The Fasting Mimicking Diet (FMD) is a dietary approach designed to replicate the benefits of fasting without the need for complete food restriction. It's an approach that lets you “trick” your body into thinking it’s fasting while still eating small amounts of carefully selected foods. It is a short-term intervention to support cellular repair processes and overall health.
Understanding the Fasting Mimicking Diet
Developed by biogerontologist Valter Longo, the fast-mimicking diet (FMD) mimics fasting effects while still allowing small food portions. The FMD is primarily plant-based and designed to produce fasting-like effects in the body by targeting blood glucose, ketone levels, and other biological markers. Unlike traditional fasting, which restricts almost all eating and drinking with the exception of water and certain other liquids, the FMD permits a limited intake of specific nutrients to sustain the body.
UCLA Health dietitian Shelby Yaceczko explains, “You can think of it as a low-calorie diet pattern designed to mimic fasting without fasting. On an FMD, you’re allowed a limited intake of calories from specific food sources at certain times, so you won’t be completely fasting.”
The diet is often set up in cycles - such as five consecutive days per month over three months. The FMD is typically followed for 5 consecutive days, once a month, for 3 months. If you’ve reached your goals in 3 months, you can stop or adjust the FMD. If you’re using it for long-term health or to manage a condition, you can continue with changes as needed based on your progress and health.
How the FMD Works
The FMD is designed to mimic the effects of fasting on the body without actually requiring complete food restriction. It involves consuming a plant-based diet for a specific period, typically 5 days.
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The FMD operates on a monthly cycle, unlike daily or weekly fasting diets. The idea is to provide the body with enough nutrients to function while still triggering some of the beneficial effects of fasting, such as cellular rejuvenation and metabolic changes.
The FMD is followed for 5 days and has a specific breakdown of macronutrients (protein, fat, and carbohydrates). The diet focuses on a macronutrient ratio of approximately 10% protein, 45% fat, and 45% carbohydrates. Generally, individuals consume around 40%-50% of their usual calorie intake on the first day, then reduce to 10%-20% for the next four days.
- Day 1: Moderate calorie intake, roughly 1,000-1,100 calories. Consume 1,100 calories, with 11% from protein (121 calories from protein), 46% from fat (506 calories from fat), and 43% from carbohydrates (473 calories from carbohydrates).
- Days 2 to 5: Consume only 725 calories per day, with a breakdown of 9% protein (65 calories from protein), 44% fat (319 calories from fat), and 47% carbohydrates (341 calories from carbohydrates).
Foods Allowed on the FMD
The FMD involves consuming a plant-based diet that is low in calories, low in sugar, low in protein, and high in unsaturated fats. A typical FMD meal plan primarily includes non-starchy vegetables such as celery, zucchini, tomatoes, and leafy greens, along with whole fruits including berries, apples, and oranges. Healthy fat sources such as avocados, walnuts, and olive oil are also allowed, providing essential fats while limiting protein and carbohydrate intake. The diet restricts high-glycemic foods such as white rice, white bread, bagels, refined sugars, processed foods, dairy, animal proteins, and starchy vegetables.
Here’s an example of foods you can eat on the FMD:
- Healthy fats: olive oil, avocados, nuts, seeds
- Plant-based protein sources: legumes, lentils, beans
- Fruits: berries, apples, oranges, other nonstarchy fruits
- Whole grains: brown rice, quinoa, oats, whole wheat products
- Noncaffeinated herbal teas: chamomile, peppermint, hibiscus
- Vegetables: leafy greens, cruciferous vegetables, tomatoes, carrots, peppers
Sample 5-Day Fasting-Mimicking Diet Food List
Here is a handy food list to take to the grocery store with you when shopping for five days’ worth of FMD meals.
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Desserts and snacks
Traditional desserts aren't really an option on the fasting-mimicking diet because of the diet's calorie restrictions. But you can certainly use berries and other fruit as a kind of dessert while staying within the parameters of the plan.
- Walnuts.
- Hummus.
- Carrots or celery sticks.
- Kale crackers.
- Specially formulated nutrition bars that conform to the prescribed macronutrient profile.
Potential Benefits of the Fasting Mimicking Diet
Research suggests this diet could offer health benefits. The fast-mimicking diet offers the benefits of fasting without the need for complete food restriction.
The FMD has been researched in both mice and a small sample size of humans with promising results. In mice, this diet has been shown to protect healthy cells, eliminate damaged cells (including cancerous ones), reduce inflammation, and enhance overall health. Similarly, in humans, the FMD was associated with reduced abdominal obesity, lower insulin resistance, and a positive impact on the immune system.
