The Fasting-Mimicking Diet (FMD) is a periodic dietary program involving a low-calorie, plant-based diet for a short, structured period. This approach provides specific nutrients while inducing a physiological state similar to water-only fasting. The FMD is a temporary intervention, typically followed for a few days each month, rather than a continuous eating pattern. It encourages the body’s natural cellular processes activated during prolonged fasting, without requiring complete food deprivation.
The Biological Mechanism of Mimicking a Fast
The Fasting-Mimicking Diet operates by carefully controlling the intake of macronutrients—specifically keeping protein and carbohydrates low while maintaining a higher proportion of healthy fats. This specific nutritional composition reduces the activity of key nutrient-sensing pathways within the body, such as the Insulin-like Growth Factor 1 (IGF-1) pathway and the mechanistic Target of Rapamycin (mTOR) pathway. Lowering the activity of these pathways signals to the body that nutrients are scarce, even though some food is being consumed.
When these nutrient-sensing pathways are downregulated, the body shifts into a state that promotes cellular cleansing and repair, a process known as autophagy. Autophagy involves the breakdown and recycling of damaged cellular components, which helps maintain cellular health and function. The FMD also appears to promote the regeneration of various cell types, including stem cells, especially during the refeeding period. This regenerative effect is partly attributed to changes in circulating factors, such as reduced IGF-1 levels.
The Structure of a Fasting-Mimicking Cycle
A typical Fasting-Mimicking Diet cycle spans five consecutive days, with specific calorie targets for each day. On the first day, individuals consume a moderately restricted calorie intake, often around 1,100 calories. This initial day helps the body gradually adjust to the reduced energy intake.
For the subsequent four days (Day 2 through Day 5), caloric intake is significantly reduced, typically to about 700-800 calories per day. The diet remains plant-based, emphasizing low protein and low carbohydrate foods, balanced with healthy fats. Common foods include vegetable soups, olives, nuts, and herbal teas, providing essential nutrients within the guidelines. Pre-packaged commercial kits are available to simplify adherence, providing all necessary meals and snacks for the five-day program.
Reported Physiological Effects
Studies on the Fasting-Mimicking Diet indicate several physiological outcomes. Participants often experience reductions in overall body weight and body fat, particularly visceral fat, which is fat stored around abdominal organs. This fat loss is supported by the body shifting to use fat for fuel during the low-calorie period.
The diet has also been associated with improvements in various metabolic markers. Research has shown potential benefits in blood sugar regulation, including reduced fasting glucose levels and improved insulin sensitivity. Studies have observed positive changes in cholesterol levels, such as reductions in total and low-density lipoprotein cholesterol, and a decrease in blood pressure. These metabolic adjustments contribute to a healthier overall profile.
The FMD appears to promote cellular rejuvenation processes, including activating cellular repair mechanisms and potentially reducing biological age by an average of 2.5 years after several cycles. The diet may also influence the immune system, with some studies indicating a more youthful immune profile after FMD cycles.
Safety and Medical Supervision
Before starting the Fasting-Mimicking Diet, consulting with a healthcare provider is highly recommended, as the diet is not appropriate for everyone. Medical supervision helps ensure the diet aligns with an individual’s health status and can identify any potential risks.
Certain populations should avoid the Fasting-Mimicking Diet without specific medical guidance. These include pregnant or breastfeeding women, individuals with a history of eating disorders, and those who are underweight or at risk of malnutrition. People with pre-existing medical conditions, such as diabetes, kidney disease, or heart disease, may experience negative effects on their health markers if not closely monitored by a doctor.
During the five-day cycle, common, temporary side effects can occur due to the reduced calorie intake. These may include mild to moderate fatigue, headaches, and increased hunger. These symptoms typically subside as the body adjusts or once the diet cycle is completed.