Paleo is generally positive about intermittent fasting, but longer fasts are a little bit dicey. The metabolic effects of fasting make it a very effective rapid fat-loss tool, but it’s hard to avoid the laundry list of problems that seem to come along for the ride: muscle wasting, potential nutrient deficiencies, weakness and dizzy spells…sure, you lose fat during a fast, but you also lose some other things you’d probably rather keep.
Enter the protein-sparing modified fast (PSMF).
A PSMF is a temporary diet to kick-start rapid weight loss in people who are already severely obese. You use it for a few weeks for initial motivation and rapid losses, always with a plan to transition to a more sustainable long-term diet once the PSMF is done. It's reasonably easy to make a PSMF Paleo-friendly, and it's an interesting option to look into for severely overweight beginners, but it's not required and it's not right for everyone - be careful and do your research before you jump in!
What’s a PSMF?
The idea of a PSMF is to reduce calories to the lowest possible threshold while still eating enough protein to preserve lean tissue mass and enough micronutrients to avoid deficiency. This is basically a kind of starvation, so you get the same metabolic benefits that you do with a “real” fast (which is also basically a kind of starvation), but the additional protein and nutrients make the whole project a little less risky, and minimize muscle loss and potential nutrient deficiencies.
Practically, a PSMF involves:
- Very few calories (typically under 1,000 per day – remember that the point is to induce a starvation response), with the vast majority coming from lean protein. Fat and carbs are minimized as much as possible.
- A few non-starchy vegetables.
- Supplemental vitamins, minerals, and salt to make up the inevitable nutrient and electrolyte deficiencies.
On a PSMF, the majority of calories entering your mouth are from protein, but the majority of calories you burn for energy come from fat; patients on a PSMF do go into ketosis. That’s because you can’t “burn” protein for energy the way you burn fat or carbs. The protein is just there to replenish muscle mass and prevent lean tissue loss – it’s used as building blocks, not as fuel. Instead of burning that protein for fuel, you’ll be burning your own body fat reserves, so you essentially are “eating” fat – your own fat.
This makes a PSMF sound a lot like keto, but they’re not the same:
| PSMF | Keto | |
| What do you eat? | Mostly PROTEIN; minimal fat and carbs. | Mostly FAT; low protein and minimal carbs. |
| What does your body burn for fuel? | Fat (from adipose tissue) | Fat (from adipose tissue and/or diet) |
| Do you have to restrict calories? | Yes, severely. | Not necessarily. |
| Is it healthy for the long term? | No; a PSMF is a short-term kick-start, not a lifestyle. | Yes |
Does it Work?
For the 668 obese patients in this study, the PSMF worked beautifully. Participants ate less than 800 calories a day during the PSMF period:
- 1.5 grams of lean animal* protein per kilogram of ideal bodyweight.
- Vitamin and mineral supplements providing the RDA of all essential vitamins and nutrients.
- At least 1.5 liters of fluids every day.
- At least 5 grams of table salt every day.
*It’s important that this be high-quality protein, e.g. fish, chicken, or egg whites. In the 1970s, several unsafe PSMF diets with low-quality protein actually killed people from malnourishment.
If you don’t think in kilograms, here’s a translation of the protein amounts:
| Your goal weight | Number of grams of protein to make 1.5 g/kg of goal weight | Sample day of food intake to get that much protein with minimal fat and carbs |
| 120 pounds | 82 | 1 skinless chicken breast, 1 tilapia fillet, and 4 large shrimp. |
| 130 pounds | 89 | 1 skinless chicken breast, 1 tilapia fillet, and 10 large shrimp. |
| 140 pounds | 95 | 1 skinless chicken breast, 3 ounces* turkey breast, and ½ cup canned tuna. |
| 150 pounds | 102 | 6oz fresh (not canned) tuna, 3 ounces turkey breast, and ½ skinless chicken breast. |
| 160 pounds | 109 | 6oz fresh (not canned) tuna and 1 skinless chicken breast. |
| 170 pounds | 116 | 6oz turkey breast, 1 tilapia fillet, ½ skinless chicken breast, and ½ cup canned tuna. |
| 180 pounds | 123 | 6oz skinless turkey breast, 6oz fresh (not canned) tuna, 1 tilapia fillet |
*That’s about the size of a small woman’s palm.
