The Skinny on Intermittent Fasting 

En español | So you’ve gained a few pounds over the holidays (who hasn't?). One of the trendiest ways to burn those fat reserves now involves both eating whatever you want and, well, not eating at all. Called intermittent fasting, the approach alternates periods of unrestricted eating with periods where you sharply limit calories according to the ratio of your choosing. For example, if you go with 16/8, you'll fast for 16 hours, then eat all your daily calories in an eight-hour window; with 5/2, you get five days of unrestricted eating followed by two days of consuming only 500 calories each day.

So does this particular yo-yo work? And should it make your list of resolutions? In a nutshell: maybe.

While there is evidence to show that intermittent fasting promotes weight loss, the latest research reveals it’s no more effective than an old-fashioned low-calorie diet. In the largest study to date, German researchers examined 150 overweight and obese nonsmoking patients over a year; some of them simply cut calories by 20 percent, while others followed a 5/2 pattern of intermittent fasting. At the end of the trial, both had lost similar amounts of weight and body fat, according to the research, published in the American Journal of Clinical Nutrition.

A  2017 study published in JAMA Internal Medicine found similar results. “At the end of the day, it’s not about when you eat, but about how much you eat over time,” explains Caroline Apovian, an endocrinologist and obesity specialist at Boston University Medical Center and president of the Obesity Society.

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That said, there may be health benefits to trying some form of intermittent fasting. One USC study, for instance, found that patients who “fasted” for five consecutive days each month (they consumed between 750 and 1,100 calories on these days) had lower blood pressure, cholesterol, blood sugar and IGF-1 levels (a compound associated with increased risk of cancer) than those who ate normally.

It also may make weight loss easier — or at least less painful. “When you gain weight, the nerves in your hypothalamus that conduct signals from your fat cells to the rest of your brain become damaged,” explains Louis Aronne, a professor and director of the Center for Weight Management and Metabolic Clinical Research at Weill Cornell Medical College. “As a result, your brain doesn’t realize you’re full, so you keep eating and gain weight. But when you go down to a very low calorie diet intermittently, it gives hypothalamic nerves time to recuperate.” As a result, you may experience less hunger than if you followed a more traditional low-calorie diet.

But as appealing as intermittent fasting may sound, it does have its downsides — including being hard to stick to. The 2017 JAMA Internal Medicine study, for example, found that almost 40 percent of intermittent fasters dropped off within just six months.

“Personally, I think it’s Draconian,” says Pamela Peeke, an assistant professor of medicine at the University of Maryland and author of The Hunger Fix. Instead, she advises her patients to eat all their daily calories within a 12-hour window — usually between around 7 a.m. and 7 p.m. “That allows your body a full 12 hours of rejuvenation, which allows it to reset itself metabolically,” she explains.

Here’s why: When you eat, glucose is stored in your liver as glycogen, which takes at least six hours to be depleted. Once this glycogen is used up, your body switches from glucose to fat as its fuel source. This fat is in turn broken down into ketones, putting you into ketosis, a state that allows your body to remove dying cells and debris from your body. “This is why we think so many people see health benefits from some type of intermittent fasting, whether it’s taking whole days off or just following time-restricted eating,” she says.  

If you still find eating within a 12-hour zone too restrictive, experts recommend trying to eat the bulk of your calories before 3 p.m. to reap some of the same benefits. Research shows that when you eat late in the evening, for example, you’re not only more likely to gain weight, but also to have higher blood pressure, cholesterol and blood sugar levels. Other preliminary animal studies have found an association between late-night snacking and memory problems. So try making breakfast — or if that’s too hard to digest, lunch — your main meal of the day in 2019, and keep your nighttime nibbling to a minimum.

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