Have you ever wondered if there’s a natural way to lower your high blood pressure, guard against Alzheimer's, lose weight, and feel better? Well as it turns out there is. Michael Greger, M.D. FACLM, founder of NutritionFacts.org, and author of the instant New York Times bestseller “How Not to Die” celebrates evidence-based nutrition to add years to our life and life to our years.

Intermittent Fasting Part 3

Today we feature part 3 of our series on the effects of intermittent fasting as we ask the question is fasting safe?

This episode features audio from Is Fasting Beneficial for Weight Loss?, Is Fasting for Weight Loss Safe?, and Alternate-Day Intermittent Fasting Put to the Test. Visit the video pages for all sources and doctor’s notes related to this podcast.


Have you ever noticed that every month seems to bring a trendy new diet? And yet obesity rates continue to rise and with it a growing number of health problems. That’s why I wrote my new book How Not to Diet. Check it out at your local public library. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger. 

Today we feature part 3 of our series on fasting starting with the question is fasting beneficial for weight loss? Let’s find out.

Fasting obviously causes consistent, dramatic weight loss, but how do fasted individuals do long-term? Some research groups reported extremely disappointing results. Here’s what they saw. Average subject started out at about 270 pounds and in the six months before the fast, continued to gain weight as obese persons tend to do. Then, after 24 days of what they called “inpatient starvation,” a dramatic 27-pound weight loss. Then what do you think happened? They gained it all back and more.

In another study with follow-ups ranging up to 50 months only 4 out of 25 so-called “superobese” patients achieved even partial sustained success. Based on these kinds of data, some investigators concluded that “complete starvation is of no value in the long-term treatment of obese patients.”

Other research teams reported better outcomes. One series of about 100 individuals found that 60 percent either retained at least some weight loss at follow-up or even continued losing. The follow-up periods varied from 1 to 32 months, with no breakdown as to who fasted how long, though making the data hard to interpret. One year after fasting, 62 patients down 16 pounds in 10 days. In another study, 40 percent retained at least 7 pounds of that weight loss.

Put six such studies together, and hundreds of obese subjects fasted for an average of 44 days, lost an average of 52 pounds, and around one or two years later, 40 percent retained at least some of the weight loss. So, most gained all their weight back, but 40 percent is extraordinary for a weight-loss study. Following 100 obese individuals getting treated at a weight loss clinic with a standard low-calorie diet, researchers found only 1 out of 100 lost more than 40 pounds, and only about 1 in 10 even lost 20 pounds, with the overall successful weight maintenance at only two patients over two years. That’s why having a control group is so important. What may look like a general failure may actually be a relative success, compared to more traditional weight loss techniques.

Researchers new to the field may find it clearly disappointing that a year later, two-thirds were “failures,” with more than a third regaining all the weight they had initially lost. But 12 percent were labeled successes, maintaining 59 pounds of weight loss two years later. They lost massive amounts of excess weight and kept it off. In a direct comparison of different weight loss approaches at the same clinic, five years after initiating a conventional low-calorie approach, only about one in five was down 20 pounds, compared to nearly half in the group who instead had undergone a few weeks of fasting years previously. By year seven, most of those instructed on daily caloric restriction were back to, or had exceeded, their original weight, but that was only true for about one in ten of the fasted group. In an influential paper in the New England Journal of Medicine on seven myths about obesity, fallacy #3 was that “Large, rapid weight loss is associated with poorer long-term weight-loss outcomes, as compared with slow, gradual weight loss.” In reality, the opposite is true. The hare may end up skinnier than the turtle.

Researchers set up a study comparing the sustainability of weight loss at three different speeds: six days of fasting versus three weeks of a very-low-calorie diet, 600 calories a day, or six weeks of a low-calorie diet, 1,200 calories a day. The question is, what happened a year later? A year later, the fasting group was the only one that sustained a significant loss of weight. That was just one year, though; how about nine years later?

