The Secrets To Sustainable Protein Sparing Modified Fast (PSMF diet)

The Protein Sparing Modified Fast (PSMF diet) is regarded by many to be the most effective way to lose body fat while preserving muscle and avoiding rebound binging due to cravings.  This article covers all you need to know to tailor a nutrient dense PSMF to suit your goals.

What is a protein sparing modified fast (PSMF)?

First developed in the 1970s, variations of the PSMF have been successfully implemented in a range of settings from weight loss clinics to the bodybuilding community.  While the details vary, a protein sparing modified fast can broadly be defined as a diet that contains adequate protein to preserve lean muscle mass while limiting energy from carbohydrate and fat.

It’s all about maximising satiety and nutrient density

As shown in the charts below in the charts developed from twenty-five thousand days of food logging data from users of the Nutrient Optimiser, we tend to eat less spontaneously when we focus on getting more protein in our diet in percentage terms.  

Conversely, we tend to consume fewer calories when more of our diet consists of less energy from fat and carbohydrates.  

As we saw in the first Nutrient Optimiser challenge, people tend to lose body fat fairly consistently when they focus on nutrient dense high satiety foods and meals with adequate protein.  

And more importantly, lean body mass can actually increase, particularly if your current protein relatively low.  

How does PSMF diet work?

Despite the peculiar name, there’s nothing magical about a protein sparing modified fast.  A focus on protein increases satiety while preserving muscle mass which is critical to maintaining metabolic health.  The reduced energy from fat and carbs allows your body fat to be used. In the absence of energy from carbs and fat, your body is forced to use body fat for fuel, and you will start to crave energy-dense fat and carb foods.  So it becomes a matter of will power to see how low you can keep your fat and carbs and ‘embrace the suck’ while you force your body to use up your stored body fat and convert protein to energy.

Is the protein sparing modified fast ketogenic? (PSMF Ketosis)

While protein intake is high in terms of percentage of calories from the diet, the protein sparing modified fast could be seen as an (endogenously) ketogenic due to the high contribution of body fat to your energy expenditure which will often cause ketones to be elevated.  

In the first Nutrient Optimiser Challenge blood ketones tended to rise initially as participants started to lose body fat, but then decreased again as their weight loss continued and they continued to lose body fat and adapted to burn fat more efficiently.  

This decrease in ketone values over time is typical.  Even the participants in the Virta study found their ketones decreased back to baseline levels after the first few weeks.   

PSMF diet guide

This article outlines the fundamental principles of the PSMF diet that can be applied to weight loss or maintenance over the long-term.

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If you want to use our PSMF calculator and determine your optimal protein, fat and carbohydrate range we recommend can get your Nutrient Optimiser Free Report.  Select ‘fat loss’ as your goal to determine your minimum protein intake and maximum carb and fat limits.  Once you adjust to these initial macros the Nutrient Optimiser Smart Macros algorithm will help you dial in your macros further to ensure you continue to lose fat while preserving muscle.


To learn more about the PSMF diet and how it works, read on.

How much protein on  PSMF diet?

In the medical version of the PSMF diet, patients obtain the majority of their energy from protein while keeping energy from both carbohydrates and fat low.[1]  

Protein levels are set at 1.2 to 1.5 g/kg of ideal body weight per day.  

  • Carbohydrate intake is restricted to less than 20 to 50 g/day.
  • Additional dietary fat beyond what comes with lean protein sources is minimised.
  • Patients in a weight loss clinic setting are restricted to less than 800 kcal/day.

Benefits of a PSMF diet

The Cleveland Clinic has carried out extensive research into the use of adequate protein low-calorie diets for aggressive weight loss and found that:[2][3][4]

  • patients are often encouraged by the initial period of rapid weight loss which reduces the dropout rate;[5]
  • while meal replacements in the form of commercial shakes or bars can be used, learning to make meals from whole foods is critical to long-term success;
  • the protein sparing modified fast is effective for people with normal glycemic control as well as pre-diabetes or Type 2 Diabetes;[6]
  • people on a whole food-based PSMF diet are significantly less hungry and preoccupied with eating compared to those on a liquid-formula based version of the PSMF; and
  • most of the weight lost during a PSMF diet is from fat tissue rather than muscle.[7]

