Should You Use a Protein Sparing Modified Fast to Lose Weight?

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Key Takeaways

  1. A protein sparing modified fast (PSMF) is a diet that involves extreme calorie restriction and eating more or less nothing but very lean protein and low-calorie, fibrous vegetables.
  2. You can lose anywhere from 2 to 8 pounds of pure fat per week while following a PSMF, depending on your starting body fat percentage, activity levels, and calorie intake.
  3. Although a PSMF can help you lose fat quickly in the short term, it’s not a good long-term solution for getting the body you want.

Want to know what protein sparing modified fasting is, why people do it, and whether or not you should try it? Keep reading.

You probably know trying to lose weight too fast is a mistake.

You’re going to run into the hurdles of extreme hunger, lethargy, irritability, and the metabolic hobgoblins attendant to all crash diets.

Plus, you’re walking a tightrope where one misstep could lead to a binge that wipes out your progress.

That said, there are advantages to losing fat as fast as possible.

You see results faster in the mirror, you don’t have to diet for as long, and you can get back to building muscle and getting stronger sooner. 

If you want to lose fat as fast as possible, though, you need to do it the right way.

The worst thing you can do is follow some half-baked diet like water fasting, the military diet, or a detox diet, which are just forms of reckless calorie restriction in disguise.

Instead, you should follow an evidence-based, structured, effective crash diet that helps you lose fat as fast as possible without losing much muscle.

And the only diet that checks those boxes is known as a protein sparing modified fast (PSMF). 

If you want to learn what a PSMF is, why it’s so effective, how it compares to other crash diets, and how to use it to lose fat as fast as possible, keep reading. 

What Is a Protein Sparing Modified Fast?

A protein sparing modified fast (PSMF) is a diet that involves extreme calorie restriction and eating more or less nothing but very lean protein and low-calorie, fibrous vegetables.

Typically, calories will be restricted to around 15 to 30% of what you’d need to eat to maintain your weight (your TDEE). For example, if you normally eat 2,500 calories, you’d eat 375 to 750 calories while following a PSMF.

Virtually all of those calories need to come from lean protein, too, like skinless boneless chicken breast, lean cuts of beef like london broil or sirloin steak, or low-to-no-fat seafood like tilapia, shrimp, crab, tuna, and cod.

This is why a PSMF is referred to as “protein sparing.” By providing adequate dietary protein, you can “spare” muscle protein from being broken down for energy. 

In other words, you’re modifying a complete fast (where you eat no calories) by adding just enough protein to avoid losing muscle. 

You’re allowed to eat small amounts of low-calorie fibrous vegetables like zucchini, squash, broccoli, leafy greens, and green beans, but no more than 100 calories-worth or so per day.

Since you’re only following this diet for a few days to two weeks, you could survive just fine without eating any vegetables. That said, eating small portions of low-calorie vegetables gives you something to chew on, improves your digestion, and helps you stay full

Finally, you’re allowed small amounts of low-calorie condiments like hot sauce, lemon juice, soy sauce, and so forth. 

Here’s what you’re not allowed to eat on this diet: 

  • High-calorie or high-carb vegetables or other plants like corn, peas, tomatoes, bell peppers, carrots, beans, potatoes, and the like. 
  • Fruit of any kind.
  • Dairy, except whey or casein isolate, or another low-or-no-fat-or-carb dairy protein supplement.
  • Nuts, seeds, or chocolate.
  • Juice or other calorie-containing beverages.
  • High-calorie sauces like teriyaki, barbecue, or sesame sauce. 
  • Grains, cereals, or other high-carb foods.
  • Any kind of fast food or sweets.
  • Added oils of any kind.

As you can see, this is an extremely restrictive, low-calorie, crash diet. You don’t get cheat meals or days, and you’re only eating enough to avoid losing muscle (and your sanity). 