This is the largest, longest follow-up study I could find. At least some of the fast-induced weight losses were maintained a year later by the great majority. A year later, 90 percent remained lighter than they started out, but then two years later, three years, four years, seven years, and by nine years later, that number dropped to fewer than 1 in 10. By then, almost everyone had regained the weight they had initially fasted away. Many patients reported they thought the temporary loss was worth it, though. As a group, they lost an average of about 60 pounds. They described improved health and quality of life, claiming re-employment was facilitated, and earnings increased during that period of time. But the fasting didn’t appear to result in any permanent change in eating habits for the vast majority.

The small minority for which fasting led to sustainable weight loss “all admit to a radical change in previous eating habits.” Fasting only works long-term if it can act as a jumpstart to a healthier diet. In a retrospective long-term comparison of weight reduction after an inpatient stay at a naturopathic center, those who were fasted lost more weight at the time, but at around seven years were back to the same weight. No surprise, since they reported returning to the same diet they were on before. Those who were instead placed on a healthier, more whole food plant-based diet were more likely to make persistent changes in their diet, and seven years later were lighter than when they started. Why can’t you have it both ways, though? You could use fasting to kickstart a big drop, and then start a healthier diet. The problem is that big drop is largely illusory.

Fasting for a week or two can cause more weight loss than calorie restriction, but paradoxically, it may actually lead to less loss of body fat. Wait, how can eating fewer calories lead to less fat loss? Because during fasting, your body starts cannibalizing itself and burning more of your own protein for fuel. Emperor penguins, elephant seals, and hibernating bears can survive just burning fat without dipping into their muscles, but our voracious big brains appear to need at least a trickle of blood sugar, and if we’re not eating any carbohydrates, our body is forced to start turning our protein into sugar to burn. Even just a few grams of carbs, like people who add honey to their water when they fast, can cut protein loss up to 50 percent.

What about adding exercise to prevent loss of lean tissues during a fast? It may make it worse! At rest, most of your heart and muscle energy needs can be met with fat, but if you start exercising, they start grabbing some of the blood sugar meant for your brain, and your body may have to break down even more protein.

Less than half of the weight loss during the first few weeks of fasting ends up coming from your fat stores. So, even if you double your daily weight loss on a fast, you may be actually losing less body fat. An NIH-funded study placed obese individuals on an 800- calorie-a-day diet for two weeks, and they steadily lost about a pound of body fat a day. Then they switched them to about two weeks of zero calories, and they started losing more protein and water, but, on average, only lost a few ounces of fat a day. When they were subsequently switched back to the initial 800-calories-a-day for a week, they rapidly replaced the protein and water. And so, the scale registered their weight going up, but their body fat loss accelerated back to the approximate pound a day. The scale made it look as though they were doing better when they were completely fasting, but the reality is they were doing worse. So, during the five-week experiment, they would have lost even more body fat sticking to their calorie-restricted diet than completely stopping eating in the middle. They would have lost more body fat, eating more calories. Fasting for a week or two can interfere with the loss of body fat, rather than accelerate it.

Here’s a good question. Is fasting for weight loss safe? Here are some answers.

Although fasting for a week or two can actually interfere with the loss of body fat, eventually, after the third week of fasting, fat loss starts to overtake the loss of lean body mass in obese individuals. But is it safe to go that long without food?

Proponents speak of fasting as a cleansing process, but some of what they are purging from their body are essential vitamins and minerals. Heavy-enough people can go up to 382 days without calories, but no one can go even a fraction of that long without vitamins. Scurvy, for example, is diagnosable within as few as four weeks without any vitamin C. Beriberi, thiamine (vitamin B1) deficiency, may start out even earlier in fasting patients, and once it manifests can result in brain damage within days which can eventually become irreversible.

Even though fasting patients report problems such as nausea and indigestion taking supplements, all of the months-long fasting cases I’ve mentioned were given daily multivitamins and mineral supplementation as necessary. Without supplementation, hunger strikers, and those undergoing prolonged fasts for therapeutic or religious purposes (like the Baptist pastor hoping to “enhance his spiritual powers for exorcism”) have ended up paralyzed, comatose, or worse.