“Adherence to a very-low-calorie, ketogenic PSMF program results in major short-term health benefits for obese patients with type 2 diabetes. These benefits include significant weight loss, often more than 18 kg, within 6 months.  In addition, significant improvements in fasting glucose and haemoglobin A1c levels are linked to the caloric and carbohydrate restriction of the PSMF diet. Insulin resistance was also attenuated, with the possible partial restoration of pancreatic beta-cell capacity.”[8]

PSMF diet in bodybuilding

Lyle McDonald popularised the PSMF diet for the bodybuilding community in 2005 with his Rapid Fat Loss Handbook.

McDonald details how someone can individualise the PSMF diet based on their goals and context.

  • Someone who is already very lean and undertaking heavy weight training will need higher levels of protein, while someone who isn’t yet lean may do better with a less aggressive approach over a more extended period.  If your protein intake is low, then all you need to do is increase your protein intake to gain muscle.
  • McDonald’s recommended protein intake ranges from 2.2 g/kg LBM to 4.4 g/kg LBM depending on context (i.e. much higher than the weight loss clinic).
  • Unlimited green leafy fibrous veggies are encouraged as they are filling and provide micronutrients with minimal calories.
  • McDonald also recommends supplementing with a multivitamin, sodium, potassium, magnesium, taurine, calcium and fish oil.  These nutrients can be harder to obtain when dieting, particularly if not prioritising veggies or seafood.
  • A severely energy restricted PSMF is typically not a long-term proposition due to the risk of nutrient deficiencies with a severe energy deficit.  Nutrient density becomes even more important when your calories are restricted.

How much protein in a calorie deficit?

If you are active and/or doing resistance training, you will have greater requirements for protein.  As shown in the chart below from a review paper by Stuart Phillips, lean muscle mass is best preserved when we have at least 2.6g/kg total body weight where there is an aggressive deficit (e.g. 35%).  However, a lower protein intake of 1.5 g/kg body weight seems to be adequate where we have a more moderate deficit.

Protein drives satiety

The body fiercely defends the loss of muscle mass by increasing appetite after periods of fasting or low protein consumption to ensure that muscle mass is retained.[10]   Conversely, prioritising protein typically leads to lower spontaneous calorie intake.[11][12]

“Protein generally increases satiety to a greater extent than carbohydrate or fat and may facilitate a reduction in energy consumption under ad libitum dietary conditions.”[13][14]

If we eat foods with a lower percentage of energy from protein, we may end up consuming more energy to obtain adequate protein.   Conversely, we can ‘hack’ our appetite by prioritising adequate protein while reducing energy from carbohydrate and fat.

The chart below (from the article Optimising macros for fat loss with less hunger) shows that diets with a higher percentage of their energy from protein tend to increase satiety and lead to less energy intake while dietary approaches with less protein tend to increase spontaneous energy intake.

Minimum carbohydrate requirement on a PSMF

While there is a need for vitamins and minerals that are often packaged with carbohydrate-containing foods (e.g. nutrient dense non-starchy vegetables), there is no minimum level of required carbohydrate intake.  While it takes more effort for the body to convert protein to ATP for use as energy, we can get the glucose we need for our brain function from protein via gluconeogenesis.

PSMF fat requirement

Most of us have plenty of body fat stores that they can draw on and hence do not have a significant need for dietary fat other than the essential Omega 3 fatty acids.  You can still get a robust micronutrient profile with 10% dietary fat (or 0.4 g/kg LBM).

While not easy, limiting dietary fat while also limiting carbohydrates allows the fat from your body to be used for energy and provides that maximum satiety on a calorie for calorie basis.  

The ‘problem’ here is that your body likes to be as efficient as possible.  Once you have eaten all the protein you need for muscle building and repair, your body will seek out fat and/or carbs for energy.  

The trick to making a PSMF diet to work is that you need to avoid fat+carb foods while prioritising protein to maximise satiety.

Thermic effect of food

Another advantage of consuming a higher protein diet is increased thermogenesis (i.e. the energy lost in the process of converting food into energy).[15]  We lose a lot more calories metabolising protein compared to fat or carbohydrates.

