PSMF diet: what it is, how it works, and who should avoid it

By Clint Tustison. Published October 15, 2026.

PSMF stands for protein sparing modified fast. You eat almost nothing but lean protein, a few low-starch vegetables, and very little fat or carbohydrate. Total intake stays under 800 calories a day. The protein is there to limit how much muscle you lose while your body burns fat for energy.

Doctors first used it in the 1970s for people with obesity who needed to lose a lot of weight quickly, and clinics still use versions of it today. It is not a diet for losing the last few pounds before summer.

The numbers clinics use

Most of what follows comes from a 2014 review in the Cleveland Clinic Journal of Medicine by Julia Chang and Dr. Sangeeta Kashyap, which describes the protocol Cleveland Clinic uses. During the main phase, a patient on that protocol eats:

• under 800 kcal per day
• 1.2 to 1.5 g of protein per kg of ideal body weight per day (the Cleveland Clinic protocol uses 1.5 g/kg)
• under 20 to 50 g of carbohydrate per day, mostly from two servings of low-starch vegetables plus lettuce
• no added fats such as butter, oil, nuts, seeds or dips, with protein foods that have under 3 g of fat per ounce
• at least 64 oz of fluid

Protein is based on ideal body weight, not your current weight. Someone who weighs 120 kg and has an ideal body weight of 70 kg would aim for 84 to 105 g a day, not 144 to 180 g. Our protein calculator does this math for you.

Because the diet can’t cover all your nutrient needs, the protocol also calls for a daily multivitamin and mineral tablet plus potassium (16 to 20 mEq), calcium (1,000 to 1,200 mg), magnesium (400 to 500 mg) and sodium (1,500 to 2,000 mg). Potassium and magnesium doses should come from your doctor, since too much potassium is dangerous for some people.

The two phases

Intensive phase

This is the strict part, with the numbers above. In the Cleveland Clinic protocol it lasts up to about 6 months. The review reports typical losses of 1 to 3 kg per week, with more in the first 2 weeks. Much of that early drop is water.

Refeeding phase

Refeeding lasts 6 to 8 weeks. You add carbohydrates and fats back slowly and cut protein down. The protocol in the review allows up to 45 g of carbohydrate a day in month 1 and up to 90 g a day in month 2, with daily protein reduced by 1 to 2 oz each month. The carbs should come from high-fiber, low-glycemic foods such as vegetables, fruit and whole-grain cereals. Potassium and magnesium supplements stop after week 2 of refeeding.

People skip refeeding because the scale has stopped moving and they want to be done. That is how a lot of the lost weight comes back.

What the studies show

The review summarizes several studies. In one of the first, 668 patients with and without type 2 diabetes started at an average of 98 kg. They lost an average of 21 kg during the intensive phase and had kept off 19 kg by the end of refeeding. In a study where every patient had type 2 diabetes, mean loss was 18.6 kg, and patients were still 8.6 kg below their starting weight after 1 year.

The same review reports drops in fasting blood glucose, HbA1c, triglycerides and blood pressure. Some of the glucose change happens within days and comes from the calorie and carb restriction itself, before much weight is lost. That is also why people on diabetes or blood pressure drugs need their doses checked before they start.

Weight regain after the diet ends is common, and the review says so. The diet is a starting point. What you eat in the months after refeeding decides how much of the loss you keep.

Side effects

Common side effects listed in the review are headache, fatigue, dizziness when standing up (orthostatic hypotension), muscle cramps, feeling cold, constipation, diarrhea, bad breath, menstrual changes and hair thinning. Most fade and can often be eased by adjusting fluids, salt and supplements.

Uric acid can rise on a high-protein diet, so gout is a concern. In one study cited by the review, gout occurred in fewer than 1% of patients. Gallstones are another known risk with very-low-calorie diets. The review notes that the fat in PSMF protein foods (roughly 45 to 70 g a day) is likely enough to keep the gallbladder emptying, which lowers that risk compared with very-low-fat liquid diets.

Who it is for

The Cleveland Clinic criteria are a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related condition such as type 2 diabetes.

Who should not do it

The review lists these as reasons not to start a PSMF:

• BMI under 27
• recent heart attack, angina, a significant heart rhythm problem, or decompensated heart failure
• recent stroke or poor blood flow to the brain
• end-stage kidney disease or liver failure
• cancer
• major psychiatric illness
• pregnancy or breastfeeding
• wasting disorders
• age under 16 or over 65

A history of gallstones and a history of gout make it riskier and need a doctor’s view first. People with type 1 diabetes are usually advised against it because of the risk of diabetic ketoacidosis and the difficulty of adjusting insulin. If you have a history of an eating disorder, tell your doctor even if it isn’t active now, and ask whether a diet this strict is safe for you.

How to start with your doctor

At Cleveland Clinic, patients get an ECG before they begin to rule out rhythm problems, plus blood work at the start, every 2 weeks for the first month, and monthly after that. They meet a dietitian and see a physician or nurse practitioner every 6 to 8 weeks.

You can bring that list to your own doctor. Ask them:

1. Am I a candidate, given my BMI and health history?
2. Which of my medications need to change before I start, and how will we track that?
3. How often should I get blood work, and which tests?
4. Which supplements should I take, and at what dose?
5. What symptoms mean I should stop and call you?

If you get the go-ahead, use the protein calculator to get your daily target, and the lean body mass calculator if you want a second reference point.