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We live in a world where food is accessible all the time, in all settings, and on-demand. Unfortunately, despite all that we know about how to prevent many lifestyle diseases, their numbers remain high. Heart disease, diabetes and Alzheimer’s remain among the top 10 causes of death in the United States. Fad diets offer quick-fix solutions for weight loss that are often not sustainable. What about long-term health outcomes?

In 1962, James Neel developed the concept of “thrifty genes” to explain how humans have the ability to survive through calorie extremes like feast and famine; that our bodies are highly effective at storing extra fuel as fat. One could make the argument that fasting has been part of lives throughout history. Fasting is still an integral part of five major religions across the world.

It’s only recently that the popularity of fasting has resurfaced among the health-conscious as a way to drop weight or kickstart some healthy new lifestyle habits. With the majority of Americans trying different forms of fasting, from intermittent fasting to time-restricted eating, and prolonged fasting diets, confusion can lead to questions that eventually end up in a doctor’s office.

Fasting is the abstention of calories from food and beverages. From a biological standpoint, fasting entails not eating foods that trigger the nutrient “sensing” pathways. Since that can be active for 24 hours after the last meal, biological fasting really begins 24 hours after the last meal. Some people think a juice fast is a fast because you are not chewing your calories, but the calories—not the state of matter—are what count.

Why fast? Some people do it for healing or spiritual purposes. Research shows that fasting can support overall metabolic health, supports cellular cleanup (autophagy) that leads to cellular regeneration, and increases in circulating stem cells.1 Also, fasting impacts markers and risk factors for aging. There are three main types of fasting: intermittent, time-restricted eating/feeding, and prolonged fasting.

During intermittent fasting, a person will refrain from consuming calories during a full day. It can be done anywhere from one to three, non-consecutive days per week. There have even been several articles and books published around the 5:2 plan. Time restricted eating is a daily pattern of calorie consumption between a window of 8–12 hours each day.

Prolonged fasting is usually done for five consecutive days. This has traditionally been done using water only. Water-only prolonged fasting demonstrates those positive biochemical markers; however, it has a high dropout rate among participants and it can be dangerous because it deprives the body of macronutrients, leads to muscle wasting and increases the risk of gallstones.

Prolonged water fasting is also challenging to do. It was this hurdle that led Dr. Valter Longo to develop a “fasting with food” concept now known as the Fasting Mimicking Diet, which he details in his book, “The Longevity Diet.” The Fasting Mimicking Diet gives you the same health benefits of prolonged water fasting, but it’s more sustainable to do over the recommended five days because the stomach sees food, while the cells see fasting. You will likely drop weight with this diet, but the other health and longevity benefits are so profound that TIME magazine named Longo as one of the 50 most influential people who are transforming health care. Benefits of prolong fasting include protection of lean body mass, maintaining healthy levels of metabolic markers and increased circulation of stem cells.2 Currently, the only Fasting Mimicking Diet available is called ProLon which has been clinically shown to result in an average of 5–8 pounds of fat loss without losing lean body mass in the process.

Here what happens on each day on the ProLon Fasting Mimicking Diet:

Day 1: Fasting State: the body is being primed to transition into the fasting state for cellular optimization;

Day 2: Fat Burning: the body, depleted of glycogen, switches to fat burning mode; cellular recycling and clean up begin;

Day 3: Cellular Cleaning: cellular clean up continues and most people reach full ketosis;

Day 4: Cellular Regeneration: cellular clean up continues and enhanced stem cell production begins; and

Day 5: Regeneration Continues: promotes self-repair through the increased circulation of stem cells.

The results have been fascinating: Reductions in circumferential body fat from 5–8 pounds in five days, preservation of lean mass (muscle mass and bone density), and decreases in the IGF-1 hormone, which has been implicated in aging and disease. In those subjects who returned to normal eating yet did the ProLon Fasting Mimicking Diet for three consecutive months (for five days per month), they were able to realize the following health benefits: weight loss, decreased waist circumference, maintained lean muscle mass and improved metabolic measures.

So now you must be wondering, what exactly are the foods that are consumed using ProLon Fasting Mimicking Diet? First of all, the entire program must be purchased online at: because the meal plan is specific and provides scientifically researched micro- and macro-nutrients in precise quantities and combinations that nourish you, but are not recognized as food by your body and therefore mimics a fasting state. This way there is no guessing or question of incorrect portions because all the food needed for the five days comes in assigned boxes for each day. It is a plant-based eating plan that consists of bars, soups, crackers, olives, and herbal teas. Just add water and you are good to go.

Technically, individuals can return to their usual "diet" in the weeks between doing the Fasting Mimicking Diet. As a clinician who attended the first global fasting summit at the University of Southern California, and has been doing some consulting work with L-Nutra, the company behind the ProLon Fasting Mimicking Diet, I can tell you I plan on using this in my practice, and for myself. I would encourage people to take a good look at their calorie intake—both in foods and beverages. I am an advocate of eating flexitarian, which is predominantly plant-based with small amounts of animal-sourced protein. I would pair all of this with time-restricted eating as a means of regular eating behavior. Be mindful of how many hours you eat each day, and what your calories consist of.

Right now, high protein diets are popular because they encourage weight loss and satiety, but in looking at long-term health outcomes, the research has shown that lower protein consumption is actually better for longevity. We know the benefits of eating more plants are that they promote a healthy gut microbiome and that the phytonutrients in plants have many chemoprotective qualities.

I'm looking forward to the continued research in the fasting space, and where it will take us in the future of healthcare. We are truly at an impasse when it comes to obesity and disease in this country. As health care professionals, we have to get to the root cause of obesity, which is essentially what we are eating or not eating.


  1. Wei et al., Sci. Transl. Med. 9, 15 February 2017
  2. Ibid

Felicia Stoler, DCN, MS, RDN

Living Skinny in Fat Genes Felicia Stoler

Felicia Stoler, DCN, MS, RDN, FACSM, FAND, known as America’s Health & Wellness Expert™ is a registered dietitian nutritionist, exercise physiologist and expert consultant in disease prevention, wellness and healthful living. She is Board Certified in Lifestyle Medicine and is a Diplomate of the ABLM/ACLM. Stoler earned her Master of Science degree in Applied Physiology and Nutrition from Columbia University and her Doctor-ate in Clinical Nutrition from Rutgers School of Health Professions. She completed her residencies at Rutgers University Athletics and ABC News Medical Unit. She has served as a part-time lecturer at Rutgers University where she teaches exercise physiology, nutrition and communications courses. Stoler has extensive media experience and hosted the second season of TLC’s groundbreaking series, “Honey, We’re Killing the Kids!” which targeted unhealthy lifestyles of families, across the country, in an effort to motivate them to make positive changes. She is the author of, “Living Skinny in Fat Genes™: The Healthy Way to Lose Weight and Feel Great” (Pegasus, 2010), featured as a “must-have” book in USA Weekend. Stoler authored the American College of Sports Medicine’s “Current Comment on Childhood Obesity.”