While there is endless debate over the “metabolic advantage” of fat vs. carbohydrates, there is actually an advantage when it comes to how many calories of protein we eat versus how much we can convert to energy.

While we can convert protein to glucose (i.e. gluconeogenesis), it is harder to do, and our body doesn’t like to do unless it has to.[17]  Satiety typically kicks in once we have had adequate protein and we go in search of fat or carbs which are easier to convert to energy.

Just think, you can only eat so much steak, but you always have a ‘dessert stomach’, even after a big meal.  

Humans are programmed to overeat foods with fat and carbs with minimal protein that are available in autumn to ensure that they survive the coming winter.  In our modern food environment though, winter never comes.

Should you just eat the highest protein foods?

So perhaps the best approach would be to simply focus on maximising the protein in your diet.  But this is not necessarily ideal because nutrient density becomes even more critical when you are trying to limit calories.   

The chart below shows the nutrients that you would obtain eating 2000 calories of the highest protein foods in the USDA database.  While you would get heaps of amino acids and B vitamins (shown at the bottom of the chart), you might struggle to get enough of some other nutrients like folate, calcium, vitamin A, vitamin C, vitamin K1 and magnesium.  

Potential problems with protein sparing modified fast without adequate attention to nutrient density

Ensuring you are getting adequate micronutrients is a crucial component to long-term success in weight loss and maintenance.  In his Rapid Fat Loss Handbook, Lyle McDonald mentions ‘The Last Chance Diet’ which was popular in the 1970s and 80s.  It was essentially a PSMF centred around liquid nutrition which led to the death of a number of devotees due to some fatal flaws.[19][20]   

  • First, they picked the cheapest protein source available, collagen; a protein that provides virtually zero nutrition to the body.  
  • Second, they provided zero supplemental vitamins and minerals (some of which would have been obtained if the dieters had been eating whole foods in the first place). This caused a couple of problems including cardiac heart loss (from the total lack of protein) and arrhythmias from the lack of minerals.

The problem wasn’t with the approach or the high protein, but rather the nutrient-poor food choices.  Conversely, a PSMF based around whole foods (which provide high-quality proteins as well as vitamins and minerals) and with adequate mineral supplementation can actually be one of the most nutrient dense dietary approaches there is.[21]

Bruce Ames’ Triage Theory

Nutrient density becomes even more critical when we are trying to eat less.  Obtaining adequate micronutrients can help to mitigate metabolic/mitochondrial slowdown and adaption to the severe calorie deficit.  

If we are getting the range of micronutrients we need, the body is more likely to keep on feasting on our fat stores without thinking there is a famine.

Similar to the protein leverage hypothesis, it seems if we provide the body with low nutrient density food it is driven to consume more energy to ensure that it gets the nutrients it needs.

Bruce Ames’ Triage Theory suggests that if we are low in critical nutrients, the body will prioritise those nutrients for functions essential to short-term survival rather than longevity and preventing the diseases of ageing (e.g. cancer, heart disease, Parkinson’s, Alzheimer’s, etc.).

“The triage theory posits that some functions of micronutrients (the approximately 40 essential vitamins, minerals, fatty acids, and amino acids) are restricted during shortage and that functions required for short-term survival take precedence over those that are less essential.  Insidious changes accumulate as a consequence of restriction, which increases the risk of diseases of ageing.”[22][23]

So, while we might do OK with poor nutrition for a limited period, we will probably do better if we obtain a substantial amount of all the essential nutrients.  

Ideally, we would get these nutrients from whole foods which are more likely to contain all the non-essential but also beneficial vitamins and minerals that we don’t track.

The most satiating and nutritious foods

We can use the Nutrient Optimiser to identify foods with the nutrients we want to obtain more of.   Prioritising amino acids is usually unnecessary because maximising vitamins and minerals generally leads to more than adequate protein.   However, in a PSMF where we are severely limiting energy, we want to increase protein as well.

Rather than just targeting the highest protein foods, a more optimal approach is to target the most satiating foods that also maximise your nutrient intake.  The chart below shows the nutrient profile for these foods. While they contain heaps of non-starchy veggies to fill you up, they also provide plenty of protein to preserve your lean muscle.  

