The Fasting Mimicking Diet, Step by Step: Five Days a Month

1. Learn what the fasting mimicking diet puts on your plate

The numbers come from the best-known trial, published in 2017. Day one supplied about 4,600 kJ, roughly 1,100 kcal, with 11% of energy from protein, 46% from fat and 43% from carbohydrate. Days two to five dropped to about 3,000 kJ, roughly 725 kcal, with protein down to 9%. In kitchen terms: vegetable soup at lunch and dinner, a small handful of nuts or olives in between, and herbal tea.

Low protein and low sugar are meant to lower blood glucose and IGF-1, a growth signal, as fasting does, while unsaturated fat keeps some fuel coming in. After the first cycle in that trial, IGF-1 and fasting glucose were down and blood ketones were up. Halving your usual portions is a different diet. Longo built the plan because an extended fast on water alone is hard to repeat every month and calls for close supervision.

2. Read what the fasting mimicking diet trials measured

Five studies, most of them using the boxed kit, cover what is known:

• Wei, 2017. 100 generally healthy adults in the Los Angeles area were randomized to three months of normal eating or three monthly cycles. The diet group lost 2.6 kg (about 6 lb) on average against 0.1 kg in controls, with a spread of 2.5 kg, and part of the loss was lean mass. Waist size, systolic blood pressure (by 4.5 mmHg) and IGF-1 also fell. Glucose and cholesterol improved mostly in people who started high, in an after-the-fact analysis. Dropout was 25% on the diet and 10% in controls.
• Brandhorst, 2024. An exploratory reanalysis of that trial's blood samples found lower insulin resistance, less liver fat on MRI and a calculated biological age 2.5 years lower at the median. That is a score from lab values, and lifespan was not measured.
• van den Burg, 2024. In the Netherlands, 100 adults with type 2 diabetes on diet alone or metformin added a monthly cycle to their family doctor's care for a year. Compared with usual care, they needed less medication, with HbA1c 0.3 points lower and weight 3.6 kg (about 8 lb) lower.
• Mishra, 2023. In Tennessee, 84 adults with high blood pressure and excess weight did four monthly cycles or four months of a Mediterranean diet. Weight, waist and cholesterol fell in both groups, with no significant difference between them at the end.
• Sadeghian, 2021. In Iran, 60 women with obesity did the diet or a daily 500 kcal deficit for two months. They lost 1.1 kg on the diet and 2.3 kg on the deficit, a gap that was not statistically significant.

These trials were small, 60 to 100 people, and short, two months to a year. Longo co-authored all but the Iranian one. He holds equity in L-Nutra, which makes the kit (his disclosures say it goes to charity), and the company supplied the food or co-funded several trials. No study in people has shown that the diet prevents disease or extends life.

3. Check whether you should skip it or ask a doctor first

In the 2017 trial the most common complaints were fatigue, weakness and headaches, mostly mild or moderate, and no serious side effects were reported. The authors still advise supervision by a health professional who knows prolonged fasting. Skip it, or plan it with your doctor, in these cases:

• Diabetes on insulin or a sulfonylurea such as glipizide or glimepiride. Five days this low can drive blood sugar too low, and the Dutch trial excluded these drugs for that reason.
• Pregnancy or breastfeeding. Skip it; your energy and protein needs are higher.
• Underweight, meaning a BMI under 18.5, the floor in the 2017 trial. Skip it.
• A history of an eating disorder, even years ago. Counted portions can restart old patterns.
• Under 18. The trials enrolled adults only.
• Kidney or heart disease, or blood pressure pills. Blood pressure fell in the trials, and with medication that can mean dizziness when you stand up. Decide with your specialist.
• Over 70. The 2017 authors wrote that safety at 70 and older still has to be established.

4. Choose between the boxed kit and a home version

The kit used in these trials is sold as ProLon: one box per day with vegetable soups, bars, chip-style snacks, tea and a vitamin and mineral supplement, eaten whenever you like. The trial results apply to that product. A home version can copy its numbers, though no published trial has tested one.