In terms of exercise, it’s generally recommended you do no more than two to three weightlifting workouts per week. You won’t have the energy to recover from much more than a handful of 30 to 60-minute workouts, and you only need to do a few sets of compound exercises to maintain your muscle when cutting.

Exercising more than this can also be counterproductive by increasing your fatigue and hunger, making the diet harder to stick to. 

Protein sparing modified fasts have been used in studies and weight loss clinics to help morbidly obese people lose weight as fast as possible for years, but they were first codified and promoted by writer, researcher, and coach, Lyle McDonald in his book, The Rapid Fat Loss Handbook.

Lyle’s PSMF recommendations have become more or less the standard, and much of the information in this article is from his book.

Summary: Protein sparing modified fasting (PSMF) is a form of crash dieting that involves eating very few calories, mostly from protein, with a small amount of low-calorie, fibrous vegetables, for several days to two weeks.

Wait, Isn’t Crash Dieting Always Bad? 


protein sparing modified fast keto


As Lyle is the preeminent authority on protein sparing modified fasting, I’ll let him answer this question: 

“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 I’m at it. My point? When idealism and reality slam together it’s never pretty. People are going to crash diet no matter what I or anybody else tell them.”

I wholeheartedly agree. 

Ideally, no one would crash diet. You’d stop reading this article, and read this one instead: 

The Complete Guide to Safely and Healthily Losing Weight Fast

Still here? 

If so, then you can count yourself as part of the majority of people who knows crash dieting isn’t ideal, but still feels the pros outweigh the cons. 

If you’re going to crash diet, though, you need to do it correctly. 

Whether or not crash dieting is bad for you mostly boils down to how you use it. 

Technically, there’s nothing unhealthy about severely restricting your calorie intake for brief periods of time to speed up fat loss. 

The problem with crash dieting, though, is people use it as a long-term weight loss strategy, instead of a short-term measure to move the needle a little faster. 

People let themselves get overweight until they reach a point where they’re thoroughly disgusted with themselves. Then, they decide to take drastic action, losing weight “once and for all” by starving themselves for a few weeks. 

They slash their calories, often eating far too little protein and losing muscle in the process. They keep this up until they can’t stand it anymore. 

They soon fall back into the same habits got made them overweight in the first place, and they gain weight until they’re ready to start the cycle anew. 

This is the worst way to lose weight.

It’s a classic case of yo-yo dieting, and it’s one of the reasons most people never manage to get the body they want. 

If you’re one of these perennial dieter types, crash dieting isn’t for you. Instead, you should commit to a long-term weight loss plan that helps you lose weight consistently and quickly with as little effort and discomfort as possible. 

Now, if you don’t have a history of yo-yo dieting, there are a few strong arguments in favor of a crash diet like protein sparing modified fasting: 

1. You see results FAST.

The biggest benefit of a PSMF is that you get to see changes on the scale and in the mirror much faster than you would with traditional weight loss diets. 

While many nutritionists, coaches, and fitness gurus claim fast weight loss is always bad, many studies show people who lose the most weight at the beginning of a diet lose the most weight overall and keep it off the longest. 

The best motivation to stick to your diet is fast results, and that’s what you’ll get with PSMF. 

Again, this isn’t a long-term weight loss solution, but it’s an effective way to kickstart your diet before transitioning to a more sustainable approach.  

2. You don’t have to spend as much time losing weight. 

The worst part about cutting is being consistent day in and day out. 

In fact, many people find that maintaining a moderate calorie deficit is just as annoying as maintaining a large calorie deficit, and the sooner they can get back to eating at maintenance, the better.

And PSMF helps you do exactly that.

3. You can get back to gaining muscle and strength faster.

When you’re cutting, you probably aren’t going to gain much muscle or strength to speak of no matter what kind of strength training program you follow. 

(There are a few exceptions to this rule, but it’s true in most cases). 

So, if you can’t make any strength or muscle gains, you might as well maximize the one form of progress you can achieve when cutting: fat loss. 

