Intermittent fasting without calorie reduction shows no metabolic advantages, study finds - Video

Skipping breakfast might not be the miracle health hack it's made out to be. A new study from the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité – Universitätsmedizin Berlin has found that time-restricted eating—one of the most popular forms of intermittent fasting-offers no measurable boost to metabolism or heart health when calorie intake stays the same. Published in Science Translational Medicine, the “ChronoFast” study, led by Professor Olga Ramich, reveals that while meal timing can shift the body’s internal clock, it doesn’t necessarily enhance key metabolic markers.

Time-restricted eating (TRE) limits daily meals to a short window—typically eight to ten hours—followed by long fasting periods. The idea gained traction after early studies in animals and some human trials suggested that it improves insulin sensitivity, stabilises blood sugar, and helps with weight control. But scientists have questioned whether these benefits come from eating at specific hours or from people unintentionally cutting calories when fasting.

To find out, the ChronoFast team designed a carefully controlled, randomised crossover trial involving 31 women with overweight or obesity. Each participant followed two schedules for two weeks: early TRE (8 a.m. to 4 p.m.) and late TRE (1 p.m. to 9 p.m.). Importantly, every meal was identical in calories and nutrients, removing the variable of energy intake. Researchers then tracked insulin response, blood sugar, fats, and inflammation through blood tests and glucose monitoring. Physical activity was measured with motion sensors, and participants’ internal biological rhythms, or circadian clocks, were assessed using a specialised “BodyTime” blood assay.

The results? Despite adjusting their eating windows, participants showed no significant improvements in blood sugar control, insulin sensitivity, or lipid levels. However, meal timing did influence circadian rhythms—late eaters’ internal clocks shifted by about 40 minutes, and they also went to bed later.

The researchers conclude that future studies should explore combining calorie control with meal timing to truly optimise metabolic health.

REFERENCE: Beeke Peters, Julia Schwarz, Bettina Schuppelius, Agnieszka Ottawa, Daniela A. Koppold, Daniela Weber, Nico Steckhan, Knut Mai, Tilman Grune, Andreas F. H. Pfeiffer, Andreas Michalsen, Achim Kramer, Olga Pivovarova-Ramich. Intended isocaloric time-restricted eating shifts circadian clocks but does not improve cardiometabolic health in women with overweight. Science Translational Medicine, 2025; 17 (822) DOI: 10.1126/scitranslmed.adv6787