Precision PSMF Calculator: Tailor Your Macros for Optimal Results

Embarking on a weight loss journey? Leverage the prowess of a Protein Sparing Modified Fast (PSMF) with our meticulously designed PSMF calculator.

This tool, inspired by insights from Lyle McDonald, allows you to calculate your precise PSMF macros, propelling you towards your desired physique while preserving your muscle mass.

Delve into this insightful guide to understand the science behind PSMF and how our calculator can be your companion in achieving your fitness goals. Your transformation awaits!


Summer is just around the corner, and if you’re like many, you’re harbouring dreams of a trimmer waistline, sculpted abs, and an overall fit physique that turns heads.

But guess what? You’re not alone in this journey!

If you’re on the hunt for a speedy and effective fat loss strategy, it’s time to dive into the world of the Protein Sparing Modified Fast, or PSMF. This isn’t just any diet; it’s the go-to choice for both bears (yes, you read that right) and bodybuilders who want to shed fat while holding onto their hard-earned muscle.

So, what’s the scoop on PSMF? Well, it’s all about providing your body with enough protein to safeguard your precious muscle (your metabolic powerhouse) while keeping fat and non-fiber carbs on a tight leash. In essence, it’s like fasting, but with a twist—it spares the protein in your body, preserving your muscles and organs.

But that’s just the tip of the iceberg! In this article, we’ll give you the lowdown on why PSMF is the real deal and show you how to put it into action using our handy, free PSMF calculator.

And if you’re hungry for even more details, check out our sister article, “Crash Diet or Miracle Weight Loss? What You Need to Know About the PSMF Diet.”

It’s time to kickstart your summer transformation journey!

PSMF is Like Spring for Your Metabolism

In nature, animals like Beadnose the Bear — pictured in both images below, six months apart — naturally cycle their weight depending on the seasons. 

Foods available in autumn provide abundant easily accessible fuel from fat and carbs that allow animals to fatten for winter.  But with access to mainly lean protein and fibre in spring and less energy from fat and non-fibre carbohydrates, they quickly shed their winter blubber.

Unfortunately, our modern food-processed system has left us in a perpetual autumn, with unlimited access to tasty processed fat+carb combo foods.  Winter never comes for most of us, let alone spring.  

To continue with this analogy:

  • we could see winter as a lower-carb diet, with fewer carbs and more energy from fat and protein, meanwhile
  • summer provides more energy from carbohydrates and less fat. 

While many people maintain their weight on a lower-carb or lower-fat diet, the greatest satiety and hence most effective fat loss occurs when we dial back energy from both carbs and fat while prioritising protein and other nutrients. 

How Do You Do a Protein Sparing Modified Fast?

George Blackburn coined the term ‘protein sparing modified fast’ (PSMF) in the 1970s.  Blackburn found very high protein feeding with obese, bed-bound patients extremely effective for weight loss while minimising muscle loss and other adverse changes to metabolism that often accompany low-calorie diets.  

A protein-sparing modified fast in a clinical weight-loss setting involves no more than 800 calories per day while maximising their protein intake and minimising energy from fat and carbs.  

Due to oxidative priority, your body uses more fat when glycogen is depleted after eating fewer carbs. So then, if your dietary fat is low enough, you will start using your stored body fat.

What Makes the PSMF Special?

Most diets simplistically focus on restricting calories, which can lead to a loss of metabolically active muscle and a decreased metabolic rate over the long term 

In contrast, a PSMF focuses on protein to intentionally preserve metabolically active lean muscle mass, with most weight loss coming from your body fat.

Restricting energy from fat and non-fibre carbs while prioritising protein and fibre pushes your body to use your stored fat for fuel and ‘spares’ your metabolically active lean muscle mass and your body’s protein stores. 

Protein is also the most thermogenic macronutrient.  We use between 25 and 35% of the energy contained in protein simply by digesting it and turning it into usable energy and muscle.  So, if you’ve ever felt the ‘meat sweats’ or the warm feeling experienced after eating a lot of protein, you’re experiencing thermogenesis at work.  Many people who practice some form of extended fasting report they start feeling cold because they get less energy from their diet and a lot less protein. 

How Long Can You Do a PSMF?

How long you can—or should—stay on a PSMF depends mainly on how much weight you have onboard and how severe your deficit is.

Even though a PSMF prioritises protein, a PSMF is still an aggressive weight loss approach.  Most people tend to be able to maintain 0.5 – 1% over weeks and months. If someone is willing to increase their calorie deficit, even greater rates of weight loss are possible in the short term. 

However, a more aggressive approach tends not to be sustainable over the long term.  Whether your PSMF is self-inflicted or a result of being stranded on a desert island, the body perceives a calorie deficit as starvation, which is a stressor.

