Scientists tested intermittent fasting without eating less and found no metabolic benefit
Time-restricted eating has been widely promoted as a simple way to boost metabolic health, but new research paints a more complicated picture. When calorie intake stayed the same, an eight-hour eating window did not improve insulin sensitivity or cardiovascular markers. What did change was the body’s internal clock, which shifted based on meal timing and altered sleep patterns. The results suggest calorie reduction, not the eating window itself, may be the real driver of health benefits.
A new study from the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité -- Universitätsmedizin Berlin challenges a widely held belief about intermittent fasting. The research shows that time-restricted eating does not lead to measurable improvements in metabolic or cardiovascular health when calorie intake remains unchanged. However, the timing of meals did affect the body's internal clocks. These findings come from the ChronoFast study led by Prof. Olga Ramich and were published in Science Translational Medicine.
Time-restricted eating (TRE) is a form of intermittent fasting that limits daily food intake to a window of no more than ten hours, followed by a fasting period of at least 14 hours. The approach has become popular as a simple strategy to support weight management and metabolic health. Animal studies show that TRE can protect rodents from diet-related obesity and metabolic problems. In humans, earlier studies have reported benefits such as improved insulin sensitivity, healthier blood sugar and cholesterol levels, and modest reductions in body weight and body fat. As a result, TRE has been widely viewed as a promising tool for preventing insulin resistance and diabetes.
Conflicting Evidence From Earlier Studies
Despite its popularity, past research on TRE has produced mixed results. Many studies have not been able to determine whether observed health improvements came from shorter eating windows, unintentional calorie reduction, or a combination of both. In addition, most earlier trials did not carefully track calorie intake or control for other factors that could influence metabolic outcomes.
To address these gaps, Prof. Olga Ramich, Head of the Department of Molecular Metabolism and Precision Nutrition at the DIfE and Professor at the Charité -- Universitätsmedizin Berlin, designed the ChronoFast trial. The goal was to test whether an eight-hour eating window could improve insulin sensitivity and other metabolic markers when calorie intake was kept constant.
How the ChronoFast Study Was Conducted
The study used a randomized crossover design and included 31 women with overweight or obesity. Each participant followed two different eating schedules for two weeks at a time. One schedule involved early time-restricted eating between 8 a.m. and 4 p.m.(eTRE). The other followed a later schedule from 1 p.m. to 9 p.m. (lTRE). Throughout both phases, participants ate nearly identical meals with the same calorie and nutrient content (isocaloric).
Researchers collected blood samples during four clinic visits and performed oral glucose tolerance tests to assess glucose and fat metabolism. Continuous glucose monitoring tracked blood sugar levels over 24 hours while food intake was recorded in detail. Physical activity was monitored using a motion sensor. In collaboration with Prof. Achim Kramer from the Charité -- Universitätsmedizin Berlin, the team also examined changes in the body's internal clock using isolated blood cells.