PSMF: the intelligent crash diet for fat loss?

written by Philip Stefanov  |  AUGUST 15, 2023

PSMF stands for protein-sparing modified fast and is an approach people generally consider decent if someone needs to lose a lot of fat quickly.

But what exactly is this modified fast, how does it work, and is it good? Let’s talk about it.

What is Protein-Sparing Modified Fast?

Protein-sparing modified fast (PSMF) is a very low-calorie nutritional approach that promotes rapid weight (and fat) loss.

The objective is to reduce your calorie intake significantly and create a sizeable deficit while also getting enough protein to support muscle mass. By keeping protein intake high, the body would be less likely to break down muscle for energy and would instead oxidize more fat.

Here’s how PSMF works in a nutshell:

  • High protein intake - 0.8 to 1.2 grams per pound of body weight
  • Fat and carb intake should be as close to zero as possible
  • Vitamin/mineral supplementation to avoid nutrient deficiencies

Let’s illustrate how this diet might look for a 220-lb person: Jim. Based on the established rules, Jim should aim for 176 to 264 grams of protein daily. That would translate to a daily calorie intake of 704 to 1056 calories since a gram of protein has four calories.

(Protein technically has 3.2 calories of net metabolizable energy per gram, but that doesn’t matter much here, given the colossal calorie deficit PSMF creates anyway.)

Even on a strict PSMF protocol, Jim is bound to get some calories from the trace amounts of carbs and fats he will inevitably consume. However, that amount is too small to impair his progress.

Who Can Benefit From a Protein-Sparing Modified Fast?

In short, a PSMF can be beneficial for people who:

  • Have more fat to lose (15+ percent body fat for men and 25+ percent for women)
  • Have experience with strict dieting and are not afraid of severe restrictions
  • Know how to track their calories and macronutrients
  • Don’t have a history of binge eating behaviors or yo-yo dieting
  • Want to kickstart a longer weight loss journey and want to see some quick initial results

It’s worth noting that PSMF is an incredibly restrictive and demanding dietary approach that requires careful planning and a good understanding of the associated risks, even in completely healthy people.

One thing to be mindful of is that PSMF forces the body into a state of ketosis, which can be pretty unpleasant until the body adapts.

For some people, that never really happens, and they experience symptoms like extreme hunger, lethargy, brain fog, and loss of motivation to do anything until they increase their carbohydrate intake.

Who Should Not Go On a PSMF

PSMF is undeniably powerful but not suitable for everyone. Here are some people who would be better off on a more moderate fat loss diet:

  • Leaner individuals - the risk of muscle loss increases as body fat percentage drops
  • People who have previously dealt with eating disorders
  • Folks with no experience tracking calories or doing severe nutritional restrictions
  • Those prone to hypoglycemia (low blood sugar levels)

Regardless of where you stand, talking with your healthcare provider before going on such a restrictive diet is always best.

How PSMF Might Fit Into a Long-Term Fat Loss Protocol

Every fat loss journey is different, but most share common elements. Let’s take a look at a couple of scenarios:

1. Rapid (Initial) Fat Loss

Use PSMF at the start of your diet for some quick initial results and motivation. For example, you can go on PSM for three to four weeks before gradually increasing your calorie intake and re-introducing dietary fats and carbohydrates.

This option works well because most people carry plenty of energy (body fat) at the start of their diet and can lose weight more rapidly without risking much muscle loss.

2. Cycling Approach

The second option is to alternate between one to two weeks of PSMF and one to two weeks of a higher calorie intake (mild deficit or maintenance). This approach works well for overweight and obese individuals because it includes short periods of massive restrictions followed by time off.

For example, here is how it might look for someone with a TDEE of about 3,000 calories:

Weeks 1-2: PSMF (800-1,000 calories/day)
Weeks 3-4: mild deficit (2,700-2,800 calories/day)
Weeks 5-6: PSMF (800-1,000 calories/day)
Weeks 7-8: mild deficit (2,650-2,750 calories/day)
Weeks 9-10: PSMF (800-1,000 calories/day)
Weeks 11-12: mild deficit (2,600-2,700 calories/day)

And so on.

You should drop PSFM if it becomes too difficult to sustain or you get quite lean (visible abs, veins, etc.). As discussed above, a lower body fat percentage increases the risk of muscle loss.

Thanks for sticking around. I'll catch you next week!


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