores of protein found in your muscles and organs.
How Does a Protein Sparing Modified Fast Work?
Most diets simply limit calories to lose weight. But, unfortunately, trying to eat less of the same foods that made you fat in the first place usually doesn’t end well.
Because your muscles and organs are critical for survival, your body will respond with an aggressive appetite increase to push you to get the protein required.
So, for long-term success on a PSMF diet, you need to modify WHAT you eat to:
- ensure you lose mostly fat while sparing as much precious lean mass as you can, and
- reduce your risk of nutrient cravings and rebound bingeing.
Preserving your muscles is not just about looking good naked or shredded on the bodybuilding stage. Maintaining strength and adequate lean mass is critical to your long-term metabolic health and assures resilient aging.
The Key Principles of the Protein Sparing Modified Fast
The key principles of the protein-sparing modified fast are:
- Reduce non-fibre carbohydrates.
- This allows your blood glucose stores and liver and muscle glycogen to be depleted.
- Reduce dietary fat.
- This allows your body to utilise stored body fat for energy.
- Prioritise dietary protein.
- This minimises the loss of metabolically active protein from your muscles and vital organs on a PSMF diet.
While a protein-sparing modified fast is typically low in calories for rapid fat loss, these key principles are still relevant for successful fat loss.
Why Is It Called Protein Sparing Modified Fast?
The weight you see on your bathroom scale is made up of:
- stored glucose, and
The PSMF diet does just what it says: it intentionally prioritises dietary protein to ‘spare’ the protein on your body during an aggressive calorie deficit.
While a PSMF doesn’t forego food entirely, it’s essentially a type of muscle-sparing fasting.
Because we are able to maintain muscle mass during a severe calorie deficit, we can maintain a higher metabolic rate (BMR).
How Can I Measure the Protein in My Body?
The term ‘lean mass’ or ‘lean body mass’ is often used to denote the non-fat components of your body.
Your lean mass is calculated by subtracting your fat mass from your weight.
Lean Mass = Total Weight – Fat Mass
Measuring your lean mass is more helpful than simply tracking your weight on the scale. However, it is even more useful to consider your body’s protein stores because it also subtracts the weight of water, bone, and minerals.
During weight loss, your goal should be to decrease your fat mass and your lean muscle mass to remain as constant as possible.
Some loss of lean mass from your body’s protein stores is typical, but you should do everything in your power to minimise it.
Unfortunately, your body’s protein stores are hard to measure precisely. A DEXA scan is the most accurate way, but it is relatively expensive and, therefore, hard to do regularly.
Many modern bioimpedance scales estimate your body’s protein stores to allow you to monitor the trend over time.
For a more detailed discussion, see What Do the Numbers on My Bioimpedance Scale mean (and How Can I Manage Them)?
Why Would I Want to ‘Spare Protein’?
Unfortunately, simply tracking the weight you see on the scale can be misleading. Most people don’t just want to lose weight; they want to lose body fat and maximise muscle.
Protein is not your body’s preferred fuel source because it is hard to convert to usable energy (ATP). But if you’re not feeding and using your muscles, your body will willingly sacrifice your metabolically active lean mass from your organs and muscles when food is scarce.
When Would a PSMF Be Better Than a Water Fast?
In the early stages of water fasting, your body uses around 400 calories (100 g protein) per day of stored protein. Over time, this decreases to 250 daily calories (60 g protein) once your body adapts to using fat and ketones for fuel.
You not only require protein to maintain and build your muscles, but you also need it to constantly repair your organs, create neurotransmitters, make hormones, synthesise enzymes, and aid in the many other functions that amino acids are critical for.
While your requirement for protein decreases during fasting, the amount of your body’s protein stores consumed daily during a fast is still significant, particularly in the first few days.
If you fast regularly or for days at a time, your appetite will increase when you eat again to compensate for the protein deficit you’ve self-inflicted. At this point, you will be extremely motivated to make up for the lack of energy and micronutrients like amino acids your body needs to carry out its daily activities.
Unfortunately, most people don’t gravitate to grilled chicken breast, egg whites, and broccoli after not eating for longer than normal. Instead, we tend to gravitate towards energy-dense, nutrient-poor, low-protein foods that help us refill our energy stores quickly.
When we eat again, most of us find it difficult to compensate for the shortfall of dietary protein created during the fast. Aside from uncontrolled cravings, this often ends in a loss of precious muscle, which can lower someone’s metabolic rate, make them ‘skinny fat’, or cause them to gain more weight than before.
To address these risks and minimise the rebound effects of an energy deficit, a PSMF prioritises protein during the aggressive period of energy restriction.
Why Would I Want to Do A PSMF?
While most people pursue a PSMF for weight loss, there are some other associated benefits, including:
- decreased LDL cholesterol,
- improved blood sugar control,
- lowered blood pressure,
- protection against metabolic syndrome,
- improved immune function, and
- boosted autophagy.
Who Shouldn’t Do A PSMF?
Depending on your personal history, past eating habits, and the way you intend to execute your fast, it might be worth talking to your doctor if you have:
- a heart arrhythmia,
- an eating disorder or have had one in the past,
- cancer (certain cancers feed on protein),
- liver or kidney failure, or
- gallbladder disorder or do not have a gallbladder.
And if you’re pregnant, a PSMF would, of course, be inappropriate. Instead, your focus should be on providing plenty of nutrients and energy to grow a new life!
How Long Can I Do a Protein Sparing Modified Fast?
So, how long can you stay on PSMF?
As with everything that has to do with nutrition, it depends.
Many factors come into play to decide how long you want to stay on your protein-sparing modified fast, including:
- your starting weight,
- your intended calorie deficit,
- your target rate of weight loss and
- how much weight do you have to lose?
