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Time-Restricted Eating: Is It Worth the Wait?

By Merna Hussien, MD

Time-restricted eating, which is a form of intermittent fasting, has become a popular strategy to lose weight. It was shown to be effective in animal models and small studies involving humans. However, a study recently published in the Journal of the American Medical Association showed that time-restricted eating led to minimal weight loss and no difference in metabolic health markers. Also, most of the weight lost was muscle weight rather than fat.

In the study, the researchers randomly divided 141 overweight or obese adults into two groups for three months. One group followed time-restricted eating, which means consuming food and drinks without restrictions between 12 p.m. and 8 p.m. daily, and fasting for the following 16 hours, until 12 p.m. of the next day, except for calorie-free beverages, such as tea and black coffee. This is commonly referred to as the 16:8 diet.

The other group was instructed to eat three meals per day, with snacking allowed in between. The study was conducted through a mobile application. The participants were given a scale that linked to the application with Bluetooth wireless technology. They were instructed to weigh themselves daily in the morning.

Researchers followed a group of the participants in person and monitored markers such as fasting blood sugar level, blood pressure, and blood cholesterol levels. The researchers compared the amount of weight loss over the three months between the two groups as the main outcome of the study.

Despite good adherence to the 16:8 diet (more than 80%), the participants in the time-restricted eating group lost on average only 2 pounds after three months. Also, this weight loss was not significantly different from that in the control group. Moreover, more than 60% of the weight lost was from lean mass, which includes body muscle mass and not fat mass. This is far more than the expected lean mass loss during weight loss. 

It was thought that with time-restricted eating, participants ate less protein, which worsens muscle mass loss. Losing muscle mass over time can lead to weakness and disability. There was also no difference between the two groups in monitored fasting blood sugar level, blood pressure, and blood cholesterol levels.

The results of this study were surprising, even to the researchers themselves, who anticipated weight loss and metabolic benefits with time-restricted eating, based on prior studies. This study concluded that the benefits of time-limited eating, if present, are probably very limited. Also, it led to more excessive loss of muscle mass. Therefore, it is recommended that followers of this diet consume adequate amounts of protein.

Questions that remain unanswered include: Does a longer duration of fasting during the day increase the benefits of this diet? And, does consuming the calories earlier in the day improve the weight loss and metabolic benefits? Further studies are needed to address these questions.

Merna Hussien, MD, is a resident in the Osler Medicine Training Program at the Johns Hopkins Hospital in Baltimore, MD. 

To learn more about heart-healthy eating, visit CardioSmart.org/EatBetter.

Read the journal study: “ Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity,” JAMA Internal Medicine, Sept. 28, 2020. 

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