Fasting-Mimicking Diet: Fad or Science-Based?

A fasting-mimicking diet (FMD) is gaining popularity among health-conscious individuals. This new nutritional concept aims to mimic the physiological effects of traditional fasting, characterized by low-calorie intake with nutrient-dense foods at specific macronutrient ratios. The diet is applied cyclically for three to five consecutive days, followed by normal healthy dietary practices.

Unlike traditional fasting, FMD allows for calorie intake, making it less challenging to follow while providing similar suggested benefits ranging from improved metabolic health and increased longevity to a reduced risk of chronic diseases.

What is the fasting-mimicking diet?

The fasting-mimicking diet was designed by Dr. Valter Longo, whose research aimed to understand the mechanisms behind aging and longevity and the effect of nutrition on health span. The FMD aims to trigger the beneficial mechanisms of fasting without needing complete food abstention. This fasting-like diet is based on three key principles:

  • Caloric restriction. The diet reduces calorie intake to about 40–50% of normal calorie needs.
  • Cycling. The diet is incorporated within cycles. FMD lasts for three to five consecutive days, followed by the return to a normal diet for the rest of the month.
  • Nutrient composition. The diet follows specific ratios of macronutrients (proteins, fats, carbohydrates) and micronutrients (vitamins, minerals) to mimic the metabolic state of fasting. It primarily incorporates plant-based foods, healthy fats, unprocessed sources of carbohydrates, and moderate protein intake.

How does it work?

The effectiveness of FMD is based on triggering similar metabolic and physiological responses usually observed with complete fasting. This includes stimulating cellular autophagy (the body's ability to clear damaged cell components, giving space to cell renewal) and reducing inflammatory markers usually associated with chronic diseases. FMD may also promote ketosis in the body by reducing glucose levels and promoting the adaptation of body cells to stress, enhancing overall cellular resistance.

The benefits of a fasting-mimicking diet

Numerous studies involving human participants and animals have been dedicated to investigating the benefits of the fasting-mimicking diet. The main potential benefits of incorporating this dietary practice into one's routine are specified below.

Weight loss and metabolic health

One potential benefit of the FMD is its ability to promote weight loss and metabolic health. The reduction in calorie intake during the diet cycle encourages the body to utilize fat stores for energy, leading to a possible reduction in body fat and weight.

Research has shown that 71 out of 100 individuals participating in three cycles of 5 days of FMD per month for three months, compared to individuals with an unrestricted diet, resulted in a reduction in body weight, waist circumference, and lowering of body mass index (BMI).

While research has shown positive effects of FMD on weight loss, it is important to highlight that these effects may vary from individual to individual and can be influenced by other dietary practices and exercise.

Positive effect on gut health

Several studies have pointed out the potentially positive effects of FMD on gut health due to its impact on gut microbiota diversity. Science has shown that a diverse gut microbiome is linked with better digestive health, improved immune function, and reduced inflammation. The reduced calorie intake offered by FMD can potentially alter the composition of the gut microbiota, promoting the growth of beneficial bacteria.

Studies performed on mice with inflammatory bowel disease showed that exposure to FMD can reduce disease activity and decrease inflammatory markers, leading to reduced gut inflammation, an increase in stem cell numbers, stimulation of protective gut microorganisms, and reversal of intestinal pathology caused by the disease.

Reduced risk of chronic diseases

Incorporating FMD into dietary practices can potentially bring positive effects in reducing risk factors for cardiovascular disease and improving outcomes for certain types of cancer or diabetes. Incorporating this diet into health practices can potentially reduce elevated blood sugar levels, improve cholesterol profiles, and reduce blood pressure, which are risk factors associated with diabetes, heart disease, and stroke.

Two clinical trials showed that participants exposed to FMD cycles reduced insulin resistance, pre-diabetic markers, and lowered hepatic fat, reducing cardiometabolic risk factors and decreasing median biological age.

A few studies also highlighted the potential involvement of FMD in reducing cancer risk. In mouse cancer models, FMD potentiated the effects of chemo-, radio-, and immunotherapy, suggesting that it can induce changes in cancer cell metabolism, making the cancer cells more susceptible to treatment.

Potential risks and side effects

FMD, like any diet, should be implemented after consultation with a qualified professional, as it can come with potential risks and side effects, particularly for certain individuals, or when not followed correctly.

Given the reduced calorie intake, the diet can enhance feelings of hunger and fatigue. Therefore, those following the FMD should ensure that they consume food rich in fiber and healthy fats to enhance the feeling of satiety.

While FMD provides, to a certain extent, macro- and micronutrients, there is still a risk of nutrient deficiency and electrolyte imbalance if the diet is not carefully planned or followed too frequently. Consult with healthcare professionals and incorporate a variety of vegetables, seeds, nuts, and plant-based proteins that can provide a broad spectrum of minerals and vitamins during your fast.

While this fasting-like diet is a promising alternative to traditional fasting and can provide health benefits, it is not suited for everyone.

Pregnant, breastfeeding women and young individuals should avoid it as it can restrict the nutrients required for development. People with medical conditions and those under medication should first consult with a healthcare provider to determine if the FMD diet can negatively affect their medical condition or lower the medication absorption rate. Finally, individuals with eating disorders should avoid FMD as it might potentially trigger unhealthy eating patterns.

How to implement the fasting-mimicking diet

To better implement FMD and reap all the benefits of this diet, it is important to follow a structured approach.

Here are the main steps:

  • Consult with a healthcare professional. Discuss your diet goals and health conditions.
  • Familiarize yourself with the FMD principles. Understand the amount of calories and specific amounts of micro- and macronutrients.
  • Schedule your cycle. Choose the week on your schedule when you plan to implement FMD, taking into account that FMD might lower your energy levels.
  • Prepare your meals. Prepare a shopping list and cook recommended foods in advance to facilitate the process and keep you on track.
  • Stay hydrated. Give your body plenty of water or herbal teas.

Aim to incorporate these food types to fully enjoy the benefits of FMD:

  • Vegetables. Non-starchy vegetables such as spinach, kale, zucchini, or bell peppers (serving: 2–3 cups).
  • Healthy fats. Olive oil (serving: 1 tablespoon), avocado (serving: ¼), nuts or seeds (serving: 1 ounce).
  • Proteins. Cooked legumes (serving: ½ cup).
  • Complex carbohydrates. Cooked grains or starchy vegetables (serving: ½ cup).

Ensure that the total calorie intake of these foods does not surpass the FMD limit of approximately 700–1,100 calories, which corresponds to 40–50% of normal calorie intake. Remember to stay hydrated, drinking at least eight cups of water or herbal teas (values may vary depending on individual needs).

Drinking water will help you stay on track. Apart from that, aim to spread out your meals into smaller portions and embrace slow, mindful eating to increase satiety and improve digestion. The post-diet transition after the FMD cycle should also be taken with care, slowly returning to healthy diet practices to avoid potential digestive discomfort associated with a sudden increase in calorie intake.

The last word

FMD is a good alternative for individuals looking for less strict dietary practices compared to traditional fasting. It can potentially offer similar benefits, such as weight loss and improved metabolic health, alongside the potential for reducing the risk of chronic diseases and supporting gut health.

The individual responses to FMD can vary based on personal health conditions and nutritional needs. As with every diet, it should be consulted with a healthcare professional to maximize benefits while minimizing risks. Finally, to explore the full potential of FMD, it should be combined with other healthy dietary practices, exercise, and good sleep.


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