Fasting: Is it Healthy or Effective?

6 min read  |  August 01, 2018  | 

Fasting sounds like the easiest and fastest way of getting rid of a few stubborn pounds, which may explain why this dieting method is exploding in popularity, its wonders lauded by celebrities, its many varieties detailed in magazines.

But not all fasting is created equal and the science backing some public claims is sketchy.

“I have mixed opinions [based on scientific studies],” says University of Miami Health System endocrinologist Dr. Gianluca Iacobellis. “We don’t even have a consensus about what exactly fasting is.”

Part of the problem is that consumers and diet-preneurs often use the word loosely, and in some cases it means not a total caloric restriction but merely a simple reduction in calories. True fasting is abstinence from all food — and, in some cases, all fluids — for a defined period, be it a few hours (before surgery, for example), a full day (often for religious reasons) or for several days (sometimes for protest).

But in most fasting diets, the restrictions tend to be partial. Particular foods may be limited, for example. Or, no eating is allowed during certain hours, but drinking water is okay.

While the practice of fasting is a centuries-old tradition in most world religions, fasting as a way to diet and detox has become popular in the U.S. more recently. In fact, it’s believed to be the most widely adopted diet by young Americans these days.

Fasting diet trends

Alternate day fasting (ADF) is the most widespread variation, Dr. Iacobellis says.

ADF, however, is a misnomer.

ADF participants do not stop eating completely on fast days.

Instead, the person will limit her intake of food to 500 calories (600 for men), on alternate days, while eating normally the rest of the time.

“It’s not actual fasting, but really a type of low-cal dieting,” Dr. Iacobellis explains. “I think it’s the most popular because it’s a schedule you can track and you have a set number of calories you can eat.”

A variation of this is known as the 5:2 Fast Diet.

This plan requires caloric restriction of 500 calories a day for women and 600 a day for men two days a week and then consuming the recommended calories the other five days.

Other popular fasting practices include eating the standard amount of calories within a limited time (usually from 10 a.m. to 6 p.m.) as well as periodic fasting, which is abstaining completely for continuous days or several days during the week, sometimes for a stretch of several months.

More research is needed

Fasting has been credited with boosting the immune system and improving cognitive abilities in lab animals while helping the brain adapt to stress, injury and disease. Animal research by a neuroscientist for the National Institute on Aging found that a 40 to 50% calorie reduction in the diet of rats and mice led to a reduction in the size of their heart, liver, gut, and muscles, yet their brain remained the same size.

One 2015 analysis of women involved with the U.S. National Health and Nutrition Examination Survey found that longer nighttime fasting duration — more than 13 hours — appeared to reduce the risk for breast cancer, and a team of biologists discovered that mice fasting for 24 hours regenerated their intestinal stem cells at double the normal rate.

A 2014 study reported that mice and cancer patients who went without eating for four days got rid of old blood cells and generated new ones, a boon for potential cures.

But while such reports may be encouraging, dieters hoping for a miracle may be too desperate to lose pounds and tame hunger pangs. Dr. Iacobellis, who is also director of the diabetes service at UHealth Tower, says information is still very preliminary and inconclusive.

Part of the problem is that research in this area has involved small groups of patients, usually without a control group, and most of what we know about fasting comes from scientific studies on mice, not humans.

Whether fasting is more effective or healthy [than a daily caloric-restricted diet] has been the subject of several studies, but they have not been long-term randomized clinical studies. And it is those that give you the best results.

Dr. Iacobellis

He knows of only one study, published in JAMA in 2017, that compared alternate-day fasting and daily calorie restriction by tracking 100 participants, 86 of them women. Researchers found that mean weight loss for those doing ADF and those on the traditional diet plan was about the same.

There also was no significant difference between groups in heart rate, blood pressure, fasting glucose and fasting insulin, and triglycerides, markers of cardiovascular health that proponents claim improve on fasting diets.

But there also were several “other surprising results that must be taken into account,” Dr. Iacobellis says.  ADF adherents ended eating more than the allotted 500 or 600 calories on fast days, but less than what they were allowed on non-fast days. Also, the dropout rate was highest with the ADF group.

Fasting as a weight-loss plan is likely not effective in the long-term either because the body compensates by slowing down and using fewer calories when it senses a deprivation of calories.

“There might be short-term loss, but long-term body weight plateaus or the person regains weight,” he adds.

Also, fasting is not for everyone. It won’t work on the morbidly obese, those wanting to lose 30 to 40 percent of their weight, for example.

People on certain medication, including insulin or other anti-diabetic medicine, should not fast, nor should those with impaired liver or kidney function or a compromised immune system. Some physicians also nix exercise when fasting for a full day, because it could deplete the body of important electrolytes.

However, fasting as a weight-loss method seems to have certain psychological benefits. “You feel you have more freedom and it sounds less restrictive if you know you can eat more the following day,” Dr. Iacobellis says.

So, what’s the final verdict?

For relatively healthy people Dr. Iacobellis considers the ADF a fine alternative to the daily calorie restriction diet if one wants to lose up to 5 percent in a short period of time — with a doctor’s blessing of course.

“Is it effective for people wanting to lose a few pounds? Yes,” he says. “But is it superior? No. In the end losing weight comes down to increasing your output (of energy) and decreasing your input [of calories] on a daily basis. It has to become a habit.”

Ana Veciana author

Ana Veciana-Suarez, Guest Contributor

Ana is a regular contributor to the University of Miami Health System. She is a renowned journalist and author, who has worked at The Miami HeraldThe Miami News and The Palm Beach Post. Visit her website at or follow @AnaVeciana on Twitter.

Tags: dieting, Dr. Gianluca Iacobellis, fasting, healthy eating

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