Whenever we are stressing our bodies, we release stress hormones which can interrupt the balance of other hormones.  As a result, we can see weight gain, cravings, binge tendencies, insomnia, and a lot of other side effects that could negatively impact your weight loss efforts.  Thus, it’s important to determine if you are capable of a PSMF.

A PSMF diet is usually broken into the intensive and refeeding phases.

  • The intensive phase can last up to six months, although this tends to differ depending on how much weight you have to lose.  During this period, calories are restricted, aerobic exercise is kept to a minimum and strength training is encouraged.
  • During the refeeding phase, calories are slowly increased.  In essence, this is a ‘reverse diet’ to slowly bring calories back up and avoid any rebound weight gain.

Body recomposition expert Lyle McDonald introduced PSMF to the bodybuilding world in his book, The Rapid Fat Loss Handbook.  Based on someone’s body type and body fat percentage, he recommends regular diet breaks with more frequent refeeds to avoid excessive impacts on your metabolism.

Is PSMF A Good Diet?

While most people would judge a ‘good diet’ by their weight loss—whether it be lean body mass or fat loss—there’s really much more to it than that.

Energy is always conserved, so there has to be a calorie deficit for weight loss to occur.  Thus, most diets are similar because they restrict the amount of energy you’re taking in.

However, due to the power of protein leverage, protein is also the most satiating macronutrient, meaning we will be less likely to experience cravings or unbearable hunger if our intake of protein increases.

A PSMF-style diet is ideal for fat loss because it not only focuses on the weight on the scale but gives you the best chance of losing fat while sparing the protein in your body.  

How Much Weight Can You Lose on a PSMF?

The more weight you have to lose, the more rapidly you will be able to lose weight. For example, someone with a LOT of extra weight may be able to lose 3% of their body weight per week.  However, rapid weight loss is hard to sustain unless you develop new habits and learn to eat in a new way. 

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.  But it’s important to note that after five years, the PSMF participants regained weight to similar levels to the control group. 

For long-term weight loss success and maintenance after the aggressive dieting phase, you need to learn to eat in a new way that you can sustain for the long term.  You can’t just go back to how you were eating before once the dieting phase is over. 

As you get closer to your ideal body fat levels, dieting becomes harder, and your rate of weight loss will slow. 

If you push it too hard, you risk waking up your lizard brain (i.e. your survival instincts in your subconscious amygdala).  At this point, your cortisol rises, which could lead to the dreaded weight loss plateau at best, and uncontrolled rebound bingeing at worst.

Our Macros Masterclass guides people towards a PSMF-style diet with adequate protein and less energy from fat and non-fibre carbohydrates.   While many of our Optimisers can lose weight much more rapidly in the short term, we find 0.5 to 1.0% of weight loss achievable over the longer term.  

Micronutrients are Critical to Minimise Your Cravings

A PSMF-style diet is effective because it uses the power of protein leverage.  Professors Stephen Simpson and David Raubenheimer have demonstrated that all organisms eat until they get the protein they need. 

We are always working to balance our intake of protein and energy from our diet.  When our diet contains less protein and more energy, we eat more to get the protein we need.  A PSMF-style diet reverses this paradigm in our modern processed food environment by prioritising protein.  Thus, we are satisfied with less energy.    

Similar to protein leverage, our satiety analysis has also demonstrated a “micronutrient leverage” effect for many, if not all, of the essential micronutrients.  We will continue eating until we get the nutrients we require.  So if the food we eat contains fewer vitamins, minerals, and essential fatty acids per calorie, we eat more. 

Essential micronutrients like amino acids, vitamins, and minerals are needed to use body fat for energy.  Thus, you could stall your progress if you only eat processed, nutrient-poor foods.  Prioritising nutrient density ensures you are also getting adequate micronutrients.  While you may be able to restrict your intake for a while by tracking your food and your conscious willpower, changing what you eat has a massive influence on whether or not you’ll lose weight over the long term.  

Once they have the foundation of adequate protein, our Micros Masterclass guides Optimisers to get all the micronutrients from their food.

Kill Cravings and Stay Energised with Electrolytes

While our satiety analysis shows that protein is the most critical nutrient that dictates your satiety, minerals like calcium, sodium, and potassium aren’t far behind.  If you’re tracking your food in Cronometer, you’ll quickly be able to see which minerals your diet is currently lacking. 

Considering you will be consuming low-calorie, high-protein foods on your PSMF, it can be challenging to get adequate micronutrients, particularly electrolytes, from food alone.   

As a minimum, make sure you’re salting your food to taste every day.  Many people have also found our Optimised Elecotryle Mix helpful to provide many of the harder to find minerals in the optimal ratios for increased satiety. 

