Protein-Sparing Modified Fast Review: Does It Aid Weight Loss?

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Evidence BasedProtein-Sparing Modified Fast Review: Does It Aid Weight Loss?

Medically reviewed by Marie Lorraine Johnson MS, RD, CPT — Written by Rachael Ajmera, MS, RD — Updated on December 11, 2023

The basics
Method
Benefits
Downsides
Foods to eat
Foods to avoid
Sample menu
Bottom line

The protein-sparing modified fast (PSMF) is a very low calorie diet designed for quick, short-term weight loss. It’s typically unsustainable and likely to lead to weight regain.

Fish on a cutting board with lemon pieces.

However, within the last few decades, it has gained widespread popularity among dieters looking for a quick and easy way to drop extra pounds.

The protein-sparing modified fast diet was originally designed by physicians to help their patients lose weight quickly.

Although the plan is often praised for its effectiveness, its safety and sustainability have been called into question.

This article takes a closer look at the protein-sparing modified fast and whether it’s effective for weight loss.

What is the protein-sparing modified fast?

The protein-sparing modified fast (PSMF) is a very low calorie diet designed to aid weight loss and preserve muscle mass.

It severely restricts calorie consumption while increasing the intake of protein-rich foods.

Additionally, the intake of carbohydrates and fats is very limited in this diet.

The PSMF was first introduced in the 1970s to help people with obesity lose weight under the guidance of a physician.

However, over the past few years, the diet has undergone several modifications. It’s also often followed without medical supervision, which can be dangerous.

Summary
The PSMF is a highly restrictive diet that involves severely limiting the intake of calories, carbs, and fat and increasing the consumption of protein.

How to do a protein-sparing modified fast

The diet is divided into two main phases: the intensive phase and the refeeding phase.

The intensive phase can last up to 6 months and involves limiting calorie intake to less than 800 calories daily.

To follow the PSMF, you must consume approximately 0.7 grams (g) of protein per pound (1.5 g per kilogram (kg)) of body weight. Generally, this should come from protein-rich foods like poultry, fish, eggs, low fat cheese, and tofu.

Added fats like oils or salad dressings are off-limits, and carbs are restricted to around 20 g or less daily. In addition, you also need to drink about 2 liters (L) or 0.5 a gallon (gl) of calorie-free liquid daily, such as water or tea.

During the refeeding phase, carbs and fats are slowly added back into the diet, and daily protein intake is gradually reduced by 7–14 g per month.

The refeeding phase can last 6–8 weeks. Up to 45 g of carbs are permitted per day in the first month, while up to 90 grams per day are allowed during the second month.

A multivitamin, as well as potassium, calcium, magnesium, and sodium supplements, are used during the intensive phase to help protect against nutritional deficiencies.

How many days a week should you do PSMF?

There isn’t a set number of days a week you need to stay on the protein-sparing diet. It’s a diet you stay on continuously for as long as you’re on it.

That said, it’s not advisable to stay on it long-term as it doesn’t satisfy all your nutritional needs. You should follow this diet only under a doctor’s supervision, and they would determine when and how to introduce refeeding depending on your specific needs.

Summary
A typical PSMF is divided into two phases: the intensive phase and the refeeding phase. During the intensive phase, carbs, fat, and calories are severely restricted. In the refeeding phase, foods are gradually added back into the diet. Follow this diet only under the supervision of a doctor.

What are the benefits of protein-sparing modified fast?

The primary benefit of PSMF is fast weight loss. Studies show that a PSMF can be effective for rapid weight loss when performed under proper medical supervision.

This is because the diet is extremely low in calories and high in protein, which promotes weight loss.

Weight loss

For example, one 2016 study of 12 adolescents on a PSMF found that participants lost an average of 25 pounds (lb) (11 kg) over a 6-month period. This accounted for about 10% of their total body weight.

That said, it’s unclear how effective a PSMF is for sustaining long-term weight loss and whether it may cause weight regain once a normal diet is resumed.

Another newer 2020 study found that people who followed PSMF lost an average of 3% more weight over five years compared to people who didn’t follow this diet. That said, after five years, there was no more difference in weight loss between the subjects and the control group.