In other words, the sooner you reach your fat loss goal, the sooner you can get back to the fun part of getting fit: lean bulking.

Summary: Crash diets like protein sparing modified fasting aren’t always bad. While they aren’t a long-term weight loss solution, they can help kickstart a diet by providing fast results. 

How Much Fat Can You Lose on a Protein Sparing Modified Fast?

If you’re only restricting your calorie intake, you can lose 2 to 4 pounds of pure fat in a week of following a protein sparing modified fast.

If you’re restricting your calorie intake and doing lots of cardio, you can lose up to 6 to 8 pounds of pure fat in a week.

For comparison’s sake, most people recommend you aim for about a pound of weight loss per week. This means that with a PSMF, you can lose fat eight times faster than you could with a normal fat loss diet.

Before you get too excited, though, you need to meet these criteria to expect these kinds of results: 

  1. You need to have a body fat percentage over 15%. The more overweight you are, the easier it will be to lose fat quickly. The leaner you are, the slower you need to lose fat to avoid losing muscle. 
  2. You need to follow the diet exactly as prescribed, with no deviations, cheat meals, or mistakes.
  3. You need to do a ridiculous amount of cardio if you want to lose 6 to 8 pounds of fat in a week—far more than you’d ever want to do for any length of time. 

Now, if you’re skeptical of the numbers I just shared, I understand. 

Many unscrupulous marketers have feathered their nests by promising their detox tea, crash diet, or strange elimination diet will help you lose “10 pounds of fat in a week” or other such nonsense. 

How can you trust I’m telling the truth? 

Well, I got these numbers from several scientific studies on protein sparing modified fasting. 

The most extreme example of the effects of PSMF comes from a study conducted by scientists at the University of Las Palmas de Gran Canaria.

In this study, the researchers split 15 overweight 18-to-55 year old men into two groups: 

  1. Group one follow a diet that provided 10% of their TDEE, all in the form of whey protein (around 300 calories per day)
  2. Group two did the same thing except they got all of their calories from table sugar.

Both groups followed their respective diets for four days, and then switched back to a normal mixed diet that provided enough calories for them to maintain their weight for three days. 

In terms of exercise, both groups were forced to walk on a treadmill for 8 hours (20 miles) per day, followed by 45 minutes of an arm cranking exercise as a form of upper body exercise. 

The researchers prepared everyone’s meals and observed every workout for the duration of the study. They also put all of the participants through a number of physical and blood tests, examining everything from their body fat percentage to VO2Max to insulin levels

All told, the diet and exercise regimen forced both groups to maintain a 5,000 calorie deficit per day—enough to theoretically lose about 1.5 pounds of fat per day.  

And in this case, that’s basically what happened.

After four days, both groups lost 4.4 pounds of pure fat, or just over a pound of fat per day. Strangely, they lost another 1.5 pounds of fat over the next three days despite eating close to maintenance calories. 

The researchers aren’t sure whey they kept losing weight over the following three days, but it could be they underestimated the number of calories they needed to eat to maintain their weight. 

In any case, that’s a total fat loss of 5.9 pounds in 7 days.

If they’d followed their extreme diet and training regiment for the whole 7 days instead of just 4, they would have lost closer to 8 pounds of pure fat.

They also lost 6.6 pounds of total lean mass, but most of this was water weight. After rehydrating after the extreme calorie restriction, the researchers found they only lost one pound of actual lean body mass (muscle). 

Keep in mind these people also didn’t lift weights, which would have significantly reduced muscle loss.

Also keep in mind that although group one technically was following a protein sparing modified fast, their protein intake was lower than most research would suggest is optimal for maintaining muscle. If they’d eaten more protein, they probably would have lost less muscle than they did in this study.

Now, you’re probably thinking these people were physically, mentally, and emotionally broken after this ordeal, but that’s not the case. 

