The centuries-old healing tradition of intermittent fasting is experiencing something of a renaissance, thanks to Jason Fung, MD, a highly respected nephrologist and director of the Intensive Dietary Management Clinic in Toronto, Canada.  Since the publication of his seminal work, The Obesity Code, and its highly reviewed companion, The Diabetes Code, the concept of fasting as a means to improve and regulate hormonal and metabolic functions related to calorie intake is getting widespread media attention.  Both medical and research communities continue to examine Dr. Fung’s work with great interest and optimism.  In the words of Timothy Noakes, MD, Ph.D., “Its strengths are that it is based on irrefutable biology, the evidence for which is carefully presented; and it is written with the ease and confidence of a master communicator, in an accessible, well-reasoned sequence so that it’s consecutive chapters systematically develop, layer by layer, an evidence-based biological model of obesity that makes complete sense in its logical simplicity.  It includes just enough science to convince the skeptical scientist, but not so much that it confuses those without a background in biology.”  The clinical historical evidence and accumulated research on fasting as a health-promoting practice are quite extensive.  Human beings, including other primates, have fasted since time immemorial.  Simply put, it’s as old as our species!  Every culture, ethnicity, and religious persuasion has at one time practiced or continues to practice intermittent fasting for health and spiritual reasons.  Let’s examine what fasting is, its safety, and how it might allow an individual with excess body fat to effectively lose weight.

There’s plenty of confusion and misunderstanding about fasting for health purposes, so let’s first examine what fasting is NOT, and some of the fundamental myths that continue to persist about its practice.  Myth #1: fasting is starvation.  This couldn’t be true since starvation is always involuntary and usually results in death or serious illness.  Fasting is the voluntary and deliberate act of temporarily withholding calorie intake for health or spiritual reasons.  Myth #2: fasting will cause you to lose muscle tissue and strength.  Human beings are perfectly adapted to survive periods of no food intake since we can store energy as fat.  Our bodies don’t start using muscle tissue as an energy source until body fat percentage drops below 4%.  To give you a perspective, elite marathon runners typically have a body fat percentage of 8%.  Myth #3: fasting causes low blood sugar.  If you have a pre-existing medical condition like diabetes, clinical hypoglycemia, or are taking certain medications then you would need to talk to your doctor before starting fasting, but the majority of people will not experience a drop in blood sugar.  Your body has mechanisms that tightly monitor and control blood glucose within a narrow range.  When energy is low your body will first break down glycogen (stored glucose) and use it as energy.  When that is gone your body will then turn stored body fat into energy-this happens every time you sleep at night!  Myth #4:  fasting results in overeating.  While it’s true that some people do consume more calories during the first meal period after a fast, it’s not true that you over-indulge in the hours and days following a fast and erase any weight loss benefit that fasting may produce.  Myth #5: fasting deprives the body of nutrients.  During the fasting period, you abstain from consuming macronutrients-protein, fat, and carbohydrates.  Your body will preserve essential fats by reducing their excretion and recycling existing amino acids to form new proteins. There are no essential carbohydrates for human health.  For longer fasting periods you can take a multivitamin supplement to avoid any water-soluble micro-nutrient deficiencies.  At any given time, your body is either in the fed state or fasting state, so fasting is the voluntary act of abstaining from calorie intake for a period of time that usually exceeds the fed state.  A short fast is considered any time under 24 hours (16, 20, 24), while a long fast usually lasts over 24 hours.  For our ancestors, fasting was a normal part of daily living since food was often scarce and it was uncertain when or where the next meal would be available.  It was not uncommon for early humans to go weeks without eating.  If fasting is unhealthy, and humans depended on glucose from carbohydrates to meet basic energy requirements, then our species would never have survived.  Our ancestors simply burned stored body fat as an energy source; this is a normal metabolic function that is highly adaptogenic.  Aside from pregnant or breastfeeding women, children, and those with a particular medical condition, there is not a single person who absolutely needs to eat three meals a day.  In fact, a great way to gain weight is to consume 5-6 meals a day, which is a popular recommendation promoted by many diet experts, sports nutritionists, and self-styled experts.

It can be said that every diet works and every diet fails.  How many of you have been on the diet rollercoaster, only to end up back where you started, overweight?  All diets, regardless of the type-Mediterranean, low-fat, low-carb, low-calorie-will produce some degree of weight loss in the short term, but after 6-12 months you experience a plateau, and body fat starts to creep back up.  Even Atkin’s and ketogenic diets, which have been shown to be the most effective for weight loss, can produce the same plateau and weight regain effect.  The Women’s Health Initiative study, one of the largest randomized controlled trials to date on calorie reduction as a means to weight loss, was a disappointment to researchers and the medical community.  The study followed over 50 thousand women for seven and half years.  Participants were split into two groups, the study group was put on a low-fat, low-calorie diet consisting of vegetables, fruits, and grains, with a reduction in daily calorie intake by 361.  Also, their exercise activity was increased 14%.  The control group was asked to eat their usual diet.  Researchers had predicted that the study group would lose 36 lbs./year (252 lbs. over 7.5 years).  When the study concluded the difference in weight between the study group and control group was less than 2 lbs.!  In fact, the average waist circumference in the study group increased from 35 to 35.4 inches.  Clearly, the “eat less, move more” strategy was a failure.  Do you ever wonder why there is no Biggest Loser reunion show?  During filming, contestants exercised several hours per day, six days a week in a military-style boot camp, and reduced their daily calorie intake by an average of 1200-1500.  Over a 30-week filming schedule, the average weight loss is 127 lbs., with an average drop in body fat from 49% to 28%-pretty impressive!  It makes sense that in a 2015 US News and World Report article the Biggest Loser diet ranked number three in popular weight-loss diets.  As we would expect, however, these results were not sustainable.  Six years after filming thirteen of the fourteen contestant’s studies had regained the weight back; this is a failure rate of 93%!  If you do a literature search, you will find many well-designed studies that confirm this result-the “eat less, move more” campaign and the “calories in, calories out” model of dieting are false advertising, they don’t work!  Since the 1970’s Americans have been following the government’s nutritional guidelines and have reduced their intake of red meat, eggs, and animal fat, while at the same time increasing their consumption of vegetables, fruit, and grains.  Eating more fruits and vegetables is certainly a good thing, however, eating more grains is disastrous!  According to current estimates, 65% of the typical American diet consists of highly refined, grain-based carbohydrates.  It shouldn’t surprise anyone that over the last 20 years’ obesity rates have exploded.  In 2015, the Centers for Disease Control and Prevention reported that no state had an obesity rate below 20%!

