The Protein-Sparing Modified Fast (PSMF) diet is a very low-calorie diet (VLCD) designed for rapid weight loss while preserving muscle mass. Initially developed in the 1970s for individuals with obesity under medical supervision, it has gained popularity as a quick weight loss solution, though its long-term sustainability and safety are subjects of debate. The term now refers to many varieties of very low-calorie diets, many undertaken without medical supervision.
What is a Protein-Sparing Modified Fast?
A PSMF is characterized by a severe restriction of calories, typically derived primarily from lean protein sources, with minimal intake of carbohydrates and fats. Added fats like oils or salad dressings are off-limits, and carbs are restricted to around 20 g or less daily. This metabolic shift forces the body to utilize fat stores for energy, leading to rapid weight loss. The diet allows only 20-50 grams of carbohydrates per day. Fat is limited to whatever comes along with protein sources. Added fats of any kind - oils, dressings, spreads, etc. are forbidden.
Phases of a PSMF
A typical PSMF consists of two phases:
Intensive Phase: This phase involves a strict adherence to a very low-calorie, high-protein diet. Calorie levels on the diet are so low that nutritional deficiencies are likely. Patients receive 1.2-1.5 grams of protein per kilogram of their goal body weight per day. Carbohydrate levels are increased up to 45 grams during the first month, and up to 90 grams during the second month. Daily protein is reduced by about 40 grams per month. Most of the calories come from lean proteins, with very little carbohydrate or fat. During the intensive phase, patients typically lose 2-6 pounds (1-3 kg) per week. The fastest weight loss happens during the first two weeks of the diet.
Refeeding Phase: This phase involves the gradual reintroduction of carbohydrates and fats back into the diet while decreasing protein intake. Carbohydrate levels are increased up to 45 grams during the first month, and up to 90 grams during the second month. Daily protein is reduced by about 40 grams per month. The refeeding phase can last 6-8 weeks.
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How PSMF Works
The PSMF diet works by inducing a metabolic state called ketosis. Ketosis occurs when carbohydrate stores become so low that the body starts to break down fat tissue to use for energy instead. By drastically reducing carbohydrate intake, the body is forced to burn fat for fuel, resulting in weight loss.
Effectiveness of PSMF for Weight Loss
Obese patients typically lose weight on a PSMF. Average weight loss is about 40 pounds in 12 weeks, with men losing more than women. Studies have demonstrated the effectiveness of PSMF for short-term weight loss:
- One study found that 15 obese patients on a PSMF lost an average of 32 pounds (15 kg) of body fat over 6 weeks.
- Another study compared a PSMF with a conventional low-calorie diet. Patients on the PSMF lost more weight initially, at 12.4% vs. 2.6%.
However, it's important to note: In fact, one year later, those who went for follow-up visits weighed an average of 9.5% less than they did at the beginning of the PSMF. Many people who lose weight on a PSMF regain it after ending the diet.
Additional Benefits of PSMF
In addition to weight loss, PSMF has been associated with several other health benefits:
- 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.
Risks and Considerations
A PSMF is a restrictive diet that requires careful medical supervision to minimize potential risks. Potential risks and considerations include:
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- Nutritional Deficiencies: Calorie levels on the diet are so low that nutritional deficiencies are likely. People on a PSMF are usually advised to take supplements to prevent electrolyte imbalances.
- Electrolyte Imbalances: This puts them at risk for electrolyte imbalances , which can be very serious.
- Gallstones: Patients who have had gallstones (and potentially their gallbladder removed) should also advise their doctors before starting a PSMF.
- Not Suitable for Everyone: Note that not everyone should try this diet, though, including those with a BMI under 27 and anyone over 65 years old. The teenage years are a time of rapid growth, which can be stunted when too few calories are consumed.
- Side Effects: While the diet has side effects, they are usually mild. However, more serious complications caused by electrolyte imbalances can occur. Some of the most common symptoms of a PSMF and calorie restriction, in general, include changes in mood, nausea, decreased energy levels, and dehydration.
Who Should Consider a PSMF?
A PSMF should only be used by overweight individuals who have failed to lose weight with other methods. It is considered safe for adults and teens when overseen by a healthcare provider. In addition, a PSMF is sometimes used before weight loss (bariatric) surgery to reduce the risk of obesity-related complications.
Foods to Eat and Avoid on a PSMF
Foods to Eat
Most foods included on a PSMF are lean-protein foods, such as poultry, eggs, tofu, fish, and lean cuts of red meat. 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
Foods to Avoid
A PSMF restricts the intake of most foods high in carbs or fat. 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
Sample Meal Plan
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
PSMF via Nasogastric Tube (NGT)
Recent research explores the administration of PSMF through a nasogastric tube (NGT) as a method to enhance compliance and potentially improve outcomes. This approach involves continuous enteral nutrition (EN) of a high-protein diet, which may counteract muscle catabolism and promote satiety.
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Study Findings
A study comparing PSMF administered via NGT (ProMoFasT group) to oral administration found:
- Both groups experienced significant reductions in body weight, BMI, and waist circumference.
- The ProMoFasT group exhibited significantly higher levels of fat-free mass (FFM) and muscle mass (MM), along with lower levels of fat mass (FM).
- Insulin levels were significantly lower in the ProMoFasT group.
These findings suggest that PSMF delivered via NGT may offer advantages in preserving lean mass and improving body composition compared to oral administration.