The New Pros and Cons of Intermittent Fasting
Research shows it won't work for everyone. But could it help you lose a few pounds? Maybe
En español | If you've packed on the COVID 15 (or 20, or 30) you're probably hungry for new ways to take those excess pounds off. One weight loss strategy that's been lauded for several years now is intermittent fasting, an eating pattern that cycles between times of, well, fasting and eating. “While many diets focus on what to eat, intermittent fasting is really all about when you eat,” says Louis Aronne, M.D., director of the Comprehensive Weight Control Center at Weill Cornell Medicine in New York City. There are two main approaches, he notes: traditional intermittent fasting, where you eat normally five days a week, and either fast or sharply restrict calories on the other two, or time restricted eating, where you choose an eating window every day so that you leave a 14- to 16-hour overnight fast.
Initially, intermittent fasting was touted as the holy grail for weight loss, not only allowing you to shed girth but also protecting against chronic diseases such as type 2 diabetes, heart disease and even certain cancers. But while initial studies were promising, Aronne notes, new research suggests the approach is no more effective than a traditional low-calorie diet.
A new study calls fasting into question
A study published this past September in JAMA Internal Medicine followed 116 overweight adults who fasted for 16 hours a day and only ate between noon and 8 p.m. for three months and found that subjects didn't lose any more weight than a control group. “We found that it wasn't an effective tool for weight loss,” says study author Ethan Weiss, M.D., a cardiologist at the University of California, San Francisco, who notes that his research didn't show any improvement in metabolic markers such as blood cholesterol or blood glucose levels. This follows on the heels of earlier research that found intermittent fasting was no more effective than an old-fashioned low-calorie diet. A 2018 study published in the American Journal of Clinical Nutrition examined 150 overweight and obese patients over nearly a year, some of whom simply cut calories by 20 percent, while others followed a five-days-on/two-days-off pattern of intermittent fasting. At the end of the trial, both had lost similar amounts of weight and body fat.
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In fact, if anything, intermittent fasting may actually inadvertently sabotage your attempts at weight loss: Weiss’ study also found that the weight the time-restricted eaters shed was mainly lean mass, including muscle, not body fat. “This is more worrisome for people over 50, since maintaining muscle mass as you age gets harder,” he explains. Preserving muscle is key in this age group, not only to keep your metabolism percolating (which in turn helps keep weight off) but also because it helps improve balance and reduces risk of falls. “Before recommending to my older patients, I would want to see more research on the effects on lean mass,” adds Weiss, who had been following time-restricted eating himself since 2014.
One problem his patients run up against with intermittent fasting, says Aronne, is that it's difficult to stick to long term. A 2017 study published in the JAMA Internal Medicine found that almost 40 percent of people fall off the fasting wagon within six months. “Some people find themselves so ravenously hungry after 16 hours of not eating, or a day of fasting, that they end up consuming thousands of calories, which defeats the purpose,” he explains. If you have diabetes, you should know that the combination of fasting and the medications you may take could cause your blood glucose levels to get dangerously low.
The benefits of intermittent fasting
That's not to say this kind of restricted eating can't have value. Intermittent fasting may in fact work for certain people, Aronne adds, especially if they don't want to be bothered with calorie tracking and food records. “It's not my first choice for weight loss,” he says, “but I have found that in a select group of patients struggling to lose weight, having them eat all their food in an eight-hour period works for them, because it's easy and they don't have to think about it: They just do it.”
For everyone, it still makes sense more generally to eat to maximize your circadian rhythms — your body's inner clock that guides you to wake and sleep — as much as possible, advises Michael Roizen, M.D., chief wellness officer of the Cleveland Clinic and author of What to Eat When. “Our bodies evolved to be primed for food during the day, so that we have plenty of energy for survival,” he says. As a result, your body is most sensitive to insulin, a hormone that moves glucose from your blood into cells for energy and storage, during the day, and most resistant to it at night. Ignoring these rhythms and eating at the wrong times — say, late at night — can raise blood sugar, a risk factor for type 2 diabetes, according to a study led by researchers at Harvard University as well as Brigham and Women's Hospital.
Take an approach instead where you make breakfast (or, if you can't stomach eating a lot that early, lunch) the main meal of the day, and make your last meal a light one after the sun goes down. “This carries many of the same benefits of intermittent fasting, since you're generally not eating within a 12-hour window, but it's much easier,” explains Roizen.