Intermittent fasting, or restricting eating times to certain time windows, is a current trend people are embracing for health and weight maintenance. But can the timing of your meals truly impact obesity and diabetes risk? It’s a line of study that Daisy Duan has been pursuing.

“My patients — and even people without disease — want to know what they can do with their eating habits or activity levels to lead a better lifestyle,” says Duan, an endocrinologist who focuses on diabetes and obesity. “I have patients who say, ‘I feel like I’m eating healthy,’ or ‘I’m restricting what I’m eating but I’m not able to lose weight.’ That’s where metabolism comes in. How do our bodies process the food that we’re eating? Are there factors besides calories that impact how our body balances energy that could affect diabetes and obesity risk?”

In a study published in the Annals of Internal Medicine, researchers randomly assigned 41 adults with obesity and prediabetes or diet-controlled diabetes to one of two eating plans for 12 weeks, with all meals provided. One was a time-restricted schedule that limited food intake to a 10-hour eating window, with 80% of calories consumed before 1 p.m. The other allowed up to 16 hours for food intake, with at least 50% of calories consumed after 5 p.m. People in both groups lost similar amounts of weight — about five pounds — and had no significant differences in sugar levels, indicating that any changes were due to reductions in caloric intake.

That work, completed during Duan’s fellowship, inspired her to look more into how sleep and circadian rhythms may impact metabolism. Working with a team in sleep research, Duan has been studying how different meal timing affects blood sugar levels and dietary fat oxidation after eating and during sleep, using stable isotope tracers that can follow how the body uses up dietary fat in meals.

In one piece published in the Journal of Clinical Endocrinology and Metabolism, Duan and co-authors gave 20 healthy volunteers a meal at a typical (6 p.m.) or later (10 p.m.) dinner time followed by eight hours of sleep in a Johns Hopkins sleep lab, finding that people who ate later had an 18% higher spike in blood sugar and 10% slower dietary fat breakdown overnight compared with those who ate earlier. These effects were more pronounced in people who typically hit the sack at home by 11 p.m. Their blood sugar levels rose 30% higher after the late dinner, while their body used up 20% less dietary fat overnight. By contrast, people who usually stay up until 2–3 a.m. were much less impacted. This finding brings up the question of whether meal timing should be personalized to each person’s natural circadian rhythm, Duan says.

She and her colleagues are now recruiting healthy volunteers and people with obesity and prediabetes for a new study to evaluate the impacts of eating an earlier or later dinner according to participants’ individual circadian rhythm rather than prescribed meal times. Investigators will first determine (in a dim-light setting) when each person’s biological night begins, based on their melatonin production. Then, participants will spend 24 hours, including eating and sleeping, in a metabolic chamber at the National Institutes of Health, which provides a comprehensive and gold-standard measurement for how the body burns sugars and fats.

Ideally, Duan says, results could lay the foundation for circadian-based meal timing as a precision-medicine approach to managing obesity and metabolic dysfunction: “I’m hoping to get some clarity about how timing of the dinner meal and sleep really affect how our body uses up the food we’re eating. Teasing that out can help patients make an informed choice.”