The fasting-mimicking diet: fasting in a box, sold by the man who studied it.

Five days of special low-calorie meals, once a month, to get the benefits of fasting without the misery of not eating. The science is interesting. The conflict of interest is huge.

The short version

What it is: Five days a month of low-calorie, low-protein, mostly plant-based meals, designed to mimic a fast.
Good for: Small drops in weight, blood pressure and some blood markers in small trials.
Bad at: Independent evidence. Almost all of it comes from the inventor's lab.
Evidence: Moderate design, small trials, big conflicts of interest.
Our take: Not part of our plan. Interesting science, expensive box.

What the fasting-mimicking diet actually is

The fasting-mimicking diet (FMD) was developed by Valter Longo, a longevity researcher at the University of Southern California. The idea: give people just enough food, of the right kind, that their body still behaves as if it were fasting. You eat for five days, but very little, and mostly plant-based foods such as vegetable soups, with little protein and little sugar.

The commercial version is sold as ProLon by a company called L-Nutra. Longo holds equity in it, the university licensed the patents to it and may get royalties, and the company supplied the food for the trials.¹² Longo says his equity is assigned to charity.² Still: the same people designed the diet, ran the studies and stand to benefit from the box.

Unlike extended fasting, you're not going without food. Unlike the 5:2 diet, the low-calorie stretch comes in a single five-day block, usually once a month for a few months.

How it works in your body

The theory is that very low calories, little protein and little sugar together switch on the same signals as a real fast: insulin drops, a growth hormone called IGF-1 drops, and the body shifts towards repair and fat burning. Then you eat normally for the rest of the month.

In the main trial, three cycles of FMD lowered IGF-1, weight and blood pressure compared with a normal diet.¹ Whether lower IGF-1 in this setting means a longer, healthier life in people is still a theory, not a result.

What you eat, what you don't

Eat (on the five days)

Vegetable soups
Small portions of nuts, olives, seeds
The crackers and snacks in the kit, in the set portions
Herbal tea, water
Rest of the month: normal, healthy meals

Forget it (on the five days)

Meat, fish, eggs, dairy
Bread, pasta, rice
Sugar, soft drinks, juice
Coffee with milk and sugar
Alcohol
Hard training

A day on the fasting-mimicking diet

The trial version gave about 1,100 calories on day one and about 720 on each of days two to five, with only around a tenth of those calories from protein.¹ In real life, day two or three looks roughly like this:

Morning: herbal tea, a small nut bar from the kit. No latte.
Lunch: vegetable soup with a few crackers.
Afternoon: a handful of olives. Lots of water.
Dinner: another vegetable soup, a few nuts or seeds. Early night.

Feeling flat and cold on days two and three is normal. The real trap is day six: the box is empty, and the whole thing only makes sense if you go back to normal meals, not a reward buffet.

In Singapore

Outside the kit, the closest hawker match is a yong tau foo bowl with only vegetables, clear soup instead of noodles, and none of the fried bits. Broths are salty, so leave some. Skip the kopi with condensed milk on those five days.

In Austria, Germany and Switzerland

Soup-based "Fastenwochen" are everywhere here, but they weren't tested in the FMD trials. Supermarket soups and stock cubes (Gemüsebrühe, Bouillon) often contain celery and lots of salt: read the label. A home-made Gemüsesuppe week is a lighter week, not a proven anti-ageing protocol.

Alcohol on the fasting-mimicking diet

OK, now and then

On normal days: one glass of dry wine with a meal
Spirits with soda water and lime, with food

Out

Any alcohol during the five FMD days
Drinking on an empty stomach
Cocktails, liqueurs, sweet wine

50 Club: one glass of dry wine with dinner on a normal day, never during the five days.

On very low calories, alcohol hits harder and puts fat burning on hold: two drinks cut it by about 73% in one small study.⁴ All the details are in our alcohol guide.

What the fasting-mimicking diet is good for, with proof

Small drops in weight and blood pressure

In the main randomised trial, 100 healthy adults did either three monthly cycles of FMD or ate normally. The FMD group lost about 2.6 kg and their systolic blood pressure fell by about 4.5 points.¹ People who started with higher risk factors saw bigger changes. A real effect, but a modest one.

"Biological age" markers

A 2024 analysis from the same group reported that three FMD cycles lowered a blood-based estimate of biological age by a median of about 2.5 years, and reduced liver fat.² The average change was smaller, about 1.5 years, and the insulin result came from only 11 people with prediabetes. "Biological age" scores are estimates, not years of life you get back.