Nutrient deficiencies aren’t the only risk. Reading about all the successful reports of massive weight loss from prolonged fasting in the medical literature, one doctor decided to give it a try with his patients. Of the first dozen he tried it on, though, two died. Now, in retrospect, both the two patients that died started out with heart failure and were on diuretics. Fasting itself produces a pronounced diuresis, meaning loss of water and electrolytes through the urine. And so, it was the combination of fasting on top of the water pills that likely depleted their potassium and triggered their fatal heart rhythms. The doctor went out of his way to point out that both of those who died started out in severe heart failure, and “both had improved greatly while undergoing starvation therapy.” Small consolation, since they were both dead within a matter of weeks.

This line of treatment is certainly tempting because it does produce weight-loss, and the patient feels so much better, but the whole killing people thing must make it a very suspect line of management.”

Contrary to the popular notion that the heart muscle is specially spared during fasting, the heart appears to experience similar muscle wasting. This was noted in the victims of the Warsaw ghetto during World War II in a remarkable series of detailed studies carried out by the ghetto physicians themselves, before they themselves succumbed. In a case entitled “Gross Fragmentation Of Cardiac Fiber After Therapeutic Starvation For Obesity,” a 20-year-old woman successfully achieved her ideal body weight after losing 128 pounds fasting for 30 weeks. After a breakfast of one egg, she had a heart attack and died. On autopsy, the muscle fibers in her heart showed evidence of widespread disintegration. The pathologists suggested that fasting regimens “should no longer be recommended as a safe means of weight reduction.”

Breaking the fast appears to be the most dangerous part. After World War II, as many as one out of five starved Japanese prisoners of war tragically died following liberation. Now known as “refeeding syndrome,” multiorgan system failure can result from resuming a regular diet too quickly. See, there are critical nutrients, such as thiamine and phosphorus, that are used to metabolize food. So, in the critical refeeding window, if too much food is taken before these nutrients can be replenished, demand may exceed supply. And whatever residual stores you still carry can be driven down even further, with potentially fatal consequences. That’s why rescue workers are taught to always give thiamine before food to victims who had been trapped or otherwise unable to eat. Thiamine is responsible for the yellow color of “banana bags,” a term you might have heard used on medical dramas, used to describe an IV fluid concoction often given to malnourished alcoholics to prevent a similar reaction. Anyone “with negligible food intake for more than five days” may be at risk of developing refeeding problems.

Medically-supervised fasting has gotten much safer now that there are proper refeeding protocols, we know what warning signs to look for, and we now know who shouldn’t be fasting in the first place (such as those with advanced liver or kidney failure, porphyria, uncontrolled hyperthyroidism, and pregnant and breastfeeding women). The most comprehensive safety analysis of medically-supervised, water-only fasting was recently published out of the True North Health Center in California. Out of 768 visits to their facility for fasts up to 41 days, were there any adverse events? Yes, 5,961 of them! But most were mild known reactions to fasting, such as fatigue, nausea, insomnia, headache, dizziness, upset stomach, and back pain. They report only two serious events, and no fatalities.

Fasting longer than 24 hours, and particularly three or more days, should only be done under the supervision of a physician and preferably in a live-in clinic. In other words, don’t try this at home. This is not just a legalistic mumbo-jumbo. For example, normally your kidneys dive into sodium conservation mode during fasting, but should that response break down, you could rapidly develop an electrolyte abnormality that may only manifest with non-specific symptoms like fatigue or dizziness, which could easily be dismissed until it’s too late.

The risks of any therapy must be premised on the severity of the disease. The consequences of obesity are considered so serious that effective therapies could have “considerable acceptable toxicity.” For example, many consider major surgery for obesity to be a justifiable risk. But the key word is effective.

Therapeutic fasting for obesity has been largely abandoned by the medical community not only because of its uncertain safety profile, but its questionable short-term and long-term efficacy. Remember, for a fast that only lasts a week or two, you might be able to lose as much body fat or even more on a low-calorie diet, than a no-calorie diet.

