The Protein Sparing Modified Fast (PSMF) diet is regarded by many as the most effective way to lose body fat while preserving muscle and avoiding rebound binging due to cravings.
This article details everything you need to know to tailor a nutrient-dense PSMF to lose fat quickly while preventing loss of muscle and rebound binging due to cravings driven by nutrient deficiencies.
To learn more about the PSMF diet and how it really works, read on.
What is a protein-sparing modified fast (PSMF)?
A protein-sparing modified fast is broadly be defined as a diet that contains adequate protein to preserve lean muscle mass while limiting energy from both carbohydrate and fat.
First developed in the 1970s, variations of the PSMF have been successfully used in a range of settings, from weight loss clinics to the bodybuilding community.
Maximising satiety a PSMF
Our satiety analysis has clearly shown that we tend to eat less when we consume a higher percentage of protein. While the average population protein intake tends to be around 12 to 16%, a PSMF requires that you push your protein percentage much higher, ideally to 40% or more (see Why Does Protein Suppress Your Appetite?).
Conversely, we tend to consume fewer calories when less of our energy comes from fat and carbohydrates.
But it’s not just a matter of eating more protein. Many protein sources also come packaged with fat. It can be easy to overdo the fat if you simply eat more of your typical sources of protein.
If your goal is to lose fat from your body, your focus needs to be on pushing the percentage of energy from protein higher by reducing the easily accessible energy from refined carbs and fat.
As you can see from the data from our 6 Week Nutritional Optimisation Masterclass, people lose body fat fairly consistently when they focus on nutrient-dense high satiety foods and meals with adequate protein.
More importantly, lean body mass can increase on a PSMF dietary approach (particularly if your current protein intake is relatively low). While resistance training is always useful to ensure you build muscle or don’t lose lean mass while you are losing weight, we see many people gain lean mass when they simply start eating a little more protein.
How does the PSMF diet work?
A focus on protein increases satiety while preserving muscle mass (which is critical to your maintaining metabolic health). The reduced energy from fat and carbs allows your body fat to be used for energy. In the absence of energy from carbs and fat, your body is forced to use body fat for fuel.
In time, you will likely start to crave energy-dense fat+carb foods. So, a hardcore PSMF diet 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.
However, you don’t need to swing from one extreme to another. Most people get the best results when they slowly dial up the percentage of protein in their diet. In the Nutritional Optimisation Masterclass, we use the Smart Macros Algoythm in Nutrient Optimiser to stretch you a little more each week to ensure you are continuing to move towards your goals over the long term
How much protein do I need on a 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.
Protein levels are set at 1.2 to 1.5 g/kg of ideal body weight per day.
- Carbohydrate intake is typically restricted to 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 calories per day.
Benefits of a PSMF diet
- patients are often encouraged by the initial period of rapid weight loss, which reduces the dropout rate;
- 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;
- 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 rather than muscle.
“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.”
PSMF for bodybuilders
McDonald details how we individualise the PSMF diet based on their goals and context, noting:
- someone who is already very lean and undertaking heavy weight training will need higher levels of protein.
- someone who isn’t so lean may do better with a less aggressive approach but over a more extended period.
- unlimited green leafy fibrous veggies are encouraged as they are filling and provide essential micronutrients with minimal calories,
- an aggressive energy-restricted PSMF is typically not a long-term undertaking due to the risk of nutrient deficiencies with a severe energy deficit, and
- prioritising getting the essential nutrients you need becomes even more important when your calories are restricted.
Is the protein-sparing modified fast ketogenic?
While protein intake is high in terms of the percentage of calories from the diet, the protein-sparing modified fast could be seen as (endogenously) ketogenic due to the high contribution of body fat to your energy expenditure which may cause ketones to be elevated.
Blood ketones tend to rise initially as weight loss commences then decreased as fat loss continues and our bodies adapt to using fat (see Blood glucose, ketone and insulin changes after six weeks with Nutrient Optimiser).
How much protein?
If you are active and/or doing resistance training, you will have a greater requirement 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 2.6 g/kg total body weight where there is an aggressive deficit (e.g. 35% below maintenance calories) while a lower protein intake of 1.5 g/kg body weight seems to be adequate where we have a more moderate deficit.
Practically though, if you are wanting to lose fat, you still require an energy deficit, so you can’t consume unlimited amounts of protein. Interestingly, Jose Antonio’s protein overfeeding studies show that people tend to gain lean muscle mass and lose fat even when in a calorie surplus when consuming up to 4.4 g/kg BW of protein!
However, if you are looking to lose weight, you simply just need to focus on your percentage of protein high while ensuring a sufficient calorie deficit to ensure weight loss by reducing your fat and non-fibre carbohydrates.
To calculate your PSMF starting macros use this simple calculator, select a ‘savage’ deficit (35%) and dial-up your protein to at least 2.2 g/kg LBM. Use the carb slider to select your preferred carb intake. As you can see in the example below, once you drive your protein up in an energy deficit, there is not much room for energy from either fat or carbs.