They all reported feeling tired, beat up, and hungry after four days of their diet and training plan, but none of them got injured, overtrained, or put themselves in “starvation mode.”

Now, remember that all of these people were overweight, with a body fat percentage of 20 to 40% and a BMI well north of 25. This makes it much easier to lose fat quickly. If these people were leaner, they likely would have lost less fat and more muscle. 

This is the most extreme example of protein sparing modified fasting I know of, but the results have been replicated in similar studies. 

For example, a study conducted by scientists at West Virginia University found that untrained, overweight people lifting weights and eating 800 calories per day (with only 80 grams of protein) lost 32 pounds of fat in 12 weeks, and retained virtually all of their muscle.

And finally, a recent study conducted by researchers at the Cleveland Clinic Foundation found that obese teenagers put on a PSMF that provided 800 calories per day llost 25 pounds in 12 weeks. 

In this case the meals weren’t entirely controlled by the researchers, so it’s likely the teenagers were cheating on their diets. Still, they lost twice as much weight as the standard “one pound per week” recommendation.

Summary: Protein sparing modified fasts can help you lose 2 to 4 pounds of pure fat per week, and up to 6 to 8 pounds per week when combined with a rigorous workout routine.

Should You Try a Protein Sparing Modified Fast?


protein sparing modified fast reddit


Here’s how to decide if you should try a protein sparing modified fast: 

1. If you’re over 15% (men) or 25% (women) body fat, you can consider trying a PSMF. 

At this body fat percentage you can expect to easily, quickly, and efficiently lose fat much faster than someone who’s less than 15/25% body fat. If you’re under 15/25% body fat, though, the risk of losing muscle is much higher when following a PSMF.

One of the best uses of a PSMF is damage control after a period of overeating. For example, some people like to gorge themselves over the holidays, and use a PSMF for a week or two afterward to help strip off some of the fat they’ve gained. 

I don’t recommend this, of course, but it does work.

(Not sure what your body fat percentage is? Read this article.)

2. If you’re starting a fat loss diet, you can consider trying a PSMF.

If you’re going to try a PSMF, it’s best to do it at the beginning of a fat loss diet when weight loss is easier than at the end of a cutting phase. This is also a good way to boost your motivation before transitioning to a more sustainable fat loss diet. 

I never recommend you do a PSMF at the end of a cut, as this is when people are most vulnerable to burnout, bingeing, and blowing their diet before reaching their goal weight—problems that will only be amplified on a PSMF. 

Even if you do a PSMF at the beginning of a cut, I still recommend you only try it if you’re over 15% body fat.

3. If you have a history of yo-yo dieting, eating disorders, or binge eating, don’t try a PSMF. 

Instead, lose weight using a combination of a well-designed weight loss meal plan and intuitive eating principles.

Finally, one group of outliers who may want to try a PSMF are athletes who need to make weight before a competition. 

If you find yourself in the situation of needing to lose 2 to 4 pounds of fat in the week or two before your competition, though, then you’ve already made a major mistake, and you’re probably better off postponing the competition and trying again next time. If that’s not possible, though, then you can try a PSMF and see how things play out. 

Summary: If you’re over 15% body fat and are starting a new fat loss diet, you can consider a PSMF. If you’re under 15% body fat or have a history of yo-yo dieting, eating disorders, or binge eating, don’t try a PSMF.

How to Do a Protein Sparing Modified Fast

If you still want to do a protein sparing modified fast after everything you’ve read, you need to do it right. 

Here’s what to do: 

  1. Eat 1 to 2 grams of protein per pound of lean body mass.
  2. Eat some low-calorie vegetables.
  3. Take the right supplements.
  4. Lift weights more than two to three times per week.
  5. Do some cardio.

1. Eat 1 to 2 grams of protein per pound of lean body mass.

The goal of a protein sparing modified fast is to provide just enough protein to prevent muscle loss, and no more. 

Thus, it’s important to scale your protein intake based on your body’s needs. 

