Is intermittent fasting a healthy weight-loss plan?

PHILADELPHIA — In the past six months, Tish Johnson has lost about 30 pounds dieting — but she’s not restricting what she’s eating. This diet is all about when.

The 39-year-old mother of two, eats only between 4 and 8 p.m., and is an evangelist for “intermittent fasting,” the popular diet plan that involves designating times when the dieter can and can’t dine. The theory is that sending the body into a state of fasting will lead to better digestion, faster metabolism, and quicker fat burn that will result in weight loss.

The diet’s been around for years, though it exploded in popularity this summer, populating Facebook feeds and appearing on dieting websites and blogs alongside regimens like Whole 30 and the popular ketogenic diet. But there’s concern among some nutrition experts that fasting for extended periods of time is an extreme option that can develop into an eating disorder, while others pooh-pooh the hype around the diet, saying it’s no different from simply restricting calorie intake.

Johnson, who coaches other women trying intermittent fasting, or “IF,” said her results go well beyond weight loss — she tried other diets in the past, including the high-fat, low-carb ketogenic diet, but believes this gives her an almost boundless energy she hasn’t had for years.

There are several different types of intermittent fasting:

• Alternate-day fasting, meaning the dieter eats unrestricted one day and the next day doesn’t eat at all, or eats a small amount of what he or she normally would.

• Modified fasting, which has been endorsed by celebrities such as Benedict Cumberbatch and Jimmy Kimmel. The routine is also known as the 5:2 diet, in which the dieter eats normally for five days a week and greatly restricts food intake two days a week. • Time-restricted fasting, which is the method Johnson uses and means only eating during specific times. The most common form of time-restricted fasting is what’s known as 16:8, which means no eating during 16 hours a day and eating all meals within an eight-hour window. (For instance, 11 a.m. to 7 p.m.)

Kai Lo, 33, has been doing some version of intermittent fasting for about four months after trying unsuccessfully to lose weight by restricting calories. He started with the 16:8 version after seeing others talking about the diet on social media, and he’s now down to eating just one meal a day — albeit a big one that usually involves lean protein and four hard-boiled eggs — around dinner time. He said he’s lost about 25 pounds from 187, and feels like he’s trained his body to not feel hungry every couple hours.

Advocates of intermittent fasting say it jump-starts metabolism and burns fat at a higher rate than calorie restriction because after 12 to 14 hours of fasting, the body starts to burn stored fat instead of glucose from food — though skeptics point out that even people who eat more frequently burn excess fat if their overall calories are low enough. A study published last fall tested intermittent fasting in mice, and found that after four months, the regimen worked — the mice who did it weighed less and had more stable metabolism, even though they ate the same amount of calories as the control group.

There have been few long-term studies examining the effects of intermittent fasting on humans, though one 2015 paper concluded any form of intermittent fasting can result in some weight loss. Researchers still couldn’t conclude whether those results were any different from those of a calorie-restriction diet.

That study also concluded there’s little evidence fasting is harmful physically or mentally, though some experts disagree.

Dietitian Emily Pierce said she’d never recommend intermittent fasting to a client looking to lose weight, and said the Instagram-famous diet plan “glamorizes the extreme.” (She cautioned that diabetics, or people with other metabolic disorders, and pregnant women should avoid intermittent fasting.)

Pierce said people shouldn’t restrict eating when they feel hunger, because hunger “is our body communicating with us,” she said. “For someone to restrict a lot like that, it leads to an increased chance of binge eating. It’s a disordered eating pattern.” She said people who see results from intermittent fasting may simply be eliminating mindless, especially late-night, snacking.