How Fast Can You Lose Weight on PSMF?

The rate you’ll lose weight depends on how aggressive your caloric deficit is and how much weight you have to lose.   

But when our bodies are deprived of energy for an extended period, they start to use other tissues for energy, whether fat, muscle or even your vital organs.  Your muscle and organs are the most metabolically active, or the tissues that expend the most energy at rest.  Hence, preserving them is critical to maintaining your metabolic rate. 

To avoid unnecessary slowing of their metabolic rate, in our Macros Masterclass, we suggest Optimisers eat as much as they can while still achieving their desired rate of weight loss.  With some trial and error, they find the minimum effective dose of restriction that still allows them to lose weight at a sustainable rate. 

If you have a significant amount of weight to lose, you want to find the right balance that will allow you to be still making progress in six months, not just fall off the wagon in two weeks because you tried too hard on day one.  Unless you have a specific weight loss ‘deadline’, we recommend most people aim for a sustainable weight loss goal of 0.5 – 1.0% per week.   

How Many Calories Are in a Protein Sparing Modified Fast (PSMF)?

A PSMF diet in a clinical weight-loss setting limits energy to no more than 800 calories per day.  However, there’s no universal protein-sparing modified fast calculator or PSMF macros that suit everyone.

Your activity levels, muscle mass, age, height, weight, and other factors determine how much energy YOU require.  If your calorie goal is too high for YOU, you simply won’t lose weight.  But if your calorie goal is too low for YOU, you won’t be able to maintain it, and you’ll likely experience overwhelming hunger and cravings.

The type and quality of energy you eat also affect how many calories you consume.  By restricting the number of calories you’re taking in without changing WHAT you eat, cravings and hunger usually spiral out of control.

High protein % foods and meals tend to have a low energy density and are highly satiating.  Thus, a higher-protein, lower-carb, and lower-fat diet like a PSMF can quickly lead to greater satiety and a significant caloric deficit with less use of our conscious willpower to restrict.

Most people find satiety kicks and start eating less and losing weight once they change the quality of foods to give their body what it needs from the food they eat,

How Much Protein Should You Eat on a PSMF?

The first step to calculating your PSMF macros is determining how much protein you need. 

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 when our Optimisers also focus on nutrient density.  

But it’s critical to understand that it’s not just a matter of eating more protein, especially on a PSMF.  Because protein typically comes packaged with fat, it’s crucial to prioritise leaner protein sources during a PSMF to reduce your overall calorie intake.  You can’t just eat high-fat foods like butter and bacon to get your protein.  If you do this, you’d simply end up eating more calories. 

The simple ‘secret’ is that dialling back your intake of BOTH fat and carbs leads to improved satiety, a lower spontaneous energy intake, and fat loss while preserving lean muscle.  As shown in the chart below, as we dial back our intake of energy from both fat and carbohydrates (while still prioritising protein), we are satisfied with fewer calories.

How Much Protein Do YOU Need on a PSMF?

You might be able to get away with 1.4 g/kg lean body mass (LBM) if you’re not physically active and are relatively sedentary.  However, if you are resistance training, you should ideally aim for 1.8g/kg LBM or more.

Using our Optimisers’ data, we determined:

  • The median protein intake is 2.4 g/kg LBM.
  • The average protein intake is 2.7 g/kg LBM. 
  • A handful of people get more than 7.0 g/kg LBM protein. 
  • Very Few people get less than 0.5 g/kg LBM. 

Lyle McDonald’s The Rapid Fat Loss Handbook recommends bodybuilders to consume up to a MASSIVE 4.4 g/kg LBM protein.  But this will be difficult for most people to achieve while still achieving an energy deficit unless they are VERY active. 

As you can see from the distribution chart above, it’s not easy to maintain a VERY high protein % for long.  Your body is always trying to find the right balance of protein and energy.  While your body can convert protein to energy, it’s not easy.  So once you start to deplete your stored energy, your cravings for easy energy from fat and carbs will increase.   At this point, energy-dense low protein foods become very attractive!

What Should MY Macros Be on PSMF?

If you’re looking to lose weight in a hurry, your calorie deficit will likely be more dramatic.  The balance of fat and carbs doesn’t matter too much, as long as you’re hitting your protein goal.  

Regardless of your total daily energy intake, your focus should be on increasing the percentage of total calories coming from protein or your protein %.  

While creating a diet with more than 55% calories from protein may be possible, we tend to find people have the best long-term results when people work up to 40% protein. 

How Much Fat Should I Eat On a PSMF?

Fat is the most calorically dense macronutrient, with every gram providing nine calories.  Hence, limiting fat is critical during a PSMF to achieve a large calorie deficit. 