One of the main variables that increases your body’s protein needs is lean body mass. The more lean body mass you have, the more protein you’ll need to eat to avoid losing it.

Women generally have more body fat and less lean body mass than men, so they’ll typically need less protein.

Weightlifting is another factor that can increase your body’s demand for protein. If you lift weights, you’ll likely benefit from eating more protein than if you don’t. 

If you don’t lift weights, well, you should. Aside from eating enough protein, lifting weights is the single best way to avoid losing muscle while cutting calories. 

With that out of the way, let’s figure out how much protein you should eat. 

First, you’ll need to estimate your body fat percentage and lean body mass.

Read this article to do that: 

All about body fat percentage – how to measure, what’s healthy & not, & more…

Next, use the charts below to find which category you fall into.

Set Your Protein Intake on a Protein Sparing Modified Fast

Grams of Protein Per Pound of Lean Body Mass (LBM) Male Body Fat Percentage Female Body Fat Percentage
2 15% and lower* 25% and lower*
1.5 16 to 25% 26 to 35%
1 26% and higher 36% and higher

* If you’re under 15% (men) or 25% (women) body fat, your risk of losing muscle while following a protein sparing modified fast is much higher.

For example, let’s say you’re a 200-pound man with 20% body fat, here’s how you’d find your lean body mass: 

200 x 0.8 = 160 pounds of lean body mass

Since you’re 20% body fat, you’ll want to eat 1.5 grams of protein per pound of lean body mass.

Next, you’d multiply your lean body mass by 1.5.

160 x 1.5 = 240

You’d want to eat 240 grams of protein per day while on a protein sparing modified fast.

Now let’s say you’re a 160-pound woman with 40% body fat.

First, you’ll estimate your lean body mass:

160 x 0.6 = 96

Since you’re 40% body fat, you’ll want to eat 1 gram of protein per pound of lean body mass.

Next, you’d multiply your lean body mass by 1.

96 x 1 = 96 

You’d want to eat 96 grams of protein per day while on a protein sparing modified fast.

2. Eat some low-calorie vegetables.

While you don’t technically need to eat any vegetables while on a protein sparing modified fast, it makes the process much easier. 

You’ll be able to chew on something, your digestion will be better, and you’ll feel more full after meals. 

Plus, it helps reduce the likelihood of any nutrient deficiencies (although you shouldn’t stay on a PSMF long enough for this to be an issue anyway).

Some good options for low-calorie, high-fiber veggies are: 

  • Green beans
  • Zucchini
  • Broccoli
  • Cauliflower
  • Squash 
  • Leafy greens
  • Kale, collard greens, etc.
  • Asparagus
  • Cabbage
  • Pickles
  • Cucumbers

3. Take the right supplements.

You don’t need to take any supplements while following a PSMF, but doing so will make the process much more enjoyable. 

First of all, it’s worth taking a serving of fish oil daily.

Although you can go a few days to a few weeks without consuming essential fatty acids, like those found in fish oil, they’re worth taking. 

There are hundreds if not thousands of functions in the body that depend on these essential fatty acids, and it’s better to be safe rather than sorry while following a restrictive diet like this. 

You can take any high-quality fish oil, but I recommend Legion Triton. An effective dose of fish oil is 500 to 3,000 mg, or four capsules capsules of Triton. 

Next, I recommend you take a good multivitamin. 

You won’t be able to meet the RDI for most nutrients while following this diet, so taking a well formulated multivitamin is a good way to minimize the risk of nutrient deficiencies. 

You can take any high-quality multivitamin, but I recommend Legion Triumph. It’s specifically formulated to contain all of the nutrients that athletes are generally lacking, along with a few nutrients that most people don’t get enough of (such as vitamin D3 and K2).

