What you need to know about intermittent fasting — and who should avoid it

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Here's what you need to know about intermittent fasting, the popular weight-loss trend. (Photo: Getty Images)
Here's what you need to know about intermittent fasting, the popular weight-loss trend. (Photo: Getty Images)

The popular weight-loss trend of intermittent fasting isn’t going anywhere and has only continued to gain steam, with celebrities like Kourtney Kardashian, Hugh Jackman and Chris Pratt extolling its virtues. The latest stars to jump on the fasting bandwagon are Jenna Bush Hager and Hoda Kotb, who decided to try intermittent fasting together (and publicly weighed themselves on camera on “Today with Hoda and Jenna”).

So what, exactly, is intermittent fasting? “Intermittent fasting is a weight loss or weight control strategy where you’re cycling between periods of eating and fasting,” Sarah Adler, PhD, a psychologist with the Stanford Eating Disorder and Weight Control Clinic, tells Yahoo Lifestyle.

There are different intermittent fasting methods

There are several ways to do intermittent fasting, but it typically involves choosing a specific window of time in which you can consume food or caloric drinks. “It can be as simple as skipping breakfast and eating at noon or finishing your last meal earlier,” says Adler.

One popular method is 16:8, where people fast for 16 hours and only eat during an 8-hour window, such as noon to 8 p.m. While fasting for 16 hours does sound like a lot, keep in mind that includes (hopefully) 8 hours of sleep. There’s also the 5:2 method — which Jimmy Kimmel follows — where people eat restricted calories (such as 500-600 calories per day) for two nonconsecutive days and then eat normally for the other five days.

What are the benefits of intermittent fasting?

For many who are able to stick with it, intermittent fasting is the “magic bullet” for weight loss. “I’ve seen a lot of people who have struggled with weight loss and have done intermittent fasting, and it seems to be the magic bullet for them,” Liz Weinandy, a registered dietitian at The Ohio State University Wexner Medical Center, tells Yahoo Lifestyle. “It’s because people are not eating as much.”

They’re also eliminating late-night eating, which can include less-than-healthy options like chips and ice cream. “Once they stop eating after dinner, that alone helps a lot of people start to lose weight,” says Weinandy. “For a lot of people, they’re not eating those extra 300 or 400 calories.”

Intermittent fasting can also have a diuretic effect — when the body gets rid of excess water — which leads to some fluid weight loss as well, according to Weinandy.

However, it’s worth noting that some researchers say there’s not enough scientific evidence on the long-term weight loss effects of intermittent fasting. “The research based on the efficacy of intermittent fasting is fairly limited in humans, so most is anecdotal,” says Adler. In addition, a 2018 German study — described as the largest investigation on intermittent fasting to date — involving 150 overweight and obese people on either intermittent fasting or conventional calorie-restricting diets, who were examined over the course of a year, found that intermittent fasting wasn’t any more effective at weight loss than calorie restriction.

That said, intermittent fast has other health benefits. “Insulin levels go down,” says Weinandy, “because if you’re not taking in any food, especially carbohydrates, our blood sugar isn’t going up.”

Adler explains that when you eat carbs, for example, the body breaks it down and converts it into sugar (glucose). But if you eat more than your body can use for energy, the sugar gets stored in fat cells. "Insulin brings sugar into fat cells and keeps it there," says Adler. "Between meals, our insulin levels go down and our fat cells release the stored sugar to use as energy. Intermittent fasting allows for insulin levels to drop so that [stored sugar] gets burned off."

There are other positive metabolic effects, including an increase in human growth hormone. “It’s important for muscle maintenance, especially as we get older,” says Weinandy. Intermittent fasting also appears to help “on a cellular level to repair DNA,” says Weinandy, by triggering autophagy — the body’s way of cleaning out damaged cells to then generate new, healthy ones.

However, not all types of intermittent fasting are created equal. “Research around intermittent fasting that shows health benefits are really limited to a very specific kind of intermittent fasting — basically, the 16:8 method,” points out Adler. “The health benefits have not been shown to be associated with other forms of intermittent fasting, like the 5:2 method.”

There can be mild, temporary side effects

Most side effects are fairly minimal, notes Adler. When people first start intermittent fasting, some may experience mild headaches or lightheadedness. In some cases, people who are not following the 16:8 method may find themselves overeating at other meals. “With intermittent fasting, 16:8 has been shown to reduce overeating in other meals,” says Adler. “Outside of that is when you’re getting overeating, increased hunger, and loss of energy.”

The eating method also isn’t right for everyone, says Weinandy. For example, some may find that skipping breakfast in the morning isn’t sustainable. In that case, not snacking after dinner, such as cutting off food by 8pm (or earlier in the evening), may work better for them. “Don’t go hot and heavy into it,” suggests Weinandy. “Gradually stop eating after dinner.”

Adds Adler: "People need to use an approach that works for them and is sustainable for them.”

Who should avoid intermittent fasting?

“In general, it’s considered pretty safe for the majority of people,” says Weinandy. But women who are pregnant or are trying to get pregnant, as well as women who are breastfeeding, should not attempt to do intermittent fasting. Also, “Anyone who has a history of eating disorders should not be intermittent fasting,” notes Adler. People with heart conditions and diabetes who are interested in trying the method should be monitored by a physician, who is knowledgeable about metabolic conditions.

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