A 2017 study also found that a PSMF was more effective than a conventional low calorie diet for short-term weight loss. In this case, after only one year, the weight loss was similar between groups.

Other benefits

In addition to helping you lose weight quickly, following a PSMF has been associated with several other health benefits.

These include:

Decreased cholesterol levels: A short-term PSMF may help reduce levels of total and LDL (bad) cholesterol, but not necessarily by a significant amount.
Improved blood sugar control: Some research has found that PSMF may help lower blood sugar levels in those with type 2 diabetes and in people with obesity.
Reduced blood pressure: Research shows that calorie restriction may help reduce blood pressure levels to improve heart health.
May help protect against metabolic syndrome: Like other types of calorie-restricting diets, this diet could improve several components of metabolic syndrome, which may help reduce your risk of heart disease, stroke, and type 2 diabetes.

Summary
Research shows that a PSMF increases short-term weight loss while preserving muscle mass. However, some studies have found that it may not be effective for sustaining weight loss in the long term. A PSMF may also help reduce your cholesterol and blood pressure levels, improve blood sugar control, and protect against metabolic syndrome.

Potential downsides

Following a PSMF may be a safe and effective option for weight loss when performed under the guidance and supervision of a healthcare professional.

However, the diet is highly restrictive, low in calories, and eliminates many important nutrients. This can increase your risk of nutritional deficiencies if you’re not being closely monitored.

A PSMF is not suitable for older adults, those with a history of disordered eating, women who are pregnant or nursing, or those with a body mass index lower than 27 kg per meter squared (m2).

It’s also not advisable for those who have a history of gallstones or have had their gallbladders removed. Very low calorie diets may increase the chance of adverse side effects in those with these conditions.

Some of the most common symptoms of a PSMF and calorie restriction, in general, include changes in mood, nausea, decreased energy levels, and dehydration.

Furthermore, very low calorie diets often lead to rapid weight loss, which carries a high risk of weight regain once a normal diet is resumed.

Slow, consistent weight loss is usually a much better option for maintaining long-term results long term.

Summary
Without medical supervision, a PSMF may increase the risk of nutritional deficiencies. It may also cause mild side effects and is not advisable for certain groups of people.

What is allowed on PSMF?

Most foods included on a PSMF are lean-protein foods, such as poultry, eggs, tofu, fish, and lean cuts of red meat.

Non-starchy vegetables are also allowed as part of the diet.

Here are some of the foods you can eat as part of a PSMF:

Poultry: skinless chicken, turkey, goose, duck
Meat: lean cuts of beef, pork, lamb
Seafood: flounder, sole, cod, catfish, halibut
Non-starchy vegetables: leafy greens, broccoli, cabbage, cauliflower, Brussels sprouts, celery, tomatoes, onions
Low fat dairy: cottage cheese, cheese, skim milk
Eggs and egg whites
Tofu

Summary
A PSMF includes lean-protein foods like poultry, eggs, tofu, fish, and red meat, as well as non-starchy vegetables.

What to avoid on PSMF?

A PSMF is a highly restrictive diet that limits most carbs and fats.

Here are some of the foods you should avoid as part of a PSMF:

Fruits: apples, berries, oranges, grapes, melons, pears, peaches
Starchy vegetables: potatoes, corn, peas, parsnips
Grains: wheat, quinoa, oats, barley, buckwheat, millet
Legumes: black beans, lentils, chickpeas, kidney beans, peanuts
Processed foods: convenience meals, baked goods, potato chips, fast food, candy bars
Sweetened beverages: juice, sweet tea, sports drinks, soda
Sugars and sweeteners: honey, maple syrup, table sugar, molasses, brown sugar, high-fructose corn syrup
Fats and oils: olive oil, coconut oil, vegetable oils, salad dressings, butter, margarine
Full-fat dairy: yogurt, cheese, milk

Summary
A PSMF restricts the intake of most foods high in carbs or fat.

Sample menu

This 5-day sample meal plan demonstrates what a typical PSMF might look like.