An aggressive short-term diet or mini-cut for someone closer to their goal weight might last four weeks. However, it may take many months or years for someone with a lot of excess weight to reach their goal.
Once you’ve lost 10% of your initial body weight, the process often gets more challenging. You might find yourself plateauing or being excessively food-focused.
While you might want to get to your goal weight as soon as possible, you may encounter more lasting success if you take it slowly and take some breaks to practice maintaining your weight if you have more weight to lose.
Signs It Might Be Time to Switch to ‘Maintenance’
Dieting of any sort is a stressor, and your body has a few ways of telling you to back off. Some symptoms include:
- excessive fatigue,
- lowered thyroid function,
- excessive hair or eyebrow thinning,
- prolonged dizziness when standing,
- menstrual changes or loss of your period (for women),
- noticeable and consistent mood swings,
- sensitivity to cold,
- poor sleep,
- endless thoughts about food,
- poor wound healing or recovery,
- loss of libido or
- any feelings that you’re becoming obsessive.
If you’re getting one or several of these symptoms, it is vital to listen to your body! To avoid decreasing your metabolic rate or losing any lean muscle mass, backing off for a while may be the best thing for your long-term progress.
Is Protein Sparing Modified Fasting Safe?
Studies have shown a PSMF can be safe and effective. You should consult with your medical practitioner if you have one of the conditions listed above to see if it’s safe even to begin a PSMF.
Emphasising nutrient density is critical to ensure a safe, successful protein-sparing modified fast. This will be explained in more detail throughout the rest of the article.
Because the body requires vitamins, minerals (like electrolytes), amino acids, and essential fatty acids to carry out all of its vital activities, things can go terribly wrong when someone sacrifices these critical nutrients. Thus, it’s crucial to focus on nutrient-dense, whole foods during your PSMF (or any diet, for that matter).
It’s also essential to remember that even the most overweight person does not have an endless amount of fat to lose. Thus, as we move closer to the weight that is naturally the most optimal for our bodies—although it might come sooner for some than others—we will meet resistance as we try to lose more weight.
History of the Protein Sparing Modified Fast
Similar to ‘keto’, ‘plant-based’, paleo, and other dietary trends that have exploded in popularity, the ‘protein sparing modified fast’ has come to mean many different things to different groups of people.
To understand if any of the various versions of the protein-sparing modified fast might be appropriate for you, it’s helpful to understand the history of the PSMF and how it has evolved.
George Blackburn first coined the concept of the ‘protein-sparing modified fast’ (PSMF) in the early 1970s. After using a high-protein, intravenous infusion for bedridden, post-surgical, obese patients, Blackburn became curious about how else he could apply this concept in other contexts.
Blackburn and colleague Bruce Bistrian of Harvard Medical School prescribed a diet of 650 to 800 daily calories from nothing but lean fish, meat, and chicken. Blackburn, who later became president of the American Society of Clinical Nutrition, describes his regime as ‘Atkins without excess fat’.
In their initial trial, a hundred patients lost an average of 50 lbs (22 kg) over four months and experienced little hunger. They also reported faster wound healing, less muscle atrophy, and additional fat loss when the total parenteral nutrition (TPN) administered contained more protein than a standard dextrose (glucose) infusion.
In their 1973 paper, Protein Sparing Therapy during Periods of Starvation with Sepsis or Trauma, Blackburn and colleagues documented the results of their PSMF studies, which included minimised loss of lean mass and getting adequate amino acids for proper nitrogen balance.
The Last Chance Diet
Sadly, we often take things to extremes.
If a little is good, more must be better.
Taking the success of Blackburn’s research to the next level, Dr Roger Linn published The Last Chance Diet in 1976. It was the latest fad diet book that promised rapid and effortless weight loss.
Readers jumped on this latest fad, which recommended a 400-calorie per day ‘Prolinn’ pre-digested protein drink. This drink is not to be confused with Valter Longo’s Prolon Fasting Mimicking Diet.
Unfortunately, Prolinn was derived from cheap, low-quality collagen made from slaughterhouse leftovers that were smothered with added flavours and colours to make it palatable. While it contained some protein, the processing removed much (if not all) of the vitamins, minerals, essential fatty acids, and even some amino acids.
While many quickly lost weight, sixty deaths were attributed to this popular fad version of Blackburn’s original protein-sparing modified fast. The deaths were generally attributed to a loss of heart muscle and cardiac arrhythmias from the extreme calorie deficit and lack of minerals like potassium.
If you want to read more about the risks of calorie restriction that negates adequate nutrition, the Amazon reviews for The Last Chance Diet are enlightening.
More recently, the Cleveland Clinic has been investing more time into researching low-calorie, higher-protein diets for aggressive weight loss in a controlled setting.
They found that:
- most of the weight lost on a PSMF is from fat and not muscle;
- trial participants are encouraged by the initial rapid weight loss period experienced during a PSMF, which has reduced their dropout rate;
- although meal replacement shakes or protein bars can be used, learning to make meals from whole foods is critical for long-term success;
- a PSMF fast is effective for people with normal glycemic control, pre-diabetes, or Type-2 Diabetes, and
- people using a whole-foods-based PSMF are significantly less hungry (more satiated) and less preoccupied with eating than those on a PSMF that focuses on a liquid formula.
Overall, a protein-sparing modified fast is safe and effective for rapid fat loss in a clinical setting when done correctly. However, you will undoubtedly experience improved satiety and increase your chances of long-term success if you prioritise nutrient-dense whole foods over powders, shakes, and bars.
Lyle McDonald’s PSMF for Bodybuilders
McDonald intended to make crash dieting safer in instances where people would want to lose weight fast, like:
- an upcoming special event,
- making weight for a competitive bodybuilding show or
- the final push of a cutting phase.
