What to Know About the Protein-Sparing Modified Fast Diet

Medically Reviewed by Christine Mikstas, RD, LD on February 25, 2024
3 min read

The protein-sparing modified fast diet, also called the PSMF diet, is a diet you can follow on a short-term basis to lose a large amount of weight quickly. Doctors most often recommend the protein-sparing diet to people who are obese

The purpose of the diet is to help people lose body fat, not muscle. In one study, obese people lost an average of 31 pounds of body fat over just 6 weeks, while maintaining their muscle mass. You should only attempt this diet under the care of a doctor or a registered dietitian.

This diet is protein-sparing because it cuts out virtually all carbohydrates and additional fats from the diet. Most of the diet consists of high protein foods like chicken and other poultry, lean meats, fish, and other seafood.

Protein-sparing helps you to lose weight because it makes fat the primary source of energy for your body, rather than carbohydrates. When your body can't derive energy from the carbohydrates you eat, it will turn your body fat into energy. This is also known as ketosis.

You can eat only 800 calories a day. Since this diet doesn't provide all the nutrition you need, you will need to take vitamins that your doctor will recommend.

There are two phases of the PSMF Diet:

Intensive phase. The first phase of this diet may last up to 6 months, but your doctor will work with you to determine the appropriate length of time. In this phase, you will eat fewer than 20 grams of carbohydrates per day, all of which come from vegetables. You will eat 1.5 grams per kilogram of body weight in protein. This figure is based on your ideal body weight, not your current one.

You will only eat lean proteins that have less than 3 grams of fat per ounce. You will not add any fats like butter or oil to your food. You should drink at least a half-gallon of liquid each day.

The supplements your doctor will recommend should include:

Refeeding phase. After the first phase, you will slowly introduce carbohydrates back into your diet over the course of 6 to 8 weeks. In the first 4 weeks, you can eat up to 45 grams of carbohydrates. For the second 4 weeks, you can eat up to 90 grams. You will also decrease your protein intake by between 1 and 2 ounces every 4 weeks.

For the first month of the diet, you should see your doctor every 2 weeks. After that, you should see them once every 6 to 8 weeks. However, your doctor should order monthly lab tests to make sure you are staying healthy while following this diet.

During the course of the diet, your body will reach the state of ketosis and release ketones for fuel. Ketones are the energy source your body creates from fat instead of making glucose from carbohydrates. In rarer cases, an excess of ketones in your bloodstream may cause kidney problems. For this reason, it's important to drink the recommended amount of fluids and keep up with your monthly lab tests.

Studies show that the protein-sparing modified fasting diet works in the short term to help obese patients lose a significant portion of their body weight.

Studies also show that this diet works better than other diets in the long term. One study found that people who had followed the PSMF diet for two years weighed less on average than those who had followed conventional diets. This study also showed that people who received extra counseling for the refeeding phase of the diet were more likely to maintain their weight loss.

The PSMF diet has a better chance of success if your doctor gives you resources for PSMF meals and recipes, along with PSMF diet plan examples.

Your doctor should also give you ideas for behavior modification since the protein-sparing modified fast diet will be a big change from your usual eating habits. They may suggest consulting a mental health professional to further help with changing behaviors on a long-term basis.

Most experts recommend incorporating daily exercise into this program. Your doctor may recommend you see an exercise physiologist who can show you some exercises that will work for you.