The fasting-mimicking diet (FMD) is a 5-day low-calorie eating protocol designed by Valter Longo’s lab at USC. The premise: produce most of the metabolic effects of multi-day water fasting (autophagy, stem cell renewal, immune cell turnover, growth factor reduction) while still allowing some food intake. The commercial version is sold as ProLon. The principles can be approximated independently with careful planning.
This guide covers what FMD does, the DIY protocol, expected effects, and when this approach makes sense versus other forms of fasting.
Table of Contents
What FMD Is
A 5-day eating protocol with very specific macronutrient targets:
- Day 1: ~1100 calories
- Days 2-5: ~700-800 calories
- Predominantly plant-based
- Low protein (especially low animal protein)
- Moderate carbohydrate
- Higher fat (relative to total)
- Specific micronutrient profile
The pattern produces many of the metabolic signals of true water fasting — including IGF-1 reduction, ketosis, and autophagy activation — while being substantially easier to maintain than 5 days of pure water fasting.
How It Works
The mechanism centres on what doesn’t happen rather than what does:
- Insufficient protein to fully suppress autophagy or activate mTOR pathways
- Caloric deficit deep enough to deplete glycogen and induce ketosis
- Specific micronutrient profile to prevent malnutrition signals
- Sufficient food intake to be sustainable, unlike water fasting
The result is a metabolic state similar to days 2-3 of water fasting, maintained for the full 5 days.
What the Evidence Shows
FMD has more clinical research than most specific fasting protocols. Findings include:
- Weight loss averaging 3-5 kg per 5-day cycle (significant water/glycogen, some fat)
- Reduced IGF-1 (longevity-relevant)
- Improved insulin sensitivity and glucose markers
- Reduced inflammatory markers
- Improved lipid profiles
- Some evidence of stem cell turnover
- Promising preliminary data in oncology contexts (with chemotherapy)
- Emerging research in MS and other autoimmune conditions
- Generally well-tolerated; adverse events similar to or less than water fasting
Effects largely return to baseline by 4-8 weeks after the cycle without cumulative practice.
DIY Protocol
The DIY version approximates ProLon’s targets without the proprietary food kit. Daily targets:
- Day 1: 1100 kcal, 50g protein, 60g fat, 100g carb
- Days 2-5: 700 kcal, 25g protein, 45g fat, 60g carb
Food sources:
- Vegetables (especially low-glycemic, leafy greens)
- Olive oil (substantial portion of calories)
- Nuts (small amounts)
- Vegetable broth
- Olives and avocado
- Small amounts of plant protein (lentils, beans on day 1; smaller amounts after)
- Herbal teas, water, plain coffee
Avoid during FMD:
- Animal protein
- Sugar and refined carbohydrates
- Dairy
- Most fruits (small amounts of berries acceptable)
- Alcohol
Sample 5-Day Plan
Day 1 (~1100 kcal)
- Breakfast: Vegetable soup with olive oil, small handful nuts
- Lunch: Large salad with olive oil, half avocado, lentils
- Dinner: Roasted vegetables with olive oil, vegetable broth
Days 2-5 (~700 kcal each)
- Breakfast: Herbal tea, small portion of nuts (15g)
- Lunch: Vegetable soup with olive oil, large green salad
- Dinner: Roasted or steamed vegetables with olive oil, vegetable broth
Consistency of pattern matters more than meal-by-meal precision. The macronutrient distribution and calorie target are the key variables.
vs Commercial ProLon
- ProLon advantages: precisely formulated, no planning required, well-studied exact composition
- ProLon disadvantages: expensive (typically $200+ per cycle), processed packaged foods, taste preferences variable
- DIY advantages: 90% cheaper, fresh whole foods, customisable
- DIY disadvantages: requires planning, harder to hit exact targets, the precise micronutrient mix isn’t replicated
For most healthy adults, well-executed DIY approximates the metabolic benefits of ProLon at a fraction of the cost. For specific clinical applications (cancer support, MS), the commercial product’s precise composition may matter more.
How Often to Cycle
Standard recommendations vary:
- For metabolic health maintenance: 2-3 cycles per year
- For weight loss: monthly cycles for 3-6 months, then maintenance
- For specific clinical applications: per the studied protocol (often monthly)
- For longevity-oriented practice: quarterly
More frequent than monthly probably has diminishing returns and increasing recovery cost.
Refeeding After FMD
Less risky than refeeding from water fasting (because some food was consumed throughout), but still worth doing carefully:
- Day 6: gradual return to normal eating, smaller meals
- Days 7-10: normal intake, normal patterns resume
- Avoid: massive refeeding meal on day 6, immediate alcohol, immediate large protein/carb load
- Continue moderate sodium and fluid intake
See our refeeding recipes guide for specific meal ideas.
Frequently Asked Questions
Is FMD as good as water fasting?
For most metabolic effects, similar. For maximum autophagy specifically, water fasting probably produces stronger effects. For practicality and safety, FMD is significantly better.
Can I exercise during FMD?
Light exercise (walking, easy yoga, mobility) is fine. Heavy training is not advised — you don’t have the fuel.
Does FMD break my fast?
FMD is not strict fasting; it’s eating in a specific pattern. Calling it “fasting” is partially marketing. The metabolic effects share much of fasting’s biology.
Will I keep the weight off?
Some yes, some no. Most people regain water and glycogen weight (1-2 kg) within days of normal eating. Sustained fat loss requires sustained dietary practice between cycles.
Can I take supplements during FMD?
Plain mineral electrolytes, water-soluble vitamins are fine. Skip protein supplements, oil-based supplements (which add calories), and BCAAs.
I have a chronic condition. Is FMD safe?
Discuss with your doctor. Particularly important for diabetes, cardiovascular disease, kidney disease, anyone on multiple medications. The Longo lab’s clinical work specifically excludes some patient populations.
The Bottom Line
The fasting-mimicking diet is a 5-day protocol that delivers most of the metabolic benefits of water fasting with significantly more practicality and safety. The DIY version, executed carefully, approximates the commercial ProLon at a fraction of the cost. Cycling 2-6 times per year, depending on goals, fits well into a broader intermittent fasting practice. For specific clinical applications (cancer support, autoimmune conditions, longevity-oriented protocols), FMD has more developed evidence than most fasting variations.