In the introduction, McDonald says:
‘I’d love to live in a world where nobody crash dieted, where everybody followed sane and safe dieting strategies and stuck with it in the long term until they reached their goal and then stuck with those newfound eating habits in the long-term.
‘I also want a pony and to be six feet tall, and to be an astronaut. And how about an end to world hunger while we’re at it?
‘My point? When idealism and reality slam together, it’s never pretty. People will crash diet no matter what I or anybody else tell them.’
Depending on your starting weight and activity levels, McDonald’s PSMF bodybuilding version amounts to around 400 – 1200 calories per day.
McDonald details that you can individualise a protein-sparing modified fast for someone based on their goals and context, noting:
- someone who is already very lean and weight training heavily will need more protein;
- someone with more weight to lose may do better with a less aggressive approach over a longer timeframe;
- unlimited green, leafy, fibrous veggies are encouraged to provide essential micronutrients;
- an aggressive, severely energy-restricted PSMF is typically not undertaken for long because of the detriments that result from nutrient deficiencies and a severe energy deficit, and
- prioritising essential nutrients becomes even more critical when energy intake is severely restricted.
How Quickly Can I Lose Weight on a Protein-Sparing Modified Fast?
Various studies have shown that a PSMF yields rapid fat loss in the short term while preserving lean mass. One study found that 15 obese patients on a PSMF lost 32 pounds (15 kg) of fat in six weeks without muscle loss!
The chart below from The Effect of Starting the Protein-Sparing Modified Fast on Weight Change Over 5 Years (Rothberg et al., 2020) shows that the PSMF study participants lost significantly more weight over the first six months than the control group.
However, it’s important to note that the final weight loss between the two groups was similar after five years. While the PSMF group initially lost significant amounts of weight, they experienced greater weight regain. Unless you learn to change how you eat for the long term, the rapid results are unlikely to be sustainable.
How Quickly Can I Lose Fat?
People with more body fat will lose weight more quickly, especially in the early stages of dieting.
Often, a significant proportion of this ‘weight loss’ is water weight. For every gram of glycogen stored by the body, we store around three grams of water. Thus, we release this retained water when we switch to a lower-carb diet and our insulin and glucose drop.
Participants tend to see rapid weight loss over the first few weeks, but water weight and fat loss tend to slow as you approach your goal weight.
In 2004, Seymour Alpert estimated that humans could release energy from stored fat at a maximum rate of 290 kJ (or 69 calories) per kilo of fat per day. Once we exceed this threshold, we begin to tap into body mass that isn’t fat, like our muscles and organs.
The table below shows different weight loss scenarios for people with varying body fat percentages.
|weight (kg)||weight (lbs)||BF%||fat (kg)||max calories||max fat loss (kg/day)||% weight per week|
- Someone morbidly obese (i.e., 330 lbs with 60% body fat) could theoretically lose up to 3.2% of their body weight per week without touching their lean mass.
- However, a leaner person (e.g., 176 lbs with 12% body fat) has a much shorter threshold and would risk using excessive amounts of lean mass if they lost more than 0.8% of their body weight per week.
When we look at the maximum energy your body can take from your fat stores each day, we see that:
- while more obese people can release a massive amount of energy from their fat stores,
- leaner people will lose lean mass during a water-only fast because they cannot meet the energy demands of their body from their stored fat.
In the latter case, a protein-sparing approach would be better than water fasting, especially if you are leaner. However, this data also highlights the importance of setting realistic goals to avoid awakening your body’s survival instincts. Once you’ve passed this threshold, your risk of excessive muscle loss and rebound weight gain increases substantially.
Does the PSMF Diet Lower Your Metabolism?
Weight loss—of any sort—requires calorie restriction. Thus, any diet promoting weight loss slows your metabolic rate to some degree. If you weigh less, you’ll need fewer calories.
However, a properly executed PSMF protects against the loss of lean mass, which maximises fat loss. Because a PSMF retains optimal amounts of lean body mass or ‘spares protein’, it should also result in the most minimal reduction in your metabolism.
To ensure you don’t affect your basal metabolic rate negatively, there are a few precautions you might want to consider, like:
- maximising nutrient density throughout your PSMF (and after),
- not overdoing it,
- adding in refeeds and cycling your PSMF appropriately, and
- altering your duration if your body is telling you to stop.
For signs that you might need to cut things short, scroll up to the section titled ‘Signs It Might Be Time to Switch to Maintenance.’
Can I Lose Fat and Gain Muscle at the Same Time?
Losing fat and gaining muscle simultaneously is what the pros refer to as ‘body recomposition’ or ‘body recomp’. This is the holy grail of weight loss.
But is it possible?
The good news is yes!
We see many of our Optimisers lose fat and gain lean mass in our challenges, especially if they were under-consuming protein previously.
However, there is some bad news, too.
Body recomp is not easy. You have to pay careful attention to your training and nutrition. In other words, the balance between your energy in and energy out becomes even more critical to avoid losing muscle mass.
As someone becomes leaner and more muscular, body recomposition also becomes more difficult. Because muscle is so metabolically expensive, our bodies can’t build and store it as efficiently as we do fat!
People with more experience in dieting and resistance training tend to cycle between strength gain, where workout volume and calorie intake are higher and cutting periods, where workout intensity is lower.
On the other side of the spectrum, people new to resistance training usually respond well to nearly any new stimulus, especially with adequate dietary protein. The stored energy can fuel your activity in an energy deficit, while sufficient dietary protein can support muscle maintenance and growth.
