A five-day low-calorie eating plan designed to trigger some of the same metabolic changes as complete fasting is drawing growing attention in preventive medicine. Doctors call it the fasting mimicking diet, or FMD.
Mykhailo Petryk, a general-practice and family medicine physician who specialises in preventive medicine and longevity at the ULTRASOUND PRO medical centre and the Ecarium 360 project, says the idea behind FMD is straightforward: over five days, a person continues to eat, but the calorie count and nutritional composition are calibrated to produce a metabolic response that partly resembles what happens during fasting.
Why fasting attracts attention
Petryk says people are searching for ways to stay active and healthy for longer rather than simply treating illness after it appears. Life expectancy is gradually rising, he notes, but many people spend their later decades managing obesity, type 2 diabetes, cardiovascular disease, and daily medication regimens.
Biological age, he explains, is not a date on a passport but a measure of how well different body systems are functioning. Two people of the same chronological age can have very different states of blood vessels, muscles, heart function, and metabolism. Modern preventive medicine aims to help people preserve physical activity, mental clarity, and independence for as long as possible while keeping the risk of chronic disease low.
Autophagy and the Nobel Prize
Central to the scientific interest in fasting is a process called autophagy, which Petryk describes as a cell's internal recycling system. During autophagy, the cell identifies damaged proteins and worn-out structures, breaks them down, and reuses some of the components. He likens the cell to a small city: if the waste-disposal system stops working, it eventually disrupts transport, energy supply, and every other service.
Research into the mechanisms of autophagy was recognised with the Nobel Prize in Physiology and Medicine in 2016, awarded to Japanese scientist Yoshinori Ohsumi.
Why doctors do not simply recommend water fasting
Complete fasting, Petryk says, is a serious metabolic stress. It can bring weakness, dizziness, low blood pressure, hypoglycaemia, electrolyte imbalances, and loss of lean muscle mass alongside body fat. He says that over his career he has repeatedly seen patients begin extended water fasts based on internet videos, without accounting for underlying conditions, without monitoring blood pressure or blood glucose, and while continuing intensive training. That combination, he says, can cause more harm than good.
FMD emerged as an attempt to capture some of the metabolic effects of calorie restriction without requiring complete food withdrawal.
The protocol
Professor Valter Longo at the University of Southern California has researched FMD for many years, focusing on how restricting calories, protein, and specific nutrients affects the signalling pathways linked to ageing, metabolism, and cellular regeneration.
The diet is not simply smaller portions of ordinary food. Total calorie intake, protein quantity, and the ratio of fats to carbohydrates all matter. The plan is predominantly plant-based, low in calories, and limited in protein. During the five-day cycle, insulin levels and IGF-1, a growth factor involved in cell regulation, may fall. Glycogen stores are gradually depleted, fat burning increases, and ketone bodies may begin to form.
Day one is a transitional day with slightly higher calories as the body still draws on glycogen reserves. Days two and three tend to be the hardest, with hunger, headaches, feeling cold, weakness, or irritability common as the body shifts more actively toward burning fat. By days four and five, some people find hunger less intense, though Petryk stresses that individual responses always vary and no one should force themselves to continue if they feel significantly worse.
After the five days, re-feeding should be gradual. Petryk recommends starting with vegetables, soups, grains, and legumes rather than returning immediately to large portions, alcohol, fried food, or heavy meals.
What the research shows
Clinical studies of several FMD cycles have found moderate reductions in body weight, waist circumference, body fat, blood pressure, and certain metabolic markers. The most pronounced changes were generally seen in people who started with excess weight, elevated blood pressure, elevated glucose, or other risk factors.
One analysis found that after three cycles, the average estimated biological age fell by approximately 2.5 years. Petryk cautions that this refers to a change in a mathematical figure calculated from a set of laboratory parameters, not a literal reversal of ageing or a proven rejuvenation of all organs. He describes FMD as a supplementary metabolic tool rather than a standalone treatment for obesity, diabetes, or cardiovascular disease.
Who should avoid it
One standardised commercial version of FMD is the ProLon programme, a five-day kit of plant-based soups, nut bars, olives, herbal teas, and other components portioned by day and calculated by calorie count and nutritional composition. A significant portion of clinical FMD research has used this standardised protocol, which is now available in Ukraine. Petryk says he welcomes its availability because standardisation makes it possible to know exactly what a person is consuming, unlike recipes reconstructed from the internet or unknown imported alternatives.
FMD is not appropriate for everyone. Petryk says the programme is not recommended during pregnancy or breastfeeding, for anyone under 18, for people who are underweight, or for those with eating disorders, severe kidney or liver disease, acute infections, or who are recovering from major surgery. People with diabetes, cardiovascular disease, or cancer, older adults, and anyone taking regular medication all need individual medical consultation before starting.
The assumption that it is just food and therefore safe for everyone, he says, is wrong. Five days of significant calorie restriction affects blood pressure, blood glucose levels, and the action of medications. The goal, he adds, is not to cancel ageing but to give people an additional tool, provided they are properly screened, hold realistic expectations, and follow safety guidelines.
