PSMF Calculator (Protein Sparing Modified Fast)
Calculate your customized daily protein target, Lyle McDonald category classification, fat-loss energy chart, 7-day refeed calendar, and weekly fat loss histogram.
What is a Protein Sparing Modified Fast (PSMF)?
A Protein Sparing Modified Fast (PSMF) is a clinically researched, short-term Very Low Calorie Diet (VLCD) specifically engineered to maximize rapid adipose fat loss while sparing 100% of lean skeletal muscle mass. Originally developed in hospital settings in the 1970s (e.g., Bistrian et al.) and later refined for athletic populations by Lyle McDonald in The Rapid Fat Loss Handbook, PSMF produces the fastest scientifically safe fat loss possible.
During normal total starvation, the body breaks down precious muscle protein to produce glucose via gluconeogenesis. A PSMF "modifies" the fast by supplying an exact therapeutic dose of high-biological-value protein based on your Lean Body Mass (LBM) while keeping dietary carbohydrates and fats at strict physiological minimums (~20g each).
Because dietary energy intake is kept extremely low (typically 800 to 1,200 kcal/day), your body is forced to extract 1,200 to 2,200 kcal daily directly from stored adipose tissue to meet basal metabolic demands.
Medical Disclaimer: PSMF is an aggressive, short-term intervention designed for rapid weight reduction and is not intended for permanent lifestyle use. Individuals with kidney disease, gout, eating disorders, or pregnancy must not undertake a PSMF without clinical supervision.
PSMF vs. Standard Keto: What is the Real Difference?
While both diets eliminate carbohydrates and induce ketosis, their biological fuel sources and rates of fat loss differ fundamentally:
| Feature | Protein Sparing Modified Fast (PSMF) | Standard Ketogenic Diet (SKD) |
|---|---|---|
| Dietary Fat Intake | Very Low (≤ 20g/day) — only essential fish oils | High (70–75% of calories) — butter, oils, fatty cuts |
| Primary Fuel Source | Stored Adipose Body Fat (Endogenous) | Dietary Fat from Meals (Exogenous) |
| Daily Calorie Deficit | Extreme (-1,200 to -2,200 kcal/day) | Moderate (-300 to -500 kcal/day) |
| Rate of Fat Loss | 2.5 to 5.0 lbs of pure fat per week | 0.5 to 1.5 lbs of fat per week |
| Recommended Duration | 11 days to 6 weeks max (per category) | Months to years (lifestyle diet) |
The 3 Lyle McDonald PSMF Categories Explained
Your biological starting body fat percentage determines your metabolic category, daily protein multiplier, protocol duration, and refeed frequency:
Protein Target: 1.5–2.0g per lb LBM. Run for a strict maximum of 11–12 days with no cheat meals. Conclude immediately with a full 2 to 3-day structured high-carb refeed before a 2-week maintenance diet break.
Protein Target: 1.1–1.25g per lb LBM. Run for 2 to 6 weeks. Includes one 5-hour high-carb refeed per week and one free meal to preserve thyroid hormone (T3) and leptin.
Protein Target: 0.8–1.0g per lb LBM. Run for 6 to 12 weeks. High internal fat stores protect muscle tissue. No carb refeeds needed; allows two free meals per week.
Mandatory PSMF Supplement & Electrolyte Protocol
Because real whole food intake is limited to pure lean protein, whole-food micronutrient intake drops sharply. Daily supplementation is non-negotiable for cardiovascular safety and muscle function:
- Sodium (3,000–5,000 mg/day): Drink 2 cups of salted bouillon broth daily or salt all lean protein heavily. Essential to maintain blood pressure and prevent dizziness.
- Potassium (1,000 mg/day): Use potassium salt substitutes (e.g. NoSalt or Lite Salt) sprinkled over meals.
- Magnesium (400–500 mg/day): Take chelated magnesium glycinate before bed to prevent painful calf/muscle cramping.
- Calcium (1,000 mg/day): Take calcium citrate if you do not consume fat-free dairy.
- Omega-3 Fish Oil (3–5g/day): Supplies essential EPA and DHA fatty acids to support cell membranes without breaking the fast.
- High-Potency Multivitamin: 1 standard tablet daily to cover water-soluble B and C vitamins.