Most people experience cravings for energy-dense, low-protein foods when they increase their workout intensity to fuel muscle growth and recovery. Hence, successful body recomposition requires tracking your body composition progress and fine-tuning your macronutrient intake to ensure you’re losing fat and gaining muscle.
To help you manage this quantitatively, the Smart Macros algorithm in Nutrient Optimiser bumps up your protein target progressively if you begin losing more lean mass than body fat.
Why Does a Protein Sparing Modified Fast Work?
While the name ‘protein-sparing modified fast’ might sound mysterious or magical, it works because it applies the power of protein leverage.
Professors David Raubenheimer and Stephen Simpson have undertaken a vast amount of excellent research to show that all organisms continue to seek out energy until they obtain adequate nutrients like protein.
Humans are no different to animals in the wild. While you may think you’re in control of what, when, and how much you eat, the conscious part of your brain (cerebellum) is no match for your primitive brain (amygdala), which dictates your survival instincts.
If your diet lacks adequate protein, your body will send you in search of food by increasing your appetite until you get enough to fill your demands. Thus, a diet with a higher percentage of energy from protein will keep you satisfied with fewer calories.
The chart below shows the satiety response to protein from our analysis of 125,761 days of data from 34,519 Nutrient Optimiser users. Increasing protein from low (12%) to high (55%) aligns with a massive 55% reduction in calories!
The principles of protein leverage are straightforward. When we prioritise protein, we:
- reduce the energy we consume from fat and non-fibre carbs,
- obtain the protein we need with fewer calories,
- see our appetite decrease,
- eat less without using as much conscious willpower, and
- lose weight.
For more detail, see Protein for Weight Loss: How Much You Need and Why It Works.
Why is a PSMF Diet So Hard to Maintain?
While a higher protein percentage diet aligns with a lower calorie intake, it’s not easy to achieve or maintain.
Your body is constantly seeking nutrients and energy and requires the right balance of both.
The chart below shows that the average protein intake of Optimisers is 31%. However, only 15% of Our Optimisers sustain a protein intake of over 42%.
We find it hard to continue eating a diet with a very high protein percentage because it is so satiating; our appetite for more protein switches off once we’ve had enough. While your body can convert protein to energy, it’s highly inefficient.
To turn protein into usable energy (ATP), your body uses 25 – 35% of the energy in protein. If you’ve ever had the ‘meat sweats’ after consuming huge amounts of protein, you’re experiencing this at work!
Once you approach the rate of releasing stored energy from fat that’s comfortable for your body, it will aggressively search for other energy sources. This is why we start dreaming of peanut butter, doughnuts, pizza, and other low-protein, energy-dense foods and lose interest in eating more lean protein.
With some discipline and tracking, most people find 30 – 40% protein sustainable over the medium term. However, most find it difficult to maintain more than 40% protein for long. At first, it feels effortless! But hunger kicks in, and some will fly off the rails and binge on less-than-optimal foods the longer they bypass their body’s hunger cues.
Before committing to a very low-calorie PSMF diet in hopes of overnight weight loss, it’s important to remember that the general US population’s average protein intake is less than 12%!
As the chart below shows, protein % has declined since the 1977 US Dietary Guidelines for Americans were introduced.
While this decrease in protein % may not look like much, Professors Raubenheimer and Simpson have shown that a slight decrease in protein % aligns with a significant increase in non-protein energy and is enough to explain the obesity epidemic.
How Many Calories Are in a PSMF?
A protein-sparing modified fast is a very low-calorie diet that typically amounts to less than 800 calories per day in a clinical setting. For most people, this is just above half your basal metabolic rate (BMR) or the calories you require to fuel your basic metabolic needs.
However, your appropriate calorie target may vary depending on a range of factors, like:
- how much fat you have to lose,
- how quickly you want to lose weight,
- your activity levels,
- your current weight and
- your current body fat levels.
The chart below shows the relationship between calories, protein %, and absolute protein intake (in grams), taken from our Optimiser data.
As we move from left to right on this chart, a higher protein % aligns with:
- a lower calorie intake (left-hand axis), and
- a higher protein intake (right-hand axis, in grams).
On average, moving from one extreme to the other aligns with:
- consuming 2.4 times more protein: 60 g -> 150 g,
- an increase in protein %: 12% -> 55%, and
- a decrease in total calories: 2,000 -> 1,100 (i.e. a 55% deficit).
You can use this and the instructions found in The Ultimate PSMF Calculator to determine a good starting point for you that’s more flexible than the clinical recommendations.
Rather than jumping from your current diet to an extreme PSMF approach, we guide people through sustainably dialling up their current protein % to a more optimal one in our Macros Masterclass.
How Much Protein Do I Need on a PSMF Diet?
In a clinical setting, the protein on a PSMF is set to 1.2 to 1.5 g/kg of ideal body weight. This is the minimum protein intake required to minimise the loss of lean muscle mass in an aggressive energy deficit.
When this is applied to a standard 800-calorie PSMF, it might include the following:
- 110 g protein (440 calories, or 55% of total calories),
- 31 g fat (277 calories, or 35% of total calories), and
- 20 g net carbs (80 calories, or 10% of total calories).
This would be appropriate for someone in a clinically supervised environment with an ideal body weight of 160 to 200 lbs (or 73 to 92 kg).
If you find an extreme protein % too hard to maintain, you should consider incorporating more calories and accepting a slower rate of weight loss.
If we increase calories to 1100 and rerun the numbers with 40% protein, our target macros come out to:
- 110 g protein (440 calories, 40% of total calories)
- 55 g fat (495 calories, or 45% of total calories), and
- 40 g net carbs (160 calories, or 15% of total calories).
You can use our free PSMF calculator to adjust the macros you initially calculated and consume more of your calories from protein to reduce the risk of body protein loss, especially if you’re more active and working out.
