Experts caution that more research is needed to understand long-term benefits of the diet.
Only eating during certain hours of the day could help people lose weight and control their blood sugar, according to a new study.
The research, published in the Annals of Internal Medicine on October 1, investigated how following a time-restricted diet -- a type of intermittent fasting -- might affect people with metabolic syndrome.
More than 93 million people in the U.S. have metabolic syndrome, meaning they have some combination of high blood pressure, high triglycerides, high blood sugar, a large waist circumference, and low HDL cholesterol. This puts them at a higher risk of type 2 diabetes and cardiovascular issues.
Dietary changes could help ease metabolic syndrome and lower those risks, but most people struggle with sticking to restrictive diets.
So researchers had participants try time-restricted eating, meaning they limited their eating to a consistent eight to 10-hour time window when the body is primed for digestion. Time-restricted eating doesn't explicitly focus on calorie restriction.
They discovered that the participants who shortened their window of eating time saw greater improvement in glucose control and decreases in weight and fat when coupled with standard nutritional counseling.
Importantly, the study's results indicate that time-restricted eating only "modestly" improves blood sugar control, so the effects may not be clinically meaningful for an individual person, said Christopher Gardner, PhD, director of nutrition studies at the Stanford Prevention Research Center and a professor of medicine at Stanford University. However, he told Health, if everyone improved their glucose regulation in line with these findings, there would be a noticeable difference in population health overall.
Here's what experts had to say about how time-restricted eating could impact metabolic health, how it compares to other types of fasting, and what to keep in mind before giving time-restricted eating a try.
The authors of this new study research circadian rhythm, or the 24-hour clock that regulates sleep, appetite, and other important functions. The idea behind time-restricted eating is that people should align their eating to the hours during which the body's metabolism is most active.
"Time-restricted eating is different than other fasting interventions," the study's lead author Emily Manoogian, PhD, staff scientist in the Panda lab at the Salk Institute for Biological Studies, told Health. Intermittent fasting, for example, sometimes requires caloric restriction, does not always support circadian health, and doesn't involve eating at consistent intervals.
However, research is mixed on whether time-restricted eating can actually help improve heart health and lead to weight loss. Some studies found it helps control blood sugar, lower blood pressure, lose weight, and improve cholesterol, while other studies found no benefit.
To further investigate time-restricted eating, the researchers recruited 122 volunteers with metabolic syndrome who had elevated levels of fasting blood sugar or hemoglobin A1C (HbA1C), putting them at risk of developing diabetes.
The participants were randomized into two groups for the study. Half received standard nutritional counseling. The other half received nutritional counseling alongside a personalized eight-to-10-hour time-restricted eating plan. That time-restricted eating plan started at least one hour after a person woke up, and ended at least three hours before they went to bed.
At the end of three months, 108 participants completed the study. The results showed that time-restricted eating was associated with a 0.1% greater reduction of HbA1C, suggesting a small reduction in average blood sugar levels. Time-restricted eating was not associated with improvements in other metabolic measures, such as fasting glucose, fasting insulin, insulin resistance, or glucose levels.
But between the two groups, the differences in weight loss were more substantial. Compared to standard nutritional counseling, participants in the time-restricted eating group had 3.3% more weight loss, 2.2% greater reduction in body mass index, and 3.3% greater decrease in body fat percentage without losing lean muscle mass.
"When you're trying to get weight loss, one of the big problems is that you also usually lose lean [muscle] mass along the way," Manoogian said. "To maintain that lean [muscle] mass...that was really exciting to see."
Caroline Susie, RDN, LD, a dietician and spokesperson for the Academy of Nutrition and Dietetics, told Health that the study does indicate time-restricted eating may help with weight loss and blood sugar control, but there are a number of limitations.
In particular, she said future research should include a larger sample size and observe participants for a longer period of time to assess long-term benefits associated with the diet.
It's also not clear exactly how time-restricted eating could lead to weight loss and improved blood sugar control -- it could have something to do with the body's circadian rhythm, or it could simply make it easier for people to reduce their calorie intake.
Gardner noted that the time-restricted eating group ate 350 fewer calories daily, which might explain some of the benefits. "This would fit into a recommendation that any strategy to help people reduce calorie intake and lose weight would improve blood glucose regulation," he said.
A future study should compare time-restricted eating to a different eating pattern to see if the former is a better approach to weight loss and blood sugar control, Gardner explained. "That would have been more clinically meaningful," he said.
Questions also still remain on whether time-restricted eating is more helpful for certain groups of people. The researchers noted that there were not enough participants to determine whether sex and other factors affected how people responded to the diet.
If people with or without metabolic syndrome are interested in adopting a time-restricted eating pattern, it should be safe for most people to try.
Compared to other types of diets, time-restricted eating is less restrictive and doesn't involve picking and choosing specific foods or limiting calories. This could make it easier to adopt and stick with. However, since there isn't a lot of research in humans yet, scientists don't know for sure.
"From a practical perspective, I think time-restricted eating can work for some people, because there isn't anything to learn," Gardner said.
But the relatively lax parameters of time-restricted eating could make it more challenging for people to see the weight loss or blood sugar results they're looking for -- there's little emphasis on changing the quality of a person's diet, just when they eat. "That bothers me," said Gardner.
If you do want to try time-restricted eating, it's always smart to check in with a healthcare provider beforehand to ensure the new diet is safe, Susie recommended.
"Meeting with a registered dietitian nutritionist can ensure that, during the time window you are eating, that your diet is full of variety while addressing any potential vitamin [or] mineral deficiencies," she said.
Susie added that the diet may not be appropriate for people with a history of disordered eating and isn't recommended for those who are pregnant or breastfeeding, or young kids and the elderly.