The uncomfortable truth

"Science proves ProLon slows ageing." Science shows some blood markers shifted in small trials, run by the inventor, using food from the company he holds equity in.¹² That doesn't make the results false. It means they need independent repeats before anyone sells them as anti-ageing. Five days of eating less once a month for a few months will lower your weight and blood pressure a bit. So will eating a bit less every day.

Across fasting methods in general, the 2026 Cochrane review found no clear weight advantage over regular diet advice.³

The risks

Diabetes medication. Five days of very low calories can cause dangerous lows on insulin or similar drugs. Fasting days roughly doubled hypos in a type 2 diabetes trial.⁵ Only with your doctor.
Tiredness, headaches, dizziness during the five days. Plan them for a quiet week.
Low protein. The diet is deliberately low in protein. That's part of the theory, and not great if you're older and trying to keep muscle.
Cost. The kits are expensive compared with soup you make yourself.
Eating disorders. Any structured fasting can feed disordered eating. Not for anyone with that history.

Allergies: watch the staples

The kits are plant-based, but nuts and seeds feature heavily. Check every label.

Staple	Allergen (EU list of 14)⁶	Swap
Nut bars, mixed nuts	Tree nuts, sometimes peanuts	Pumpkin and sunflower seeds
Crackers	Often cereals containing gluten, sesame	Rice cakes, vegetable sticks
Vegetable soups	Often celery	Home-made soup without celery
Seed mixes	Sesame, sometimes mustard in dressings	Sunflower seeds, olive oil and lemon
Soy-based items	Soy	Bean or lentil soups

Don't do the fasting-mimicking diet if you…

are pregnant or breastfeeding
are under 18
have ever had an eating disorder
have diabetes or take insulin, sulfonylureas, SGLT2 or GLP-1 drugs
are underweight, frail or older and losing muscle
have kidney or liver disease
are allergic to nuts and can't check every item in the kit

Healthen me asks about all of these and gives you a different plan if any apply.

How we use the fasting-mimicking diet in your plan

We don't. It's an info article for us. The evidence is interesting but small and not independent, and the plan relies on buying a branded product. Our approach is real food from the supermarket.

What we use instead: a daily eating window from time-restricted eating, as long as you comfortably can, plenty of plants, and one fasting day after your weekly Rebel Day. If you want to try FMD, talk to your doctor first, especially if you take any medication.

This article is general information, not medical advice. If you have a medical condition or take medication, talk to your doctor before changing how you eat.

Sources

Wei M et al. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Science Translational Medicine 2017;9(377):eaai8700. doi.org/10.1126/scitranslmed.aai8700
Brandhorst S et al. Fasting-mimicking diet causes hepatic and blood markers changes indicating reduced biological age and disease risk. Nature Communications 2024;15(1):1309. doi.org/10.1038/s41467-024-45260-9
Garegnani LI et al. Intermittent fasting for adults with overweight or obesity. Cochrane Database of Systematic Reviews 2026, Issue 2, CD015610. doi.org/10.1002/14651858.CD015610.pub2
Siler SQ, Neese RA, Hellerstein MK. De novo lipogenesis, lipid kinetics, and whole-body lipid balances in humans after acute alcohol consumption. American Journal of Clinical Nutrition 1999;70(5):928–936. pubmed.ncbi.nlm.nih.gov/10539756
Corley BT et al. Intermittent fasting in type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabetic Medicine 2018;35(5):588–594. doi.org/10.1111/dme.13595
Regulation (EU) No 1169/2011 on food information to consumers, Annex II (the 14 allergens). eur-lex.europa.eu/eli/reg/2011/1169/oj

Keep reading

Extended fasting (36–72 hours)
Days without food: the hardcore end of fasting. Clinics report few side effects, but you lose muscle along with fat, and stopping needs care. Doctor territory.
The 5:2 diet
Five normal days, two days on about 500 calories. The 5:2 diet works about as well as a regular diet, with more hunger on fast days. Here's the full picture.
Time-restricted eating (16:8 and friends)
Eat inside a window, close the kitchen the rest of the day. 16:8 is simple and helps you eat less, but the clock itself is no fat-burning magic. Here's what the trials show.

From the 50 Club: Alcohol

See all diets, sorted and explained