More Protein is Ideal, Especially if You’re Active
A review paper from Stuart Phillips showed lean muscle mass is preserved best during an aggressive deficit (35% below maintenance calories) when we’re consuming at least 2.6 g protein/kg total body weight.
In practice, this level of protein intake is tough to achieve, even if you are very active and using a lot of energy to exercise. Using a 100 kg man with 30% body fat who is moderately active as an example, this would equate to:
- 2570 maintenance calories,
- 1670 calories on a diet (35% deficit),
- 260 g protein (1160 calories, or 62% of total calories),
- 55 g fat (495 calories, or 30% of total calories), and
- 30 g net carbs (120 calories, or 8% of total calories).
A diet with such a high protein % would be hard to design—let alone sustain for long—with whole foods, especially if you’re coming from a standard Western diet and not accustomed to that much protein.
Could I Eat ‘Too Much Protein’ on a Protein-Sparing Modified Fast?
It is impossible to over-consume protein if you’re also in an energy deficit where you’re losing weight.
Jose Antonio’s protein overfeeding studies showed that people gained lean muscle mass and lost fat despite consuming over 4.4 g protein/kg of BW in a calorie surplus!
The idea that our satiety response to protein depends on increasing protein % by dialling back fat and carbs is often overlooked. In other words, it’s not about simply eating more protein.
Because protein usually comes packaged with fat, simply eating more protein without adjusting your carbs and fat will lead to a higher calorie intake, as shown in the chart below. You can’t just eat more butter, bacon, and fatty steak to get more protein!
The key to successfully using a PSMF approach is to change what you eat by dialling back energy from fat and non-fibre carbohydrates and prioritising protein to increase your protein %. Unfortunately, most people find cutting back on carbs and fat harder than eating more protein!
Do I Need to Follow PSMF Macros Every Day?
Depending on how drastic your calorie restriction is and how much weight you have to lose, some people follow a PSMF-style approach a few days a week.
This could be appropriate:
- before an event where you know you’ll eat more,
- after a day where you overate, or
- on a day when you’re not as active.
While you can adjust your diet to accommodate your lifestyle daily, it’s likely counterproductive to switch between ‘extreme’ dietary approaches (e.g., high-fat keto or PSMF) in terms of effective habit formation.
Without meticulous planning, tracking, or extreme self-restraint, you’ll likely reward yourself for ‘being good’ and make up for the energy deficit you’ve created on your off days.
In our Macros Masterclass, we focus on developing consistent habits and routines. Although this takes more time and effort than just winging it, we’ve found it results in the best long-term success.
Do I Need to Track My Calories on a Protein Sparing Modified Fast?
Most people gravitate toward a lower protein % diet with more energy than they need from carbs and fat. Thus, they find tracking their food helpful to reform new habits and adhere to their macro calculations (if they’re using them), at least at first.
Most people are surprised to see how much energy they get from added dietary fat when they start logging their typical diet food during baselining week of our Macros Masterclass.
From there, we guide Optimisers through dialling back their energy from fat and carbs progressively so they can use stored body fat for fuel while prioritising protein for greater satiety.
If you don’t want to track your food, you might enjoy learning how to use your blood glucose monitor as a fuel gauge to guide your eating, as we do in Data-Driven Fasting Challenges. If your blood glucose is higher than your normal, the DDF app will guide you to prioritise energy from protein rather than fat and carbs.
Data-Driven Fasting uses your blood glucose to gamify the weight loss process.
Dietary carbohydrate raises blood sugars in the short term, but fat keeps them elevated for longer.
For more on this, check out our article on the Food Insulin Index.
Thus, reducing carbohydrates and fats and prioritising protein and fibre is the most effective way to get your blood sugar to drop so you can eat again sooner.
When people reflect on their blood glucose response to foods, it often pushes them towards more of a PSMF approach.
What is a Sustainable Rate of Weight Loss?
While rapid fat loss may sound attractive, there are downsides, as noted above, including:
- loss of lean mass, or loss of your body’s protein stores from your muscles and organs,
- rebound bingeing, and
- poor sustainability.
To help you understand what is realistically achievable, the chart below shows the weight loss rate distribution over the four-week duration of our Macros Masterclass. While some people can sustain losing 2% of their body weight per week, we suggest people are doing great if they’re losing between 0.5 to 1.0% per week.
Many people lose weight rapidly during the Macros Masterclass, especially in the first few weeks. But if someone is already losing weight at 1.0% per week, most people don’t have to reduce calories further.
After running numerous challenges with thousands of people over the past four years, we’ve adjusted Nutrient Optimiser’s Smart Macros algorithm to increase your calorie target if your weight loss exceeds 1.0% per week.
Although rapid weight loss sounds desirable, most people don’t manage to keep the weight off once the aggressive diet is over. Unless someone has a shortlist of foods, meals, and habits dialled in, rebound bingeing is more likely to occur. Competitive bodybuilders are a prime example of this, as it’s not uncommon for them to undo all their hard work in the few weeks after a show with uncontrolled bingeing.
This is not a failure of willpower or discipline. It’s just biology and your survival instincts taking over! Unless you change your eating habits progressively during the dieting phase, it’s nearly impossible to resist the allure of hyper-palatable foods once the diet ends.
Thus, you might consider taking a less aggressive approach unless you have a good reason for wanting to be lean by a specific date and not for the rest of your life!
How Can I Work Towards a PSMF Diet Approach?
A PSMF may seem mysterious, magical, and unattainable. However, it’s straightforward when you understand that any successful diet lies somewhere between the following extremes:
- mainly protein and fibre, with minimal energy from non-fibre carbs and fat and a focus on foods towards the bottom right of the chart below vs
- mainly industrial fats and refined carbs with minimal protein and fibre that are heavy in foods found towards the top left of the chart below.
