Intermittent fasting has become one of the most talked-about diet strategies in recent years, with millions embracing time-restricted eating (TRE) as a way to manage weight and improve health. But a new study from researchers in Germany suggests that when calorie intake stays the same, simply limiting the hours you eat may not deliver the metabolic benefits many expect.
Scientists from the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité – Universitätsmedizin Berlin conducted the study, known as the ChronoFast trial, to test whether meal timing alone could improve insulin sensitivity, blood sugar, and cardiovascular markers. The findings, published in Science Translational Medicine, show that an eight-hour eating window, without reducing calories, did not measurably improve these outcomes in women with overweight or obesity.
Results of previous time-restricted eating (TRE) trials have been mixed, and many have not accounted for spontaneous calorie reduction. The ChronoFast trial specifically addressed this gap. Participants consumed their usual meals in two different schedules: early, between 8 a.m. and 4 p.m., or late, between 1 p.m. and 9 p.m., while keeping calorie intake and nutrient composition nearly identical.
During the two-week periods, the researchers collected blood samples, monitored glucose levels and controlled for physical activity. At the end of each phase, they measured insulin sensitivity, blood sugar, blood lipids and inflammatory markers.
No measurable metabolic improvements were observed.
“Our results suggest that the health benefits observed in earlier studies were likely due to unintended calorie reduction, rather than the shortened eating period itself,” explained Prof. Olga Ramich, who led the study.
While the metabolic effects were unchanged, the research uncovered a different kind of shift. Using a test developed by Charité researcher Prof. Achim Kramer, the team measured participants’ internal circadian rhythms. The BodyTime assay showed that eating schedules changed participants’ biological clocks: those following the late-eating plan went to bed and woke up later, and their cellular clocks shifted by about 40 minutes compared with early eaters.
“The timing of food intake acts as a cue for our biological rhythms — similar to light,” said first author Beeke Peters.
These findings suggest that while time-restricted eating can influence circadian timing, calorie balance remains the key factor for improving metabolic health.
“Those who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their energy balance,” summarized Ramich.
The researchers plan to explore whether combining time-restricted eating with calorie reduction may enhance benefits and whether personal factors, such as genetics or chronotype, affect outcomes.
For now, the takeaway is simple: changing when you eat can shift your body’s internal rhythms, but without reducing how much you eat, it may not change your metabolism.
The study was supported by grants from the German Research Foundation (DFG), the German Diabetes Association, the European Association for the Study of Diabetes and the German Center for Diabetes Research (DZD).