However, as shown in the chart below from our satiety analysis, moving from a very high-fat diet to a lower-fat will increase satiety and aligns with a lower calorie intake.  However, you’re unlikely to achieve significantly greater satiety by reducing fat to below 40% of your calories. 

Some fat tends to come along with protein, so it’s practically difficult to get much lower than that with whole foods.  Your body also requires around 1.6 g per day of omega-3 fatty acids and requires some fat for healthy gallbladder function. 

How Many Carbs Are in PSMF?

The reality is, once you are prioritising protein and getting some fat that comes you’re your whole protein sources, there isn’t a lot of room left for carbohydrates. 

Although it’s a little harder to do, your body can create all the glucose it needs from dietary protein in a process known as gluconeogenesis.  So you can think of it as resistance training for your metabolism.

Carbs are typically limited to between 20 and 50 g per day during the intensive phase of a clinical PSMF.   If you’re more active, you may want to opt for more carbs and less fat.  However, if your blood sugars are elevated, you can dial back the carbs more. 

Our satiety analysis shows that we eat the most calories when we consume around 45% of our energy from non-fibre carbohydrates, with the remainder from fat.  Conversely, we eat much less when we reduce our intake of non-fibre carbs to between 10 and 20%.  Going lower than this does not provide any additional benefit.  Non-starchy vegetables tend to provide a lot of bulk, which helps with satiety and provides some of the vitamins and minerals that are harder to find in very high-protein foods

How Do You Calculate Macros for Carnivores?

If you’re following a carnivore diet, your carbohydrate intake will already be very low.  

Your body can produce all the glucose it needs from protein through gluconeogenesis.  But keep in mind that because your glucose is made from the protein you eat, you may need to prioritise more protein to make up for the protein that is being converted to glucose in your body. 

Many people prefer a higher fat style carnivore diet because it’s tastier and more sustainable.  However, if you want to use the fat on your body, you’ll need to dial back the fat in your diet while still prioritising protein.   

The PSMF Macro Calculator

To get started with your personalised PSMF, you can use our simple macro calculator.

  1. First, enter your current body weight.
  2. Next, enter your current body fat.
  3. Select ‘weight’ loss as your goal.
  4. Choose your intended deficit.  For a PSMF, we recommend choosing a ‘savage’ deficit.
  5. Use the slider to select your preferred net carb intake.
  6. Finally, use the protein slider to dial up your protein target.

The outputs on the right will show you your target protein, sparing modified fast macros or your calorie, protein, fat, and carb intakes.  From there, you can enter them into MyFitnessPal or Cronometer.

As long as you hit your protein target and stay below your calorie limit, the mix of fat and carbs doesn’t matter much.  In our Macros Masterclass, we guide Optimisers to focus on keeping their protein bar longer than their energy bar in Cronometer.  Each week, the Smart Macros algorithm in Nutrient Optimiser adjusts the macronutrient targets to ensure sustainable weight loss progress.

For more detail on how we guide Optimisers to dial in their macronutrients in the Macros Masterclass, check out Macronutrients [Macros Masterclass FAQ #2].

Tracking Your Progress

Aside from weight loss, tracking body fat can be a valuable tool to ensure the weight you’re losing is mainly fat and not muscle. 

Even while losing weight, we find many people are able to gain muscle mass by simply increasing their protein percentage (protein %).

What Should I Eat in PSMF?

Finally, we have created our High Protein: Energy Recipe Book for people who want to jump into the deep end with a hardcore PSMF.

Our Protein: Energy Recipe Book is packed with recipes that have super high protein percentages while still providing all the micronutrients your body requires to prevent deficiencies and cravings on your PSMF. 

Optimising Your PSMF Experience

While restricting calories is critical, it’s not the only thing you should consider when starting your PSMF. 

Aside from a calorie deficit, here are some other top takeaways to take into account when calculating your macros for PSMF:

  1. Prioritise protein and increase the percentage of calories coming from protein.
    • Most importantly, we need protein to preserve metabolically active lean muscle mass.  Without it, our bodies break down our muscle tissue responsible for burning calories at rest.
    • A high protein % can stimulate muscle growth and raise our metabolic rate, despite eating at a calorie deficit.
    • Protein is the most thermogenic macronutrient, meaning we burn the most energy breaking it down.  It is also the most satiating.
  2. Reduce energy from fat and non-fibre carbohydrates.
  3. Use nutrient-dense foods to fill in the gap.
  4. Use Cronometer to keep track of all your intakes.
    • Weigh your foods to ensure you’re accurate and on target to hit the macros you calculated.
  5. Stay hydrated with an electrolyte recipe.
    • Supplying your body with high-satiety electrolytes will keep you full and energised at the expense of a few (if any) calories.