We find that most people have the best long-term success when they move towards the PSMF extreme by progressively dialling back energy from carbohydrates and (or) fat.
In our Macros Masterclass, we help Optimisers understand their baseline habitual food intake. From there, we guide them to progressively adjust their intake of protein, fat, and carbs towards a higher protein %, PSMF-style diet to ensure their rate of weight loss is sustainable for the weeks and months ahead. Once they reach their goal, they can slowly reintroduce more energy from fat and carbs to practice weight maintenance.
If you’re more of a hands-on learner, you can start by tracking your food and meals in Cronometer using our free 7-Day Tracking Challenge. From there, you could slowly work towards your goal macros that you can calculate using our free PSMF calculator.
Is A Protein Sparing Modified Fast Ketogenic?
Because protein % is so high and calories are restricted, carbs tend to be very low on a PSMF diet. Thus, you will likely see elevated (endogenous) ketones from the breakdown and utilisation of body fat for energy if you are actively losing body fat.
We tend to see blood ketones rise as we start to lose weight. However, as fat loss continues and our bodies adapt to using our own stores for fuel, blood ketones typically decrease as extra energy from all sources is drained from your body.
For more details, see What Are Optimal Blood Glucose Levels for Ketosis?
Can I Use Protein Powders, Bars, or Other Products on a PSMF?
Protein powders, bars, and shakes can be used in a PSMF-style diet to boost protein intake as an adjunct to an already nutrient-dense diet.
Whey protein powders and products made from them tend to provide a complete amino acid profile compared to other packaged and processed high-protein products.
While this might sound like a low-cal, low-maintenance way to make all your PSMF dreams come true, keep in mind that powders are effectively pre-digested and will thus provide less satiety than whole foods.
Compared to meat, seafood, and non-starchy vegetables, they also have an inferior and often synthetic micronutrient profile.
To optimise your satiety and long-term sustainability, you should prioritise whole-food protein sources as much as possible.
How Much Fat Should I Consume on a PSMF?
Based on your activity levels and dietary preferences, the balance between your fat and carb intake will change. However, you will need to reduce your carb and fat intake to effectively lose fat on a protein-sparing modified fast.
Most of us have plenty of body fat stores to pull from. Hence, we don’t have a significant need for dietary fat aside from omega-3 fatty acids. Your gallbladder only needs around 10 g per day of fat to function, and you can still get a robust micronutrient profile with 10% dietary fat (0.4 g/kg LBM). If your protein comes from whole foods, it’s also rather hard to fall too far below this amount.
Because fat typically comes with protein, we don’t see additional benefit from a satiety standpoint if we reduce fats below ~40%. Remember, you also require some dietary fat for efficient gallbladder function, hormone production and omega-3 fatty acids for brain health.
Our analysis shows that your diet may be less nutrient-dense if less than 25% of your total calories come from fat.
To learn more about how fat impacts satiety and which ones are the best to consume, check out Does Eating Fat Make You Fat? The Surprising Truth About Cholesterol and Saturated Fat!
How Many Carbohydrates Should I Consume on a PSMF?
Although the body requires vitamins and minerals that often come packaged in nutrient-dense, whole foods like non-starchy vegetables, there is no minimum carbohydrate intake on a PSMF. A traditional PSMF typically restricts carbohydrate intake to 20 to 50 grams per day.
For more on how minerals and vitamins affect satiety and hunger, visit our respective articles, The Effect of Minerals on Appetite, Hunger, and Satiety, and Vitamins for Weight Loss and Satiety: Which Ones and How Much Do You Need?
Our analysis shows that we maximise our satiety when net carbs from non-fibre carbohydrates provide 10 – 20% of energy (i.e., net carbs = total carbohydrates minus fibre).
Fibre tends to improve satiety. Thus, consuming as much fibre from non-starchy veggies as you can reasonably tolerate will improve satiety and reduce hunger.
From a nutrient density perspective, there is no additional benefit to reducing non-fibre carbohydrates below about 10% of calories. Non-starchy vegetables provide many of the harder-to-find nutrients on a high-protein diet, and they are also highly satiating. Cutting out all carbohydrates can also push us to overconsume energy from fat.
For more on carbs and fibre and how they affect satiety, check out, How Carbs, Fat, Sugar, and Alcohol Affect Appetite.
Keeping your intake of fats and carbs to a minimum forces the body to either dip into your fat stores, or break down protein through the labour-intensive process of gluconeogenesis for energy.
Although we’re not consuming carbs directly, we can still get all the glucose we need for healthy brain function by eating high amounts of protein because of this process.
Gluconeogenesis requires considerable effort to convert protein to ATP for energy. In fact, up to 25-35% of the total energy we consume from protein goes towards converting it into usable fuel!
In contrast, carbs and fat use up much less energy and are thus easier to store as body fat (when we consume too much).
For more on this, check out Oxidative Priority: The Key to Unlocking Your Body Fat Stores.
You May Need Some Carbs to Support Intense Activity
If you’re physically active, you may need more carbohydrates than someone who is not.
However, the reality is it may be challenging to do a lot of intense activity if your overall energy intake is limited.
When cutting, most bodybuilders cut back on volume in the gym and focus on keeping their step count up because low-intensity activity relies more on slow-burning energy sources like fat rather than carbs.
Reducing their exercise intensity also decreases their risk of diving into their bodily protein stores for fuel (instead of body fat stores).
Protein % Has the Biggest Impact on Satiety!