Monday

Breakfast: eggs with spinach and tomatoes
Lunch: grilled cod with steamed broccoli
Dinner: herbed turkey breast with roasted Brussels sprouts

Tuesday

Breakfast: tofu scramble with onions, garlic, and bell peppers
Lunch: oven-baked chicken with a side salad (no dressing)
Dinner: pork chops with roasted asparagus

Wednesday

Breakfast: egg-white omelet with zucchini, tomatoes, and garlic
Lunch: baked catfish with boiled cabbage
Dinner: lettuce wrap with lean ground beef, mushrooms, garlic, ginger, and scallions

Thursday

Breakfast: low-fat cottage cheese with cinnamon
Lunch: low-carb turkey meatballs with zucchini noodles and tomatoes
Dinner: lemon garlic roasted chicken with side salad (no dressing)

Friday

Breakfast: hard-boiled eggs with salt and pepper
Lunch: baked tofu with steamed green beans
Dinner: grilled sirloin steak with oven-roasted eggplant

Summary
The sample menu above provides several meal ideas that can be consumed on a PSMF. You can easily adjust the foods included to suit your taste.

The bottom line

A protein-sparing modified fast is a highly restrictive diet designed to promote rapid weight loss by increasing protein intake and limiting your consumption of calories, carbs, and fat.

Some studies have found it effective for short-term weight loss and improved blood sugar control, cholesterol, and blood pressure.

However, it may also increase your risk of nutritional deficiencies and weight regain in the long run.

Additionally, due to its restrictive nature, it’s best to follow it under the guidance of a healthcare professional to maximize results and minimize adverse side effects.

How we reviewed this article:

SourcesHistoryHealthline has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Bakhach M, et al. (2016). The protein-sparing modified fast diet.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4784653/
Chang JJ, et al. (2017). Limited carbohydrate refeeding instruction for long-term weight maintenance following a , very-low-calorie meal plan./nutrition/ketogenic-diet-101/
Dieting & gallstones. (2017).https://www.niddk.nih.gov/health-information/digestive-diseases/gallstones/dieting
Formisano E, et al. ((2023). The real-life use of a protein-sparing modified fast diet by nasogastric tube (pro mo fas t) in adults with obesity: an open-label randomized controlled trial.https://www.mdpi.com/2072-6643/15/22/4822
Nicoll R, et al. (2018). Caloric restriction and its effect on blood pressure, heart rate variability and arterial stiffness and dilatation: a review of the evidence.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5877612/
Parvaresh A, et al. (2017). Modified alternate-day fasting vs. calorie restriction in the treatment of patients with metabolic syndrome: A randomized clinical trial.https://pubmed.ncbi.nlm.nih.gov/31779987/
Pfoh RH, et al. (2020). The effect of starting the protein-sparing modified fast on weight change over 5 years.https://pubmed.ncbi.nlm.nih.gov/31916212/
Sellahewa L, et al. (2017). A systematic review of evidence on the use of very low calorie diets in people with diabetes.https://pubmed.ncbi.nlm.nih.gov/26435354/
Sukkar SG, et al. (2021). A clinical perspective of low carbohydrate ketogenic diets: a narrative review.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8310928/

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Evidence Based

This article is based on scientific evidence, written by experts and fact checked by experts.

Our team of licensed nutritionists and dietitians strive to be objective, unbiased, honest and to present both sides of the argument.

This article contains scientific references. The numbers in the parentheses (1, 2, 3) are clickable links to peer-reviewed scientific papers.

Medically reviewed by Marie Lorraine Johnson MS, RD, CPT — Written by Rachael Ajmera, MS, RD — Updated on December 11, 2023

Protein-Sparing Modified Fast Review: Does It Aid Weight Loss?

Nutrition

Nutrition

What is the protein-sparing modified fast?

How to do a protein-sparing modified fast

How many days a week should you do PSMF?

What are the benefits of protein-sparing modified fast?

Weight loss

Other benefits

Potential downsides

What is allowed on PSMF?

What to avoid on PSMF?

Sample menu

Monday

Tuesday

Wednesday

Thursday

Friday

The bottom line

How we reviewed this article:

Evidence Based

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