In our 6 Week Nutritional Optimisation Masterclass, we guide people to get adequate protein and then progressively dial back their carb and fat intake to ensure they are getting the blood glucose and fat loss outcomes they want.
While not everyone needs to be on a hardcore PSMF to get results. In fact, most people find they get better results over the long term when they are less aggressive.
Minimum carbohydrate requirement on a PSMF
While there is a need for vitamins and minerals that often come packaged with carbohydrate-containing foods (e.g. nutrient-dense non-starchy vegetables), there is no minimum carbohydrate intake on a PSMF.
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 requirements
Most of us have plenty of body fat stores to 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). Your gall bladder only needs around 10 g per day of fat to function. Its hard to go below this amount if your protein is coming from whole foods.
While not easy, limiting dietary fat while also limiting carbohydrates allows the fat from your body to be used for energy and maximises satiety on a calorie for calorie basis.
As shown in the chart below from our analysis of half a million days of MyFitnessPal data, satiety tends to improve as we reduce fat to below 40%.
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 work over the long term is 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). We lose more calories metabolising protein (35%) compared to fat or carbohydrates (4 to 9%).
While a calorie can often cause the dieter to feel cold and tired, the higher percentage of protein leads to higher dietary-induced thermogenesis, meaning that you lose more of the calories eaten are ‘lost’ as heat that will help to keep you warm (rather than being available to be stored on your body as fat).
Even more than protein alone, foods with a high dietary-induced thermogenesis tend to be more satiating. So not only do we have less net energy available for storage on our body we tend to consume less of them.
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.
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 calories are limited.
The chart below shows the micronutrient fingerprint 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 (e.g. folate, calcium, vitamin A, vitamin C, vitamin K1 and magnesium shown towards the top of the chart).
This chart from our analysis of 467 recipes in our 29 recipes books optimised for nutrient density shows that, up to around 50%, nutrient density tends to correlate with protein. But beyond that, more is not necessarily better. However, if you prioritise foods and meals with more nutrients, you will be better plenty of protein.
Potential problems with a protein-sparing modified fast
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.
- First, they picked the cheapest protein source available, collagen; a protein that provides minimal vitamins and minerals.
- Second, they provided zero supplemental vitamins and minerals (some of which would have been obtained if the dieters had been eating whole foods).
This caused a number of problems, including cardiac heart loss 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) can actually be one of the most nutrient-dense dietary approaches.
To overcome these potential shortcomings of a PSMF diet, we have developed a high satiety nutrient-dense food list that you can download for free here.
Nutrient density is critical!
Nutrient density becomes more critical when we are trying to eat less. Obtaining adequate micronutrients can help to mitigate metabolic 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. Conversely, if we send a signal with the food we eat that there is a lack of quality food available it seems to go into storage mode, preparing for an emergency.
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.”
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.
To help you understand what nutrients you need to focus on (and which foods and meals contain them) we have created a simple free 7 Day Food Discovery Challenge.
The most satiating and nutritious foods
Rather than just targeting the highest protein foods, a more optimal approach is to target the most satiating foods that also maximise your intake of the harder to find nutrients.
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.
What about “rabbit starvation”?
“Rabbit starvation” happens to very lean people if they only have lean protein foods with minimum fat. 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 to burn, this is not going to be an issue!
How to implement a nutrient-dense Protein Sparing Modified Fast
- Target plenty of protein and fibre with less energy from fat and carbs.
- Minimum protein intake in a weight loss clinic setting is 1.2 g/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 or more is typical for someone lifting heavy).
- Focusing on nutrient-dense foods will ensure you 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
- If your goal is weight loss, the focus needs to be on limiting energy from fat and carbs.
- If you are not seeing the progress you want, 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. Between 0.5 to 1.0% per week is a healthy and sustainable target. Unless you are carrying a LOT of body fat, it can be harder to sustain a faster rate than this over the long term. As you get leaner, it gets harder to lose weight while preserving precious lean mass.
- Lift heavy / exercise
- If you are dieting and relatively sedentary, your 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. Make sure you try our Free 7 Day Tracking Challenge to get started on your journey of nutritional optimisation.
- The protein-sparing modified fast (PSMF) provides adequate 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.
- 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 same basic principles can be applied in other situations to maximise fat loss while maintaining lean muscle mass.
What do I eat on a PSMF?
One of the biggest challenges with a PSMF is to choose foods and meals with more protein and micronutrients while limiting energy from fat and carbs.
To help you get started, we have developed:
PSMF-friendly meals are often simple, but they don’t have to be bland and boring. Our Fat Loss Recipe Book provides 33 nutrient-dense recipes with a secret index of 150 recipes that have been optimised to support your sustainable nutrient-dense PSMF. Check it out here. These recipes provide a nice balance between satiety and nutrient density that will be ideal for most people who want to lose body fat at a healthy rate over the long term.
If you really want to take your PSMF to the next level you can try our book of high protein to energy recipes which will provide maximum satiety per calorie while still providing plenty of the essential nutrients you need to prevent cravings and deficiencies.