When we combine the satiety responses of all the micronutrients, we see that reducing either fat or non-fibre carbs yields a similar satiety response. However, we get the most significant satiety benefit if we reduce energy from fat and carbohydrates, which increases our protein %.
For more on protein, check out Protein for Weight Loss: How Much You Need and Why It Works.
Why Micronutrients Are SO Important!
At this point, we want to dive into why nutrient density is not only vital for your success on a PSMF but also for your safety, especially when trying to lose weight quickly.
Vitamins and minerals are the catalysts that allow us to utilise our body fat for fuel. Thus, obtaining adequate micronutrients helps us mitigate any slowing of the metabolic rate, or adapt to a severe calorie deficit.
If we’re getting the profile of micronutrients we need, our bodies are more likely to keep feasting on our fat stores without noticing famine is imminent, as it has the raw materials it requires to do the job.
Conversely, eating low-quality foods, despite eating enough or too many calories, sends a signal that famine is imminent! Consequently, the body goes into ‘storage mode’ to prepare for an emergency and might respond with ravenous hunger.
Similar to the protein leverage hypothesis, the body seems to be driven to consume more energy when it’s eating nutrient-poor food until it gets the nutrients it requires.
Bruce Ames’ Triage Theory suggests that the body prioritises nutrients required for short-term survival when nutrient intake is scarce. Subsequently, preventing age-related diseases like cancer, autoimmunity, heart disease, Parkinson’s, and Alzheimer’s are put on the back burner.
While you might fare okay if you’re eating poorly for a short period, your long-term health will be better if you consistently get plenty of every essential nutrient.
Ideally, we want to get all these nutrients from whole foods instead of supplements because they’re more likely to contain a complex of essential and non-essential vitamins, minerals, amino acids, fatty acids, and food compounds with positive health benefits we don’t track.
Many PSMF proponents strongly recommend supplementing with vitamins, minerals, and omega-3s because it can be challenging to get enough of them when calories are strictly limited. As we read earlier, extreme caloric deficit, without adequate nutrients like in the Last Chance Diet, can even be fatal!
As your energy deficit becomes more aggressive, you will need to be more intentional about getting the nutrients you need.
Depending on the duration of your PSMF, supplements can definitely be helpful. However, your goal shouldn’t be to rely on them, and you should strive to get as many micronutrients from whole foods as possible! Not only does this keep you safe, but it will also increase your satiety which will help you prevent cravings and rebound bingeing.
Similar to protein, our analysis also shows a statistically significant appetite and craving response for several micronutrients like potassium, calcium, and sodium. The charts below show these relationships.
For more on potassium and how to get more of it during your PSMF, check out High Potassium Foods and Recipes: The Ultimate Guide.
For more on calcium and where to find it, visit Healthy High-Calcium Foods and Recipes.
For more on sodium, where to find it, and how much you need, visit Sodium in Food: A Practical Guide and How Many Grams of Sodium Do You Need Per Day?
The table below shows the ranking of the nutrients that our analysis shows align with greater satiety and a lower energy intake. While the protein % impacts satiety the most, many other micronutrients elicit a stronger satiety response when we get them from our diet.
|niacin (B3) (g/cal)||32%|
|vitamin B5 (g/cal)||28%|
|riboflavin (B2) (g/cal)||28%|
|vitamin A (g/cal)||23%|
|fibre: carb ratio||21%|
|vitamin B6 (g/cal)||20%|
|vitamin K1 (g/cal)||19%|
|thiamin B1 (g/cal)||17%|
|vitamin E (g/cal)||17%|
|vitamin C (g/cal)||14%|
|omega 3 (g/cal)||11%|
|vitamin B12 (g/cal)||9%|
Data from Optimisers in our Micros Masterclass also shows that we tend to eat less as we obtain all the essential nutrients we need from the foods we eat.
Because a nutrient-dense diet supplies all the vitamins, minerals, amino acids, and fatty acids they require—and not only protein!—their bodies have all the raw ingredients they need to carry out necessary processes. They’re more than happy to use their stored energy for fuel.
To demonstrate the power of micronutrient leverage, the chart below compares protein % vs calories from Optimiser data alongside our analysis of 438,014 days of data from MyFitnessPal users.
Protein % aligns with a lower energy intake for Nutrient Optimiser and MyFitnessPal users. However, moving from a low to high protein % yielded a significantly greater reduction in total calorie intake in our Optimiser population.
The decrease in energy consumption when protein % was increased was only 13% amongst MyFitnessPal users compared to the 55% achieved by our Optimisers.
This is likely because of the much higher protein % intake our Optimisers are recommended to consume. In our Macros Masterclass, Optimisers are guided to dial-up protein to improve satiety and nutrient density.
Aside from having an adequate protein %, we believe the other key factor to increasing satiety is nutrient density. So, if you want to maximise satiety to eat less, reduce your risk of rebounding from cravings, and avoid any dangers associated with a long-term energy deficit, prioritise all the essential micronutrients—not just the amino acids from protein!
What’s the Relationship Between Protein % and Nutrient Density?
When calories are limited, nutrient density becomes even more critical.
As the chart below shows, a higher protein % up to about 50% aligns with a greater nutrient density. However, the benefit of protein is not infinite, and more protein doesn’t align with greater nutrient density once we exceed this threshold. Although you’ll be getting plenty of (essential) amino acids, you’ll miss out on other nutrients!
For more info on why amino acids are critical for satiety, check out Optimal Amino Acid Intakes for Weight Loss and Satiety.
The micronutrient fingerprint chart below details the cumulative nutrient content of the top 50 recipes from our NutriBooster recipes when protein % is prioritised. While we get an extremely high 76% protein (shown in the top right of the chart), our intake of several micronutrients like manganese, folate, calcium, omega-3s, vitamin C, and vitamin E is lacking when we mainly focus on protein %. The nutrients we’re struggling to get enough of are listed at the top of the chart.