Finally today, we put alternate day intermittent fasting to the test.

Rather than cutting calories day in and day out, what if you instead just ate as much as you wanted every other day? Or for only a few hours a day? Or fasted two days a week? Or five days a month? These are all examples of intermittent fasting regimens. That may even be the way we were built. Three meals a day may be a relatively novel behavior for our species. For millennia our ancestors may have only “consumed only one large meal a day or went several days at a time without food.”

Intermittent fasting is often presented as a means of stressing your body in a good way. There is a concept in biology called hormesis, which can be thought of as the that-which-doesn’t-kill-you-makes-you-stronger principle. Exercise is the classic example, where you put stress on your heart and muscles, and as long as there’s sufficient recovery time, you are all the healthier for it. Is that the case with intermittent fasting? Mark Twain thought it was: ‘‘A little starvation can do more for the average sick man than can the best medicines and the best doctors. Not just a restricted diet, but total abstention from food for one or two days.’’

But Twain also said, “Many a small thing has been made large by the right kind of advertising.” Is the craze over intermittent fasting just hype? Many diet fads have their roots in legitimate science, but over time, facts can get distorted, benefits exaggerated, and risks downplayed. In other words, “Science takes a back seat to marketing.” At the same time, you don’t want to lose out on any potential benefit by dismissing something out of hand based on the absurdist claims of overzealous promoters. You don’t want to throw the baby out with the baby fat.

Religious fasting is the most studied form of intermittent fasting, specifically Ramadan, a month-long period in which devout Muslims abstain from food and drink from sunrise to sunset. The effects are complicated by a change in sleeping patterns, and also thirst. The same dehydration issue arises with Yom Kippur, when observant Jews stop eating and drinking for about 25 hours. The most studied form of intermittent fasting that deals only with food restriction is alternate-day fasting, which involves eating every other day, alternating with days consuming little or no calories.

At rest, we burn about a 50:50 mix of carbs and fat; but, we run usually out of our glycogen, our carbohydrate stores within 12 and 36 hours of stopping eating. At that point, our body has to shift to rely more on our fat stores. This “metabolic switch” may help explain why the greatest rate of breakdown and burning of fat over a three-day fast happens between the hours of 18 and 24 of the 72-hour period. The hope is to reap some of the benefits of taking a break from eating without the risks of prolonged fasting.

One of the potential benefits of alternate-day fasting over chronic calorie restriction is that you get regular breaks from feeling constant hunger. But might people becomes so famished on their fasting day that they turn the next into a feasting day, and overeat? If you ate more than twice as much as you normally would, then that presumably would defeat the whole point of alternate-day fasting. Mice fed every other day don’t lose weight. They just eat roughly twice as much in one day than non-fasted mice would regularly eat in two. That is not, however, what happens in people.

Randomized to fast from 8 p.m. the night before to 8 a.m. the next day, fasting for 36 hours only led to people eating an average of 20 percent more the day after they broke the fast, compared to a control group that didn’t fast at all. That would leave them with a large calorie deficit, equivalent to a daily calorie restriction of nearly a thousand calories a day. This particular study involved lean men and women, but similar results have been found amongst overweight or obese subjects, typically only about a 10 to 25 percent compensatory increase in calorie intake over baseline on non-fasting days. And this seems to be the case whether the fasting day was a true zero calorie fast or a few-hundred calorie so-called “modified fast” day, which may lead to better compliance.

Some studies found study subjects appeared to eat no more or even less on days after a day-long mini-fast. Even within studies, great variability is reported. In a 24-hour fasting study, where folks ate an early dinner and then had a late dinner the next day after skipping breakfast and lunch, the degree of compensation at the second dinner ranged from 7 percent to 110 percent. This means some got so hungry by the time supper rolled around that they ate more than 24 hours’ worth of calories in a single meal. The researchers suggested that perhaps people first try “test fasts” to see how much their hunger and subsequent intake ramps up before considering an intermittent fasting regimen. Hunger levels can change over time, though, dissipating as your body habituates to the new normal.