Dieting and exercise fail as a strategy for long-term weight loss because of one simple reason, they don’t address the hormone insulin and the cycle of insulin resistance that occurs when you gain weight.  According to Dr. Fung, “The inability of most diets to reduce insulin resistance is exactly why they eventually result in weight regain. Fasting, on the other hand, introduces prolonged periods of low insulin levels, which breaks the cycle of high insulin and insulin resistance. Another way to look at this is that most diets ignore the biological principle of homeostasis. The body adapts to changing environments. For example, if you are in a dark room and suddenly go into the bright sunlight, you will momentarily be blinded, but within moments your eyes will adapt. The same applies to weight loss. If you maintain a constant reduced-calorie diet, the body will quickly adapt to it. Energy expenditure (metabolism) declines to match the reduced intake. Weight plateaus then is regained. This is not because you have stopped your diet but because your body has now adapted to it.” Our bodies only have two energy sources-glucose and fat-and we derive those from protein, fat, and carbohydrates.  Our bodies can’t store protein, but excess intake is converted to glucose and stored as glycogen in the liver.  Fat is absorbed directly from the intestine into the bloodstream, and any excess is stored as body fat.  Carbohydrates are turned into glucose and stored in the liver and muscle as glycogen.  When glycogen stores are full the excess glucose from carbohydrates is converted to fat in the liver through a process called de-novo lipogenesis, and stored as body fat.  Since our bodies can only use two sources of energy, imagine that we have two storage compartments, one for glucose and one for fat.  We don’t burn glucose and fat at the same time, so when glucose is readily available, we will burn that preferentially first before accessing body fat.  The hormone insulin is the prime determinant of whether we burn sugar or fat as a primary fuel source.  Insulin is the only fat-storage hormone we have and is directly associated with body fat accumulation, thus insulin is the primary driver of weight gain.

This is how the weight gain cycle works:  every time you eat food-protein, fat, and carbohydrates-your pancreas produces insulin.  Insulin allows our cells to use the glucose from food as an energy source.  When energy needs are met our bodies store the excess glucose as glycogen.  When glycogen stores are full, our bodies turn the excess glucose to fat, and that is stored in fat cells throughout our bodies. This process happens multiple times a day, every day, every week, and every month for many years.  The more insulin we produce the more resistant we become to its effects, and the more resistant we become the more insulin we produce, leading to a vicious cycle of high circulating insulin (hyperinsulinemia) and insulin resistance.  This is how we get fat-our bodies are always burning sugar because we are always eating and stimulating an insulin response!

Frequent, successive intermittent fasting can help break the cycle of high insulin and insulin resistance and allow you to access and burn stored body fat.  When you fast your body does not produce insulin.  In the absence of insulin activity, your body will first use any available glucose as energy, including glycogen which is liberated and burned as glucose.  When your glycogen stores are used up your body will start accessing fat and turning it into energy.  Simply put, this is how you lose weight-your body is living off its fat stores.  Over time, insulin levels return to normal and remain low, any cellular resistance to insulin diminishes, leptin signaling and activity increase which helps control appetite and overeating, and body fat percentage begins to drop.  As Dr. Fung explains, “Caloric-reduction strategies like “eat less, move more” fail virtually every single time, yet fasting is often effective when simple caloric reduction is not. Why? The short answer is that when you’re eating regularly, even if you’re eating fewer calories, you’re not getting the beneficial hormonal changes of fasting. During fasting, unlike during caloric reduction, metabolism stabilizes or even goes up to maintain normal energy levels. Adrenaline and growth hormone increase to maintain energy and muscle mass. Blood sugar and insulin levels go down as the body changes from burning sugar to burning fat. All this begins to address the long-term problem of insulin resistance.”

If you want to learn more about the health benefits of fasting I highly recommend reading Dr. Fung’s book, The Obesity Code.  It’s important to remember that if you are overweight or obese this is a time-dependent condition, meaning it took many years to accumulate excess body fat.  The weight-loss benefits of intermittent fasting are also time-dependent, so you will need to be patient and make fasting a part of your weekly schedule.  There are certain individuals who should not practice intermittent fasting-pregnant or breastfeeding women, children, and those with certain medical conditions and/or are taking prescription medications.  If you’re unsure it’s a good idea to check with your healthcare provider before starting a fasting program.

Jason Fung. “The Complete Guide to Fasting.” iBooks. https://itunes.apple.com/us/book/the-complete-guide-to-fasting/id1165629417?mt=11

Michael Chase, MS, NTP

Nutrition Science and Dietetics