How often should I eat on a PSMF?

It’s not so important when you eat as long as you stick to the foods that align best with your goals when you do eat.  

Recent research suggests that in the fasted state we can use a massive 3.5 g/kg/day and digest up to 4.3 g/kg/day of protein.[25]  This makes sense in an evolutionary context when there wouldn’t have been a regular supply of food, but we would have needed to be able to use the food when we came across a big hunt after a long famine.

Practically though, it can be hard to consume your minimum protein allocation in one sitting.  Eating two meals a day seems to be ideal to help you maintain a consistent deficit while maximising satiety and minimising your opportunities to continue to eat.


How many calories should I eat on a PSMF?

A PSMF is not a zero calorie fast.  You need some protein and ideally some essential fats and adequate vitamins and minerals from your diet (while minimising energy intake as much as you can sustainably achieve).

People with more fat to lose will be able to maintain a more significant deficit for longer without losing muscle than lean bodybuilders.  If you’re interested in trying a PSMF, the Nutrient Optimiser free report will give you minimum protein levels as well as a minimum calorie intake to start your PSMF journey.  

If you’re not achieving at least 0.5% per week, the Smart Macros algorithm will help you to ensure you continue to lose weight while making sure you don’t lose more lean muscle mass than fat.

What about “rabbit starvation”?

“Rabbit starvation” happens to very lean people if they only have lean protein foods with minimum fat available.  They just can’t get enough energy to sustain high levels of body fat. However, for most of us who have plenty of body fat, this is not going to be an issue!  

How to do a nutrient dense PSMF

  • Target plenty of protein and fibre with less energy from fat and carbs as per the foods and meals recommended your Nutrient Optimiser free report.  
    • Minimum protein intake in a weight loss clinic setting is 1.2g/kg total body weight.  
    • Your appetite will likely drive you to eat more protein if you are active (2.2 to 2.4 g/kg lean body mass is typical for someone lifting heavy).
    • Focusing on nutrient-dense foods will ensure you still get adequate protein as well as vitamins and minerals while minimising energy consumption.
    • Eat only carbs that come with non-starchy veggies (i.e. no processed grains or foods with added sugars).  Eat only the fat that comes with lean protein foods.
  • Don’t eat too much
    • It will be hard to overeat nutrient dense high satiety foods if you stick to only these foods.
    • It may be beneficial to track or plan your energy intake to ensure you are achieving your goals.
    • Ratchet down your maximum energy intake until you achieve your desired rate of weight loss (e.g. at least 0.5% per week).
  • Lift heavy / exercise (optional)
    • If you are dieting and relatively sedentary the body will see your muscle as unnecessary and expensive.  
    • Resistance training will help you to use the protein to build lean muscle and keep your metabolic rate up.
  • Pay attention to your micronutrients
    • To improve your chance of long-term success, it’s essential to pay attention to both your protein and your micronutrient intake.    
    • If you want to maximise your chance of success you can track your food in Cronometer which will sync with the Nutrient Optimiser to identify optimal foods and meals for you.


  • The protein sparing modified fast (PSMF) provides adequate levels of protein to support lean muscle mass while restricting energy from carbohydrates and fat.
  • Protein intakes vary widely depending on the goals and the level of energy restriction between.
  • Providing adequate nutrients, ideally from whole foods, is critical to long-term weight loss and maintenance.
  • It is crucial to prioritise nutrient-dense foods to improve your chances of long-term success.
  • While the PSMF is commonly used in weight loss clinics and the bodybuilding community, the principle can also be applied in other situations to maximise fat loss while maintaining lean muscle mass.

Further reading

Nutrient Optimiser



























post updated August 2017

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  • eismarsh says:

    Personally using LCHF, but keeping nutrition high, I am now working through the Nutritional Ketosis (NK) which proposes lower protein to stop gluconeogenesis ( for readers: ). What are your thoughts on gluconeogenesis in the whole mix ? and the g/kg ratio ?