Question	Boxed kit	Home version
Tested in trials?	Yes, in the trials from Longo's group	No, it only copies the published numbers
Calories and protein	Fixed by the box	Depend on your measuring spoons and a kitchen scale
Work involved	Open the day's box	Shop, batch-cook on Sunday, measure oil and nuts
Food choice	Set menu	Seasonal vegetables and your own spices
Vitamins and minerals	Supplement included	Ask your doctor or pharmacist about a multivitamin

For a first cycle, the box shows you the portions without any math. If you cook and own a kitchen scale, the home version is workable.

5. Build your five days from the grocery store

A home version copies the kit's pattern: vegetable soups as the base, with most of the energy from nuts, olives, avocado and olive oil. Fruit, juice, meat, fish, eggs, dairy, bread and pasta stay out for their sugar, protein or starch, and beans go in by the spoonful. Days two to five could look like this:

Time	What you eat	Calories, roughly
8 a.m.	Unsweetened herbal tea, 1/2 oz walnuts (about 7 halves)	90
12:30 p.m.	2 cups vegetable soup (carrots, celery, zucchini, canned diced tomatoes, kale) with 1/4 cup rinsed cannellini beans and 2 tsp olive oil	230
3:30 p.m.	8 Kalamata olives, 1 cup cucumber slices	80
7 p.m.	1 1/2 cups butternut squash soup with 2 tsp olive oil and 1 tbsp slivered almonds, a small baked sweet potato	320
Day one only, add	Half an avocado, 1 more tbsp olive oil, 1/2 oz almonds, an extra cup of soup	400

That comes to about 725 calories, and about 1,100 on day one, with roughly 16 g of protein on days two to five. Cook both soups on Sunday and freeze what you won't eat by Wednesday in quart containers. Drink water and herbal tea through the day. If you plan to drop coffee, taper it the week before so a caffeine headache doesn't land on day two.

To check a plate, the meal scan in FastTrack estimates calories and macros from a photo; for oil and nuts, measuring spoons are more reliable.

6. Get through days two to five and know when to stop

Put the five days in a quiet week, Monday to Friday for example, with no business trip or Thanksgiving dinner in it. Many people find day two or three the hardest and day four easier. Keep to walks and easy rides until the following week, and get up slowly from bed and from the couch.

Stop and eat a normal meal if you feel dizzy, shaky, sweaty or confused, if your heart races, or if you vomit. If you check your blood sugar at home, repeated low readings are a reason to stop too. If you faint or have chest pain, call 911. After any of these, talk to your doctor before you try another cycle.

7. Break it on day six and plan the next cycle

Day six is a normal day with a light start: oatmeal or Greek yogurt with berries for breakfast, soup with rice or a turkey sandwich for lunch. Keep the burger and fries for day seven. Coming off five days with some food is easier than coming off a water fast, and our guide on how to break a fast covers the longer cases.

Skip the scale on day five, since part of that drop is water; the 2017 trial weighed people 5 to 7 days after they returned to normal eating. The trials repeated the diet about monthly, for two to four months in adults without diabetes and up to twelve in the diabetes trial. Effects over years are unstudied, so plan three cycles, then review with your doctor.

8. Compare the fasting mimicking diet with daily intermittent fasting

With 16:8 fasting you eat within the same eight hours every day, for example noon to 8 p.m., with no calorie targets. The fasting mimicking diet asks for five hard days a month and nothing in between.

Which works better is an open question. We found no trial in PubMed that compares the two head to head in healthy adults, and against a daily 500 kcal deficit or a Mediterranean diet the results were similar. Pick the approach you can keep up for a year.

Some people combine them: 16:8 in ordinary weeks, and one five-day cycle a month with no eating window. The FastTrack timer follows your 16:8 days and shows which fasting phase you are in, up to ketosis.

The short version

Five plant-based days a month, about 1,100 calories on day one and 725 after that. In small trials, mostly from Longo's group, weight and blood pressure came down, with wide differences between people. The kit is what was tested; a home version only comes close. On diabetes drugs or with heart or kidney disease, plan it with your doctor. If you are pregnant, underweight, under 18 or have had an eating disorder, skip it.