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

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

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.

Rating Score Breakdown
  • Overall score: 2.25
  • Fast weight loss score: 4
  • Long-term weight loss score: 1
  • Easy to follow: 2
  • Nutrition quality: 2
BOTTOM LINE: While the protein-sparing modified fast (PSMF) aids short-term weight loss, it’s highly restrictive and very low in calories. Thus, it carries a risk of nutritional deficiencies and weight regain.
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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 on 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.

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 per day.

To follow the PSMF, you must consume approximately 0.7 grams of protein per pound (1.5 grams per 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 grams or less per day.

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

The refeeding phase can last 6–8 weeks. Up to 45 grams of carbs are permitted per day in the first month, while up to 90 grams per day are allowed during the second month (1).

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

Summary A typical PSMF is divided into two phases: the intensive phase and 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.

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.

One small study in 12 adolescents on a PSMF found that participants lost an average of 25 pounds (11 kg) over a 6-month period. This accounted for about 10% of their total body weight (1).

Another older, 6-week study in 15 people showed that following a PSMF reduced body fat by 32 pounds (14 kg) without significantly altering muscle mass (2).

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.

In fact, most studies report that dieters regain more than 50% of the weight they lose within 2–3 years of completing a PSMF diet (3).

One study in 127 people found that a PSMF was more effective than a conventional low-calorie diet for short-term weight loss.

However, after one year, weight loss was similar between groups, suggesting that a PSMF may not be as effective for weight maintenance in the long-run (4).

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.

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

Some of the potential benefits of a PSMF include:

  • Decreased cholesterol levels. One study showed that a short-term PSMF reduced levels of total and LDL (bad) cholesterol by 20%. However, the diet also decreased levels of HDL (good) cholesterol (5).
  • Improved blood sugar control. Some research has found that very-low-calorie diets like the PSMF may help lower blood sugar levels in those with type 2 diabetes (6, 7).
  • Reduced blood pressure. Studies show that calorie restriction may help reduce blood pressure levels to improve heart health (8).
  • May help protect against metabolic syndrome. A very-low-calorie diet could improve several components of metabolic syndrome. This may help reduce your risk of heart disease, stroke, and type 2 diabetes (9, 10).
Summary A PSMF may help reduce your cholesterol and blood pressure levels, improve blood sugar control, and protect against metabolic syndrome.

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, very 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 breastfeeding, or those with a body mass index lower than 27 (1).

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 risk of adverse side effects in those with these conditions (11).

Some of the most common symptoms of a PSMF and calorie restriction, in general, include changes in mood, nausea, decreased energy levels, and dehydration (1).

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 (12).

Slow, consistent weight loss is usually a much better option for maintaining 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.

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 (1).

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.

A PSMF is a highly restrictive diet that limits most carbs and fats (1).

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.

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 a 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.

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 improvements in 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.