Fasting-like diets have also shown promise in activating protective processes in cells, which could be beneficial for preventing or treating Alzheimer’s disease. Both animal and human studies have shown benefits for memory and overall well-being, but further research is needed to support these findings.
Here are some potential benefits of a fasting-mimicking diet based on research:
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- Weight loss: FMD can lead to reductions in body weight and body fat.
- Improved metabolic health: FMD may improve markers of metabolic health, such as blood pressure, fasting glucose levels, and insulin sensitivity.
- Reduced inflammation: FMD has been shown to reduce levels of inflammatory markers like C-reactive protein (CRP).
- Lowered risk factors for age-related diseases: FMD may decrease risk factors for diseases associated with aging, such as cardiovascular disease and diabetes.
- Cellular rejuvenation: FMD can activate cellular stress response pathways and autophagy, which may promote cellular repair and rejuvenation.
- Improved cognitive function: FMD may improve cognitive function, which could be beneficial for brain health and Alzheimer’s disease prevention.
According to Dr. Yaceczko, documented benefits include weight loss, decreased belly and liver fat, reduced cholesterol and blood sugar levels, and lowered blood pressure. Additionally, the diet may help reduce inflammation and cell damage in the body, processes associated with aging and disease.
One of the unique aspects of the FMD is its potential influence on cellular health, particularly through a process known as autophagy. “FMD may potentially speed up autophagy, which eliminates bad cells in the body that cause disease and promote aging processes,” notes Dr. Yaceczko. This regenerative effect on cells is a key reason the diet is being studied for its effect on age-related diseases and cellular health.
Cardiovascular Health and Disease Prevention
The FMD has also been linked to cardiovascular benefits. Some early findings suggest it may help reduce risk factors associated with heart disease, improve cholesterol levels and regulate blood pressure, which are vital components of heart health. Research on the FMD’s effects on chronic diseases such as diabetes, cancer, and Alzheimer’s disease is also ongoing, though most studies have been conducted only on animals.
The potential of the FMD to support cancer treatment and recovery has drawn attention, as well. Though early studies have been promising, Dr. Yaceczko emphasizes caution. “The FMD is being studied for its ability to extend median lifespan, reduce inflammation and cancer incidence, and increase results from anti-cancer treatments, but much of the research has only been done in animal studies,” she says. As with any dietary intervention, clinical trials are essential to determine the FMD’s full effects on human health, Dr. Yaceczko says.
Potential Risks and Side Effects
The FMD is generally considered safe for healthy individuals when followed as instructed. However, like any restrictive diet, it does come with potential side effects and risks.
Here are a few of the more well-known side effects:
- Hunger and discomfort: The low-calorie nature of the diet may lead to increased hunger, which can be uncomfortable. During the initial days of FMD, you can expect hunger pangs.
- Fatigue and weakness: Some people may experience fatigue or weakness due to the reduced calorie intake. The FMD involves a significant calorie reduction, which means your body has less fuel than usual. As your system shifts into fat-burning mode, you may experience dips in energy, especially during the first couple of days.
- Potential for disordered eating: Following a restrictive diet like the FMD could potentially exacerbate unhealthy or disordered eating patterns in people with eating disorders. Some people may begin to associate food with guilt or feel anxious about deviating from the plan. For some, the FMD can trigger cycles of strict adherence followed by overeating on non-fasting days.
- Adverse effects on certain medical conditions: People with certain medical conditions, such as diabetes, may experience negative effects on blood sugar levels.
- Reduced food intake can also affect your digestive system: During the FMD, some people report feeling bloated, constipated, or experiencing changes in their intestinal health.
- Some people may experience headaches or mood swings due to lower calorie intake and potential shifts in blood sugar. These symptoms, often called “fasting headaches,” tend to resolve after the first few days.
- Long-term or frequent diet cycles without nutritional balance during non-fasting days can risk B vitamins, iron, and calcium deficiencies.
In a study of 100 generally healthy participants trying the FMD, 54% to 100% reported no adverse effects during the FMD cycles. The most common self-reported symptoms were mild to moderate fatigue, weakness, and headaches. No severe adverse effects were reported. Overall, after three cycles of the FMD, participants reported only mild to moderate side effects.
Overall, the diet resulted in reductions in body weight, body fat, blood pressure, and insulin-like growth factor 1 (IGF-1). The FMD was found to be safe, feasible, and beneficial for participants at risk for disease, with improvements in various health markers.
Who Should Avoid the FMD?
The Fasting Mimicking Diet isn’t suitable for everyone. It is important to consider safety precautions and contraindications before starting the FMD.