I also recommend you heavily salt your food with either table salt or a combination of table salt and potassium salt (like No Salt). There isn’t much scientific evidence behind these supplements, but it’s well established that low-carb diets (like a PSMF) can deplete electrolytes. Taking these minerals also seems to help reduce the extreme lethargy caused by extreme calorie restriction. (Oh, and it’s best to take potassium with your food, not in tablet form, as it’s easy to take too much accidentally.)

It’s also worth taking 600 to 1200 mg of calcium and 500 mg of magnesium daily, for the same reasons. Any brand of these minerals will work, but I like Natural Calm magnesium and Kirkland calcium tablets.

Finally, I generally recommend you avoid protein powders, bars, and other calorie-containing supplements.

Supplements typically aren’t as satisfying as whole foods, and you don’t have any spare calories to “spend” on them when following a PSMF.

If you insist on getting some of your daily protein needs from protein powders, I recommend you take a high-quality casein isolate protein. Casein takes longer to digest than whey, and generally does a better job at helping you stay full. 

You can take any high-quality casein protein isolate, but I recommend Legion Casein+.

So, in summary, here are the supplements I recommend while on a PSMF: 

  1. At least 500 to 3,000 mg of high-quality fish oil (like Legion Triton) per day
  2. A full serving of a high-quality multivitamin (like 8 capsules of Legion Triumph) per day
  3. Generous amounts of table salt and potassium (on your food) daily
  4. 600 to 1200 mg of calcium per day
  5. 500 mg of magnesium per day

4. Lift weights more than two to three times per week.

Lifting weights is one of the single best ways to maintain your muscle when restricting your calories for fat loss, and it becomes more and more important the more you restrict your calories. 

Luckily, though, it takes far less weightlifting to maintain your muscle than it does to build it in the first place.

This is why you can reduce your training volume by anywhere from ½ to ⅓ of your normal training volume for several weeks without losing strength or muscle.

So, when following a PSMF, lift weights two to three times per week, sticking to relatively short workouts involving compound exercises like the squat, bench and military press, and deadlift.

If you want a simple and effective 3-day per week workout routine, check out this article: 

The Definitive Guide on How to Build a Workout Routine

5. Do some cardio.

You don’t need to do cardio while following a PSMF, but doing so will help you lose fat faster. 

Another side benefit of cardio is it gives you something to do when you might normally be eating, preparing food, or cleaning dishes.

Even a 20 to 30-minute daily walk will help you burn some additional calories, with more being better. 

You can do any kind of low-impact, low-intensity cardio you want, but it must be low intensity. You shouldn’t be doing any HIIT or anything that makes it difficult to breathe while during your workout.

The Bottom Line on Protein Sparing Modified Fasts


protein sparing modified fast results


A protein sparing modified fast (PSMF) is a form of crash dieting that involves eating very few calories, almost entirely in the form of protein and some low-calorie vegetables. 

The idea of a protein sparing modified fast is to provide just enough dietary protein to the body to avoid losing muscle, while keeping total calories as low as possible. 

Although crash dieting isn’t an ideal way to lose weight, if you insist on doing so a PSMF is the best way to go about it. 

If you play your cards right and you meet a few key criteria, you can lose anywhere from 2 to 8 pounds of pure fat per week. 

You should only do a PSMF if you’re over 15% (men) or 25% (women) body fat. You can try it if you’re leaner than that, but the risk of muscle loss, lethargy, and weight regain is much higher the leaner you are when you start. 

If you’re over 15/25% body fat, you don’t have a history of binge eating, eating disorders, or yo-yo dieting, you can try a PSMF at the beginning of a cutting phase to kickstart your progress.

Here’s how to set up a PSMF: 

  1. Eat 1 to 2 grams of protein per pound of lean body mass.
  2. Eat some low-calorie vegetables.
  3. Take the right supplements.
  4. Lift weights more than two to three times per week.
  5. Do some cardio.

Keep that up for a few days to two weeks (depending on how much fat you need to lose), and you’ll lose weight faster than you ever thought possible.

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