In an eight-week study in which obese subjects were restricted to about 500 calories every other day, after approximately two weeks, they reportedly started feeling very little hunger on their slashed calorie days. This no doubt helped them lose about a dozen pounds, on average, over the duration of the study, but there was no control group with which to compare. A similar study with a control group found a similar amount of weight loss. About ten pounds over twelve weeks in a group of normal weight (meaning overweight, on average) individuals. For these modified regimens where people are getting prescribed 500 calories on their quote-unquote “fasting” days, researchers found that from a weight loss perspective, it did not appear to matter whether those calories are divided up throughout the day or eaten in a single meal.

Instead of prescribing a set number of calories on “fasting” days, which many people find difficult to calculate outside of a study setting, a pair of Iranian researchers instead came upon a brilliant idea of unlimited above-ground vegetables. Starchy root vegetables are relatively calorie-dense, compared to other vegetables. But vegetables that grow above the ground, including stem vegetables like celery and rhubarb, flowering vegetables like cauliflower, leafy vegetables like, well, leafy vegetables, and then all the fruits we tend to think of as vegetables, like tomatoes, peppers, okra, eggplant, string beans, summer squash, and zucchini. So, instead of just prescribing a certain number of calories for their “fasting” days, subjects alternated between their regular diet and helping themselves every other day to an all-you-can-eat above-ground vegetable feast (along with naturally non-caloric beverages, like green tea or black coffee). After eight weeks, subjects lost an average of 13 pounds and two inches off their waist.

The same variability discovered for calorie compensation has also been found for weight loss, though. In a twelve-month trial in which subjects were instructed to eat only one-quarter of their calorie needs every other day, weight changes varied from a loss of about 37 pounds to a gain of about 8 pounds. The biggest factor differentiating the low-weight-loss group from the high-weight-loss group appeared to be not how much they feasted on their regular diet days, but how much they were able to comply on their fast days with the calorie restriction.

Overall, ten out of ten alternate-day fasting studies showed significant reductions in body fat. Small short-term studies show about a 4 to 8 percent drop in body weight after 3 to 12 weeks. How does that compare with continuous calorie restriction? Zero-calorie alternate-day fasting was compared head-to-head to a daily 400-calorie restriction for eight weeks. Both groups lost the same amount of weight (about 17 pounds), and in the follow-up check-in six months later, after the trial ended, both groups had maintained a similar degree of weight loss (still both down about a dozen pounds).

The hope that intermittent fasting would somehow avoid the metabolic adaptations that slow weight loss or improve compliance don’t seem to have materialized. The same compensatory reactions in terms of increased appetite and a slower metabolism plague both continuous and intermittent caloric restriction. And the largest, longest trial of alternate-day fasting found that it may even be less sustainable than more traditional approaches. By the end of a year, the drop-out rate of the alternate-day fasting group was 38 percent, compared to 29 percent in the continuous calorie-restriction group.

We would love it if you could share with us your stories about reinventing your health through evidence-based nutrition. Go to NutritionFacts.org/testimonials. We may share it on our social media to help inspire others.

To see any graphs charts, graphics, images or studies mentioned here, please go to the Nutrition Facts Podcast landing page. There you’ll find all the detailed information you need plus links to all of the sources we cite for each of these topics.

For recipes, check out my How Not to Diet Cookbook. It’s beautifully designed, with more than 100 recipes for delicious and nutritious meals. And all proceeds I receive from the sales of my books goes to charity.

NutritionFacts.org is a nonprofit, science-based public service, where you can sign up for free daily updates on the latest in nutrition research via bite-sized videos and articles.

Everything on the website is free. There’s no ads, no corporate sponsorship. It’s strictly non-commercial. I’m not selling anything. I just put it up as a public service, as a labor of love, as a tribute to my grandmother whose own life was saved with evidence-based nutrition.  Thanks for listening to Nutrition Facts. I’m your host, Dr. Michael Greger.

Pin It on Pinterest

Share This