Intermittent fasting and calorie counting

As a weight loss plan, it's easy to see why intermittent fasting is appealing: it involves eating what you want only during certain windows of the day (often eight hours a day).

Courtney Peterson, a nutrition researcher at the University of Alabama at Birmingham, said that instead of counting calories or measuring portion sizes, dieters can simply pay attention to the clock.

“There's a very simple rule: either eat or don't eat,” Peterson says.

The technology has skyrocketed in popularity in recent years and has become a major trending topic on social media.

But can time-restricted eating, a type of intermittent fasting, really help you lose weight and improve your health?

Here's what you need to know about this practice:

What is Intermittent Fasting?

Intermittent fasting is an eating strategy that alternates between fasting and regular eating schedules, and is defined as not eating for at least 14 hours, says Peterson. Variations of intermittent fasting include eating every other day, eating five days a week and fasting two days, or restricting eating to certain times each day.

Time-restricted eating is the most common form of intermittent fasting, in which you condense your daily eating into a 10-hour window or less: delaying breakfast until 10am or noon, eating dinner by 6pm or 8pm, and abstaining from eating for the rest of the day.

How does that help?

The theory behind time-restricted eating is that it supports your body's circadian rhythm – your body's internal clock – and scientists say that extending your fast, for example, may boost the body's processes that regulate blood sugar and fat metabolism.

Early research in mice beginning in 2012 seemed to suggest that time-restricted eating could have health benefits, and a small study in obese people suggested the practice could help with weight loss and improve other health markers.

Is time restricted feeding effective for weight loss?

Research has shown that people on time-restricted eating plans tend to eat fewer calories, which may lead to weight loss.

Results from multiple studies suggest that obese adults who engage in time-restricted eating without worrying about calories naturally reduce their daily energy intake by 200 to 550 calories, which is 3 to 5% of their baseline body weight.

However, larger studies with people observed over longer periods have shown that time restrictions alone may not be the problem.

A study published in the New England Journal of Medicine in 2022 followed 139 obese people for a year. Participants either consumed a calorie-restricted diet at certain times of the day or ate the same calories throughout the day. Both groups lost an average of 14 to 18 pounds, but the strategies were not significantly different.

“Our data to date suggests that time-restricted eating is no better or worse than calorie restriction,” Peterson says, adding that the method also doesn't help you burn calories.

Still, the simplicity of time restriction may make it easier to stick to than a typical diet, Peterson says.

“Very few people like counting calories,” she says.

Is intermittent fasting safe?

Early clinical trials involving eating windows of six to 10 hours have found time-restricted eating to be “generally safe,” the researchers reported in the journal Obesity.

But a high-profile study presented at the American Heart Association's Scientific Sessions this year suggested that people who follow an eight-hour time-restricted diet have a much higher risk of death from cardiovascular disease than those who eat over a 12- to 16-hour period.

Dr. Francisco Lopez Jimenez of the Mayo Clinic noted that the study has not been published in a peer-reviewed journal.

But he says there's reason to be cautious: Years of evidence suggest that skipping breakfast can lead to cardiovascular disease and death, and people should consult with a health care provider before attempting any dietary restrictions, especially if their fasting period lasts until noon.

“We should pause for a moment and think before recommending a particular diet,” Lopez Jimenez says.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Science Education Media Group. The AP is solely responsible for all content.