New Johns Hopkins study challenges benefits of intermittent fasting

When it comes to weight loss, how many calories you consume might be more important than when you consume them, challenging the popularity of intermittent fasting, according to a new study of time-restricted eating by researchers at Johns Hopkins University published April 19 in Annals of Internal Medicine.

The Johns Hopkins researchers randomly assigned 41 adults with obesity and prediabetes to either time-restricted eating (TRE) with a 10-hour eating window or a regular eating pattern for 12 weeks to compare weight loss and other measures of metabolic health, according to a study summary provided by Annals of Internal Medicine. Their randomized, controlled trial found that both TRE and a more traditional daily eating pattern resulted in weight loss when calories were held constant in both groups. It did not seem to matter whether participants consumed most of their calories early in the day or in the evening, suggesting that overall calories may be more important than meal timing when it comes to weight loss.

"It makes us think that people who benefit from time-restricted eating—meaning they lose weight—it's probably from them eating fewer calories because their time window's shorter and not something else," said the study's lead author, Nisa Maruthur, an associate professor of medicine at Johns Hopkins University, in an interview with NBC News. Maruthur presented the findings at the inaugural scientific plenary session "New in Annals of Internal Medicine: Hear it First from the Authors" in Boston last week during the annual American College of Physician's Internal Medicine Meeting.

Image caption: Nisa Maruthur

Similar to intermittent fasting, patients using TRE limit their eating to a window of time during the day and then fast for the remaining hours. During the eating window, patients are not required to count calories or monitor food intake and during the fasting window, patients are limited to water and calorie-free beverages. Evidence shows that when adults with obesity limit their eating window to four to 10 hours, they naturally reduce caloric intake by approximately 200 to 550 calories per day and lose weight over two to 12 months. Whether TRE induces weight loss independent of reductions in calorie intake, as seen in rodent studies, is unknown.

At the beginning of the study, the researchers assessed participants' history and activity level to estimate baseline caloric needs. The participants received prepared meals with identical macronutrient and micronutrient compositions based on their individual calorie estimates and instructions on when to consume the meals. They ate the same number of calories daily throughout the study. The time restricted eating group was instructed to eat only between the hours of 8 a.m. to 6 p.m. and consumed most of their calories before 1 p.m. each day. The usual eating pattern group ate between 8 a.m. and midnight and consumed most of their calories after 5 p.m. each day. After 12 weeks, both groups lost about the same amount of weight and there were no real differences in fasting glucose, waist circumference, blood pressure, or lipid levels. According to the authors, these findings suggest that if or when TRE interventions induce weight loss, it is likely in part due to a reduction in calories, and, therefore, clinicians can counsel patients that TRE may help them lose weight by decreasing their caloric intake.

According to the authors of an accompanying editorial from the University of Illinois Chicago, these results have important clinical implications. While TRE was not found to be more effective for weight loss, it may be easier for patients to follow because it allows them to continue consuming familiar foods. This simplified approach to treating obesity could help patients who don't do well counting calories.