    P.S. I am waiting eagerly to use your NO with NK 🙂

    Thanks for all your efforts, information and detail 🙂

  • Thomas says:

    When your body is in a nutritional ketosis state your body naturally goes into a muscle sparring mode. It seems redundant to overload your body with protein. If your body has any stored fat that’s where you will get the energy.. your body is not going to start breaking down your muscles for energy when there is stored fat to be burned. You don’t loose muscle because of your diet. You loose muscle because you are not using them.

    • definitely. as discussed in the article, people who are just looking for aggressive weight loss aim for lower end protein intakes (1.2g/kg ideal body weight) while people who are lifting heavy and turning over a lot of protein typically target more to prevent loss of lean muscle mass in fasting. if you just focus on getting adequate nutrients it’s hard to not get enough protein.

  • Marty, are the nutrients inside the box with red dashed lines the ones that are to be focused on for each of the different diets? In other words, if I focus on these specific nutrients and use your food list for the diet it references, then I don’t need to worry about tracking the other nutrients?
    Thank you!

    • I need to write a post to flesh that out more. A full red rectangle equates to a score of 100% (i.e. you would have two times the DRI for all nutrients). There are always nutrients that are harder to find, but they are different for different approaches. If we focus on foods that contain more of the harder to find nutrients we typically get more than enough of the other ones. I hope that helps.

  • eismarsh says:

    I have still to fully understand and utilise this great data, once I get the MyFitnessPal (MFP) data out and into Cronometer, then to Nutrient Optimiser (NO), can you flesh/spell-out the data stages I have just mentioned ? to get to really use your NO
    Thanks Marty

    • sure. let me know if you have specific questions.

      • eismarsh says:

        I finally got my data from MFP, as I log all foods/macros AND MFP pulls in exercise and body stats from Runtastic app, and I have a lot of data with MFP:

        1) What is the better way to import the data (directly or via a few moves) ?

        2) and then how do I setup Nutrient Optimiser ?
        to pull data from MFP and/or Cronometer

        3) Are there any other sources I can use, e.g. HeadsUpHealth, Ketonix (breath Ketone reader app), AppleHealthKit, etc.

        Thanks Marty

      • eismarsh says:

        Hi Marty, I appreciate you are a busy man.. did you get a chance to review my 3 questions regards my navigation towards your NO.
        Thanks in advance
        Steve Marshall
        Nottingham UK

  • I may be misunderstanding this,but shouldn’t the end of this say “which means you DON’T? need to eat a lot of them?……….While these vegetables have a very high nutrient density score (ND) in terms of nutrients per calorie, they also have a low energy density which means you need to eat a lot of them to get the nutrients you need. Sorry if it is just me being confused…….

  • Beata says:

    A very low calories diet (high in proteins) is indicated here to lose weight. Recently I was following the 5000 calories a day intake (fats and proteins) by Jack McAnespy where there was no weight gain. Any thoughts?

    • It seems that some people can increase their metabolism and activity for a period of time to burn off excess energy to defend a body weight set point. For example, if my son eats a ton of sugar at a party he’ll get hyper and jumpy until he clears the energy. I don’t think this work for everyone though and likely not most people in the long term.
      Many people will find their blood sugars stabilise and they find a natural level of satiety when they stop eating processed carbs and switch to a whole foods low carb diet. Lots of people will lose some weight. But it seems that many people stall with an ad libidum ‘eat fat to satiety’ approach and it’s the people who actively manage their intake that achieve leanness.

      This video with Luis and Allesandro is worth a watch. Interestingly, it appears that we require less energy intake once we’re adapted to a higher fat diet, so unlimited fat, fat to satiety or ketone chasing with more fat may not end well in the long term.

  • Rian says:

    This article is awesome! I stumbled upon this PSMF concept recently but there isn’t a ton of easily accessed info out there. This article set me straight on a lot things. What other PSMF references are people using? Are there some books and podcasts I should be looking for? I want to know more but I’m having trouble getting my hands on additional detailed information. Thank you!

  • Anna says:

    Well written nutrition article, with an occasional muscly arm photo. Frankly my Saturday morning couldn’t get much better! 🙂

  • Great article. I’ve been doing PSMF for 4 weeks now, and have shed the pounds. I found another great resource to be for recipes and also a calculator to get my lbm for protein. Reddit /psmf is also a great community

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