Non-Paleo PSMF diets also use protein foods like tofu, cottage cheese, and protein shakes. Some other PSMF diets use a little less protein than that, so it would probably be safe to nudge those amounts down a little bit, especially if your goal weight is towards the higher end of the scale.
So what was the result of all that chicken breast and tuna in the study? The patients lost an average of 21 kilograms (46.2 pounds), and an average of 56% of their “excess” body weight (“excess” being defined as all the pounds above the ideal weight for their height). They also improved their blood pressure, blood sugar control, and blood lipid numbers. Other studies have also found that a PSMF is helpful for diabetes and effective in diabetic patients.
In terms of side effects many patients felt dizzy during the first two weeks, but that they could easily fix the problem by eating more salt. Some of them also got constipated, and a few showed hair loss, but overall the researchers noted that the patients didn’t get ravenously hungry, and that “Most patients felt well on the PSMF and were encouraged by their ability to lose weight.”
Is it Safe?
If you’re familiar with the idea of rabbit starvation, the low-calorie, all-protein diet might sound very familiar. “Rabbit starvation” is a problem described by explorers like Vilhjalmur Stefansson, who noticed that hunters on long trips in the winter got very sick when rabbit was the only meat available to eat. Rabbit is extremely lean, so the men ended up eating a low-calorie, very high-protein diet. And it made them pretty sick: they started getting GI symptoms, constant ravenous hunger, headaches, and fatigue: they could even starve to death if it went on long enough.
A PSMF is basically a controlled, medically-supervised version of rabbit starvation with added vitamin and mineral supplements. That might raise some serious questions about the safety of a PSMF: is this really safe?
The differences between a PSMF and uncontrolled rabbit starvation are that people doing a PSMF already have enough fat stores to burn for fuel: they’re not lean to start with, so they don’t have to start breaking down muscles and organs right away. The explorers in Stefansson’s reports were basically trying to run off stored fat when they didn’t have any stored fat to run off – of course they got sick. A safe PSMF stops well before the person’s fat reserves are exhausted. A PSMF diet is not recommended for people who are already lean, or even slightly chubby; it’s for people who are actually obese.
People doing a PSMF also aren’t running around outside in the winter all day, and they’re taking vitamin and mineral supplements to make up for everything they get from food. And they’re doing it temporarily, not as a permanent change – that can’t be stressed enough. This is a short-term strategy for people who understand what they’re doing and know the risks, not a diet you’ll be on for the rest of your life.
Several official PSMF programs are going strong and showing good results – for example, at the Cleveland Clinic - and patients at these programs do fine. They have some side effects from the weight loss (feeling cold, low energy, hunger, constipation…) but they aren’t getting seriously ill.
Like most weight-loss programs, PSMF programs struggle with helping participants maintain their weight loss: a PSMF isn’t a magic bullet for healing a lifetime of obesity. But it is a pretty good kick-start, if you follow it up with healthy eating habits for the long term, and all the available evidence suggests that it’s safe when done intelligently.
Summing it Up
A protein-sparing modified fast is a low-calorie, very high-protein diet designed to help minimize the muscle wasting and nutrient deficiency problems of traditional fasting. It’s used as a temporary way to kick-start the beginning of a weight-loss program in people who have severe obesity to start with. The ultimate goal is always to transition to a healthy diet; the PSMF is just an initial strategy.
It does seem to be effective for weight loss, and few serious side effects have been reported among people who used high-quality protein (meat, fish, or egg whites, not commercial protein shakes) and did the PSMF under proper supervision.
A PSMF doesn’t have to be Paleo, but you could easily do it with Paleo foods, and then later transition into a basic Paleo diet for the long haul. Of course, nobody has to do a PSMF at all, but it’s an interesting tactic to have in your weight-loss arsenal in case you want to try it.
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