Individuals who are pregnant or breastfeeding, those with a history of eating disorders, and people with existing medical conditions such as diabetes, low blood pressure, kidney disease, or heart disease, should avoid it or consult with their health care provider if considering the diet. “Individuals who are considered at high risk of malnutrition or undernutrition should typically avoid fasting diet patterns,” Dr. Yaceczko warns. Additionally, those on medication should be sure to consult with a doctor before starting a fasting diet.
- Pregnant or breastfeeding women should avoid it, as reducing calories and altering macronutrient intakes might harm their nutritional needs. Fasting-mimicking diets may not provide adequate nutrients for expectant or breastfeeding mothers and their infants.
- Growing individuals require a consistent and balanced diet to support their development.
- Fasting-mimicking diets may exacerbate eating disorder behaviors and thought patterns.
- People with nutritional deficiencies or those who are underweight.
- People with a history of eating disorders. Restricting your food intake can be triggering for people who have or have had an eating disorder and could lead to unhealthy behaviors.
- People with a preexisting medical condition, such as hypertension, diabetes, liver disease or another chronic condition, should consult a physician prior to starting this or any other new diet.
Tips for Navigating the FMD
Navigating a fasting-mimicking diet can be both rewarding and challenging. Effective meal planning is the key to a successful fasting-mimicking diet.
Here are some helpful tips:
- Hydration is crucial during fasting-mimicking days. Dr. Yaceczko recommends consuming at least 70 ounces of water daily to prevent dehydration.
- Avoid intense workouts: High-intensity workouts can be challenging while fasting.
- Practice mindful eating by savoring each bite, chewing slowly, and being present during your meals to enhance the sense of satisfaction.
- Consider consulting a nutritionist or doctor to determine if this is a safe option for you.
FMD vs. Other Fasting Protocols
Unlike other intermittent fasting methods that require complete abstention from food for specific time intervals, the FMD allows for a limited amount of food intake. Dr. Yaceczko points out that this dietary flexibility can make the FMD more appealing to those who find traditional fasting too restrictive. “For some, the FMD may be easier to adopt, given it allows for certain foods and nutrients during fasting days,” she explains. By offering a structured yet less restrictive fasting experience, the FMD has opened a door for individuals who wish to explore the benefits of fasting without entirely eliminating food.
The ProLon Diet
The ProLon diet is the most well-known commercial version of the fasting mimicking diet.
The Prolon Fasting Mimicking Diet was pioneered by Dr. Valter Longo, a biogerontologist and cell biologist at the University of Southern California. Prolon has undergone 32 clinical trials with 18 universities around the globe, including USC’s Longevity Institute, and it has more than 130 issued patents. Prolon features a meal program you can buy and easily follow. It includes all the foods you need in specifically formulated quantities and ratios for five days.
“(The) Prolon prepackaged meal kit consists of plant-based whole foods and does not include the use of any meat or dairy foods, gluten, GMOs or processed foods,” Dilley explains. It also contains a supplemental energy drink made from vegetable glycerine as well as a plant-based omega-3 supplement from algal oil.
Currently, Prolon offers several product subscriptions and bundles. The basic five-day program currently sells for $175 when set up as an every-four-months subscription or $195 when ordered just once.
Prolon recommends users follow the plan once per month for anywhere from one to six consecutive months, depending on your health goals and current health status. After this initial onboarding period, the company claims you can maintain the cellular rejuvenation and healthy aging benefits by following the program just three times a year thereafter.
DIY Approach
Because the Prolon program can be a financial commitment, some are trying a do-it-yourself approach to the fasting-mimicking diet. Theoretically, this is possible, but Melanie G. Murphy Richter, a registered dietitian nutritionist and director of communications for L-Nutra, the company that owns Prolon, warns it’s not a good idea.
“As much as I'd love to say it's possible to DIY, there is no way to ensure that a person's composition (of nutrients) is precise,” Richter says. “Our FMD is based on 25 years of diligent research and specially formulated ingredients. It's not something that can be perfectly replicated, and I think it's important to call that out.”
Wroe also cautions that on your own, there’s a greater risk of failing to meet your nutrient needs. It may also be more difficult to safely enter ketosis - when the body burns fat for fuel - because if your ratios of macronutrients aren’t exactly right, the diet may not be as effective as if they were precise. These two potential problems are less likely to develop when using the tested and prepackaged program, she says.
Despite those warnings, if you’re still determined to try a fasting-mimicking diet on your own, the rules are fairly straightforward. You’ll be tricking your body into thinking you’re fasting by removing most of the calories you’d typically eat each day.
The general idea on the fasting-mimicking diet is to consume fewer calories on fasting days and to adhere to a ratio of macronutrients that, as mentioned, are roughly 10% protein, 45% fat and 45% carbohydrates. You’ll also need to restrict high-glycemic ingredients, Wroe says. High-glycemic foods are those that rapidly elevate blood sugar.