Rather than consistently cutting calories, adherents to the intermittent fasting approach to weight loss alternate between periods of unrestricted eating and periods that allow for few calories. The 5:2 Diet, for instance, allows five days per week of normal eating while limiting you to around 500 to 600 calories on the other two days. Alternate-day fasting is one day on, one day off. Time-restricted eating limits food intake to a certain number of hours per day, every day of the week. Biohacking, popping up recently in Silicon Valley circles, takes intermittent fasting to an extreme, with plans that have adherents going without food for as long as seven consecutive days.



Does Intermittent Fasting Work?

Fasting every other day may be as effective for weight loss as old-fashioned calorie cutting (but not more so), according to a 2017 study from the University of Illinois at Chicago published in JAMA Internal Medicine. Researchers asked 100 obese adults to follow a reduced-calorie diet, an intermittent fasting plan, or no regimen at all. At the end of the year, both the reduced-calorie and alternate-day fasting groups had lost roughly the same amount of weight—5.3 percent and 6 percent, respectively. Another study published in 2011 in the International Journal of Obesity that compared a 5:2 approach with a less drastic but consistent calorie-cutting approach found similar, comparable weight loss between the two.

The Upside

Short-term studies suggest that intermittent fasting may activate immune responses that improve blood lipids, blood sugar, blood pressure, and inflammation, says Frank Hu, M.D., Ph.D., chair of the nutrition department at the Harvard T.H. Chan School of Public Health. Some studies show that intermittent fasting may also preserve learning and memory functioning—in animals. The research on the effects of intermittent fasting on human brains is lacking, says Mark Mattson, Ph.D., though he is currently studying its long-term impact on cognition and markers of human brain health at the National Institute on Aging in Maryland.

The Downside

It’s hard to stick with—almost 40 percent of the intermittent fasters in the University of Illinois at Chicago study dropped out, compared with 29 percent in the calorie-restricted bunch.

And people may react differently to fasting. “A person might feel light-headed, tired, and nauseous, and will not have the energy to exercise,” says Consumer Reports nutritionist Ellen Klosz. Experts say those with diabetes, hypoglycemia, or a history of disordered eating, as well as teens and pregnant and nursing women should sit this one out.

The Middle Ground

Close the kitchen after dinner’s over and skip the late-night snack, so you actually “break the fast” with your morning meal the next day. Putting even a small limitation on the hours in which you eat can help rein in the calories.