The Science Behind Fasting Diets

white plate with one pea on it
Place setting with handful of peanuts on white plate.

Have you heard of intermittent fasting? In recent years, weight loss diets touting some variation of intermittent fasting have trended on bookstore shelves and lifestyle blogs, so what is all the buzz about?

Intermittent fasting makes alluring promises of dramatic weight loss, reduced chronic disease risk, and even longevity. In exchange, devoted followers are charged with one small task, and small indeed it is: Consume a devastatingly small number of calories on one or more days of the week while eating as you normally would on other days of the week.

Deprivation Doesn’t Equal Fat Loss

One variation suggests fasting for two days and eating normally for five days, also known as the 5:2 approach. Advocates of intermittent fasting often explain that the secret to weight loss requires eating so few calories that you trick your body into fat-burning mode. Depending on the brand of intermittent fasting you follow, a “successful” fasting day may ask you to deprive yourself of anywhere from 1,000 to 2,000 calories below your recommended energy intake.

A popular interpretation dictates that fasting days exceed no more than 500 calories for females, 600 calories for males. Put another way, proponents of the diet may advise you to consume as low as 25 percent of your daily calorie recommendation on your fasting days. Twenty-five percent.

What is wrong with this picture? Aside from the foreseeable hunger pangs, cranky mood and poor cognitive performance you will likely encounter, this advice may actually put more distance between you and your goal of lasting weight loss. Intermittent fasting believers claim that your body decides to burn fat storage when faced with this extreme deprivation of calories. In reality, the body stockpiles the energy it does have on hand, such as stored fat, but gets rid of metabolically expensive lean tissue, such as skeletal muscle, in response to this calorie deficit crisis, making it harder for us to lose weight but easier for us to stay alive, a simple protective mechanism.

Intermittent fasting actually compels our body to degrade muscle proteins at a faster rate, chipping away at lean mass instead of fat mass, precisely the opposite of our desired effect. This diet unfortunately abandons the well-substantiated scientific finding that eating too few calories for our daily metabolic function results in a consistent response by the body to squirrel away and hoard energy, not burn it off. In fact, the diet only gets us as far as what is called the post-absorptive state or early fasting state, and that really matters. Proponents of this approach insist that the diet is accompanied by metabolic changes that simply do not occur under its prescribed conditions.

How are Fasters Losing Weight?

What, then, of the success stories and all of its followers? Given the extreme calorie restriction prescribed, this diet demands very low-calorie options for its fasting days. It is no wonder that fruits and vegetables easily top the list of low-calorie foods recommended for fasting days because they fill the stomach with their high fiber and water content in exchange for relatively few calories.

This upsurge in fruits and vegetables may well displace more calorically dense, nutrient-poor foods in their diet. This inadvertent substitution of nutrient-dense for calorie-dense foods, a typical reduction in sodium intake from eating fewer processed foods resulting in less water retention, and the loss of skeletal muscle and its accompanying carbohydrate and water storage, all helps to explain some of the weight loss that a number of people report on this diet.

This diet might also have an advantage in its ability to rekindle an individual’s awareness of hunger as a bodily sensation. On fasting days, perhaps you reach new depths of hunger that you would otherwise never allow yourself to reach, so your awareness of physical hunger is heightened on the days you eat normally. In a society where food availability is high and cues to eat and snack follow us everywhere we go, it is all too easy to eat in response to emotional and social cues more often than internal physiological cues.

And yet, even with these theorized advantages, intermittent undereating is still undereating. Unfortunately for intermittent fasting supporters, slim research exists to verify the claims of the diet. As of this writing, not one randomized controlled trial – long considered the gold standard in population health research – has been published to date, and the little evidence the diet does rely upon was based on studies that lacked a randomized study population, a control group against which to compare the intervention, or both, according to The American Journal of Clinical Nutrition.

Further, it is impossible to eat those 500 daily calories in a way that does not jeopardize your body’s ability to maintain critical functions and acquire enough dietary fat for cell membrane integrity or enough dietary glucose for vital brain function, for example. Most of us require a bare minimum of 1,200 calories to keep up with the many, many necessary processes that our magnificent bodies perform (even during sustained weight loss) on a daily basis.

Even in its most faithful adherence, intermittent fasting is shown to be no more of an effective weight loss strategy than run-of-the-mill calorie restriction, yet it carries the added burden of significantly increased appetite on both non-fasting and fasting days, according to an article in Molecular and Cellular Endocrinology.  

Eating healthy and feeding our families can be complicated enough, and such extreme caloric deficit and psychological deprivation can really interfere with a healthy and wholesome relationship with food. Extensive research reviews point to the frequent finding that these dieters do not eat sufficient amount or quality of calories to counteract the body’s stockpiling strategy discussed earlier. In other words, even if we manage to meet our depressingly low calorie target on fasting days, we do not do it right on the non-fasting days, so we suffer through those fasting days for nothing, and our body continues to protect our precious fat stores despite our best intentions.

Potential Dangers

Intermittent fasting diets may encourage continuation of physical activity on fasting days, despite the acknowledged caloric deficit on those days. Such undereating during physical activity, particularly if moderate or vigorous activity as many dieters will wish to do to speed their efforts, places them at risk for dangerously low blood sugar, dehydration, injury and compromised immune function.

It is also well acknowledged that the diet can be downright dangerous for several subpopulations, including pregnant women, people with type 1 diabetes, people with a history of disordered eating, and people who take one or more prescription medications, among others.

You may have heard of fasting as a therapeutic treatment for particular medical conditions, and this does occur – but under meticulous oversight and supervision by a physician. Despite the popularity of intermittent fasting in the diet industry, a lack of evidence exists to support its safety and efficacy as a weight loss diet for the masses, which is how intermittent fasting diets are routinely promoted.

How to Think Smart, Not Fast

Anything that asks you to turn your back on the tremendous edible bounty of the Northwest for any length of time, intermittent or otherwise, deserves a critical look. If you really want to tempt your body to shed excess fat, insist on getting a truly full night of sleep, as this is the only kind of fasting that is well demonstrated to aid lasting weight loss efforts, according to The American Journal of Clinical Nutrition.

Better yet, consider a “kitchen closed” rule for yourself where you take your last bite of food about two hours prior to bedtime. This can help you avoid late-night snacking that tends to be overly indulgent in calories anyway, and this simple rule may lead you to feel more refreshed and alert the next morning, since your body wasn’t working double-duty to digest your midnight snack and perform important overnight repair and recovery processes.

Spare yourself the increased muscle protein degradation, psychological deprivation, and destructive hormonal changes that can accompany extreme approaches like intermittent fasting. Prioritize high-quality sleep instead. Think smart, not fast.

By Lora Silver, a Seattle-based writer, recipe developer, and graduate student in the Master of Science in Nutrition program at Bastyr University. When she is not in class learning cutting-edge, whole foods nutrition science, she playfully experiments with recipes in her kitchen and explores the Seattle area with her wonderful husband, who fortunately shares her love of whole foods. Reviewed by Cristen Harris, PhD, associate professor at Bastyr University.