Approved PSMF Foods: Pure Lean Proteins & Vegetables
A valid PSMF food must be almost entirely pure protein with virtually zero fat and zero sugar. The classic "PSMF Shopping List" includes:
- Skinless chicken & turkey breast
- Egg whites (liquid or separated)
- White fish (cod, tilapia, halibut, pollock)
- Canned tuna in water (drained)
- 99% lean ground turkey / ground beef
- Fat-free cottage cheese & non-fat Greek yogurt
- Whey protein isolate (mixed in water)
- Spinach, kale, romaine, arugula
- Broccoli, cauliflower, cabbage
- Cucumbers, celery, zucchini
- Mushrooms, asparagus, green beans
- Mustard, hot sauce, vinegar, lemon juice
- Black coffee, green tea, zero-calorie soda
Training & Lifting Guidelines During PSMF
Because glycogen and systemic recovery are constrained, training volume must be reduced to avoid overtraining and muscle loss:
- Maintain Heavy Weight Intensity: Continue lifting heavy weights in the 6–10 rep range. Heavy mechanical tension signals your body that muscle mass is functionally necessary.
- Cut Workout Volume by 50% to 66%: Perform only 2 to 3 full-body lifting sessions per week with only 2–3 heavy work sets per exercise.
- Strictly Avoid High-Intensity Cardio (HIIT): HIIT on an aggressive deficit elevates cortisol and triggers muscle breakdown. Stick entirely to low-intensity walking (8,000–10,000 daily steps).
Frequently Asked Questions
What is a Protein Sparing Modified Fast (PSMF)?↓
A Protein Sparing Modified Fast (PSMF) is a medically developed Very Low Calorie Diet (VLCD) designed to maximize rapid adipose fat loss while sparing skeletal muscle mass. By providing high protein based on Lean Body Mass (LBM) while keeping dietary carbohydrates and fats at strict minimums (~20g each), your body draws 70% to 90% of its daily energy directly from stored body fat.
What is the difference between PSMF vs. Keto?↓
While both diets induce ketosis, a standard keto diet is high-fat (70-75% fat, 20-25% protein) with a mild calorie deficit. In contrast, a PSMF is very low fat and very low carb, providing only pure lean protein (65-80% protein, <10% fat). This creates a massive 1,000–2,000 kcal deficit where your body burns its own stored fat instead of dietary fat.
What are the Lyle McDonald PSMF categories?↓
Based on Lyle McDonald's Rapid Fat Loss protocol, individuals are divided into three categories based on body fat percentage: Category 1 (men under 15%, women under 24%) runs for 11-12 days with high protein and full post-diet refeeds; Category 2 (men 16-25%, women 25-34%) runs for 2-6 weeks with weekly 5-hour refeeds and 1 free meal; Category 3 (men 26%+, women 35%+) runs for 6-12 weeks with 2 free meals weekly.
How much weight can you lose on a PSMF?↓
Because daily caloric deficits frequently reach 1,200 to 2,000 kcal below maintenance TDEE, individuals typically lose 3 to 7 lbs of scale weight in week 1 (a mix of glycogen, water, and fat) followed by 2 to 4 lbs of pure adipose fat per week thereafter.
How do carb refeeds and free meals work on a PSMF?↓
Structured carb refeeds and free meals restore leptin levels, replenish muscle glycogen, support thyroid (T3) production, and prevent metabolic slowdown. Category 1 uses a 2 to 3-day high-carb refeed; Category 2 uses a 5-hour high-carb refeed once weekly plus 1 free meal; Category 3 uses 2 free meals per week.
What supplements and electrolytes are required on PSMF?↓
Due to severe calorie restriction, daily supplementation is mandatory: Sodium (3,000–5,000 mg via bouillon broth/table salt), Potassium (1,000 mg), Magnesium (300–500 mg elemental), Calcium (1,000 mg), a high-potency multivitamin, and 3–5g of Omega-3 Fish Oil to provide essential fatty acids.
What can you eat on a PSMF diet?↓
PSMF nutrition is centered strictly around very lean protein sources: skinless chicken breast, egg whites, 99% lean ground turkey, white fish (cod, tilapia, halibut), canned tuna in water, and fat-free cottage cheese, accompanied by non-starchy fibrous vegetables (spinach, lettuce, broccoli, cucumber, celery).
How should I lift weights and exercise on a PSMF?↓
Cut your workout volume by 50% to 66% (perform only 2 to 3 full-body sessions per week with 2–3 heavy sets per exercise). Maintain heavy weights to signal muscle retention, but avoid exhausting high-volume sets or intense cardio.
How long should you stay on a PSMF before taking a diet break?↓
Category 1 individuals should diet for no longer than 11 to 12 days before taking an 11-12 day maintenance break. Category 2 individuals can diet for 2 to 6 weeks followed by a 2-week maintenance break. Category 3 individuals can safely diet for 6 to 12 weeks before taking a 2-week diet break.