However, when we maximise nutrient density, we’re still getting plenty of protein, AND we’re also getting plenty of ALL of the micronutrients. As we’ve mentioned earlier, micronutrients are critical for satiety, safety, and sustainability in any diet!
What Should I Eat on a PSMF Diet?
To help you kickstart your journey toward a PSMF approach, we’ve included a ‘PSMF food list’ full of popular nutrient-dense, high-protein % foods from our High Protein:Energy NutriBooster recipe book below.
As you add more of these foods to your current diet, your satiety will improve, and you will be less drawn to less-than-optimal foods.
Optimal Foods for a Protein-Sparing Modified Fast
- egg white
- sirloin steak
- chicken breast
- pork loin
- cottage cheese (low fat)
- chicken liver
- chicken thigh
- arugula (rocket)
- Brussels sprouts
- coriander (cilantro)
- green beans
Nutrient-Dense Protein Sparing Modified Fast Recipes
To give you a taste of some of the nutrient-dense high protein % recipes included in this High Protein:Energy NutriBooster recipe book, we’ve also listed a few links to some of our protein sparing modified fast recipes below:
- Turkey & Spinach Egg White Omelette
- Blackened Fish Stir Fry
- Lettuce Wraps with Prawn Salsa
- PSMF Flan
- Spinach Stuffed Chicken Breast
- Coconut Chicken Kebabs
If you’re not ready to jump into the deep end of super high protein % recipes, we have a range of other recipe books optimised for nutrient density that you can check out here. Any of the nutrient-dense recipe books shown in the top right provide plenty of protein and the micronutrients you need to thrive!
Key Takeaways for a Successful PSMF
There is no one-size-fits-all Protein Sparing Modified Fast that works for everyone.
While there are many ways to apply a protein-modified fast, the list below includes a detailed step-by-step summary to ensure you optimise your protein-sparing modified fast results.
1. Focus on increasing protein %.
- In a clinical setting, the minimum protein intake required for weight loss is 1.2 g/kg of total body weight.
- Your appetite will likely drive you to eat more protein if you’re active. Thus, you should aim for 2.2 to 2.4 g/kg lean body mass (LBM) or more if you’re lifting heavy.To ensure you’re getting adequate protein, vitamins, and minerals while your energy intake is restricted, focus on nutrient-dense foods and use more processed ones like protein powders as adjuncts.
2. Keep fibre high, and minimise energy from fat and carbs.
- Eat only carbs from non-starchy veggies. In other words, avoid processed grains or foods with added sugars when compiling foods for your PSMF meal plan.
- Only eat fat that’s packaged alongside lean-protein foods.
- To learn how to dial your macronutrients in, you may be interested in our four-week Macros Masterclass.
3. Don’t overeat.
- If your goal is weight loss, limit energy from fat and carbs.
- If you’re not seeing the progress you want, tracking or planning your energy intake in Cronometer may be beneficial to ensure you’re moving toward your goals.
- If you want to track your calories, use our PSMF macro calculator and follow the directions to calculate your intakes to move towards your goals.
- Ratchet down your maximum energy intake until you achieve a desired, sustainable weight loss rate. This might allow you to eat more or less than the macros you’ve calculated from our PSMF calculator. A healthy and sustainable target is between 0.5 to 1.0% per week. Unless you carry a LOT of body fat, a faster rate can be hard to sustain for extended periods. As you lean out, losing weight while preserving lean mass will become harder.
4. Lift heavy and strength train.
- If you are dieting and relatively sedentary, your body will see your muscle as unnecessary and expensive. Thus, they will be on the list of body stores to use for energy.
- Resistance training will help you use dietary protein to build lean muscle so you can maintain (and sometimes improve) your metabolic rate.
5. Pay attention to your micronutrients.
- To optimise your protein and micronutrient intakes and ensure long-term success, use our free PSMF food list to formulate a PSMF meal plan that fits your tastes and preferences.
- Focus on our nutrient-dense, PSMF-focused NutriBooster recipes like those found in our High Protein:Energy NutriBooster recipe book and add them to your protein-sparing modified fast meal plan.
- To ensure your electrolytes stay balanced, incorporate an electrolyte drink and salt your food regularly. Be sure to include foods rich in potassium, magnesium, calcium, and sodium.
- If you are going to do a long-term PSMF or are using nutrient-poor foods, a multivitamin may be helpful.
- To get a jump start on maximising your success, you can use our free 7-Day Tracking Challenge to track your food in Cronometer and sync it with Nutrient Optimiser to identify optimal foods and meals to fit your goals. We look forward to being a part of your journey toward Nutritional Optimisation!
- A protein-sparing modified fast (PSMF) provides adequate protein to maintain lean muscle mass during a weight-loss period while restricting energy from carbohydrates and fat.
- Depending on the user’s goals and level of energy restriction, protein intakes can vary widely. However, a high protein % using high-quality protein sources is the key to a successful PSMF or any weight loss plan, for that matter.
- To improve your PSMF results and increase your chances of long-term success, prioritising nutrient-dense foods rich in fibre and protein is crucial.
- Obtaining adequate nutrients—ideally from whole foods—is critical for long-term health, weight loss, satiety, and keeping the pounds off.
- While the PSMF is used frequently in weight loss clinics and the bodybuilding community, we can apply the same basic principles to other situations requiring a calorie deficit to maintain lean muscle mass.
- Aiming for a sustainable rate of weight loss of 0.5 to 1.0% body weight per week is critical to long-term success and will increase your chances of maintaining the weight you have lost. The Macros Masterclass is an excellent place for guidance on this principle and others.