Will Intermittent Fasting Help You Lose Weight?

Are you having trouble losing weight? So are a lot of other people. Have you tried intermittent fasting? Should you?

According to a survey by the International Food Information Council, intermittent fasting was the most popular weight loss diet in 2018. Some have called it a fad whose adverse effects have not been sufficiently studied, and the US National Institute on Aging does not recommend it. Many religions observe periods of fasting, and some of them claim health benefits. There are several nonreligious regimens of intermittent fasting, including alternate-day fasting, time-restricted feeding where eating is limited to certain hours each day, and the 5:2 diet where people fast for two days a week.

There is some preliminary evidence that intermittent fasting is effective for weight loss, decreases insulin resistance, improves indicators of cardiovascular and metabolic health, and may improve chronic pain and mood disorders. A Cochrane systematic review found only low-quality evidence that was insufficient to determine whether intermittent fasting could prevent cardiovascular disease. They found inconsistent results on the benefits of intermittent fasting and found that comparisons between intermittent fasting and calorie restriction diets were not conclusive.

When the benefit is uncertain, it is premature to try to determine the mechanism for the benefit, but that hasn’t stopped people from speculating. Proposed candidates for the mechanism have included a “metabolic switch” from lipid synthesis to fat mobilization, melatonin, the circadian rhythm, genetic predispositions, and changes in the gut microbiome.

It seems plausible that if you are only allowed to eat during certain hours, you will tend to eat fewer calories than if you are on an unrestricted or “seafood” diet (“If I see food, I eat it.”) So, the first thing we would like to know is whether intermittent fasting improves weight loss by reducing total calorie intake.

Thanks to a recent study published in The New England Journal of Medicine, “Calorie Restriction with or without Time-restricted Eating in Weight Loss” by Liu et al., we now have an answer to that question. Researchers were from China and Tulane University; subjects were recruited in Guangzhou, China. They studied 139 obese patients for twelve months. All were instructed to follow the same calorie-restricted diet (1,500—1,800 kcal a day for men; 1,200–1,500 kcal a day for women), and all were closely monitored by dietary health coaches. After randomization, those in the time-restricted group were instructed to eat only between 8 a.m. and 4 p.m. while those in the calorie-restriction-only group were allowed to eat any time of day.

Results

At twelve months, subjects in the time-restricted group had lost 8 kg and those in the calorie-restriction-only group had lost 6.3 kg. This was not a statistically significant difference (p = 0.11). Both groups had similar changes in waist circumference, body-mass index, body fat, blood pressure, glucose levels, lipids, and other measurements. Reports of mild side effects were similar in both groups.

In this study, improvements were seen with calorie restriction, and time-restricted eating did not significantly improve results. So, does that mean time-restricted eating doesn’t work?

Unfortunately, there is a big flaw in the study design. In Guangzhou, cultural habits already limit the hours people are likely to eat. At baseline, the participants typically ate only during a ten-hour-and-twenty-three-minute time frame. So, the restriction to an eight-hour period was only a minor change of a couple of hours. An editorial accompanying the study pointed this out and suggested that the results might have been different in other cultures where people typically ate during longer periods. It speculated that time-restricted eating might be a useful strategy for weight loss in some situations and that more study is needed.

Conclusion: calorie restriction works; there’s no good evidence that intermittent fasting does.

We know that reducing calorie intake results in weight loss and improvement in cardiac risk factors. We don’t have any such certainty about the benefits of intermittent fasting. If time-restricted eating helps people reduce their calorie intake, that is obviously a good thing. But we don’t even know for sure if it does that. Science will continue to study this; time will tell.

Harriet Hall

Harriet Hall, MD, a retired Air Force physician and flight surgeon, writes and educates about pseudoscientific and so-called alternative medicine. She is a contributing editor and frequent contributor to the Skeptical Inquirer and contributes to the blog Science-Based Medicine. She is author of Women Aren’t Supposed to Fly: Memoirs of a Female Flight Surgeon and coauthor of the 2012 textbook Consumer Health: A Guide to Intelligent Decisions.