Intermittent fasting may not work for everyone without a calorie deficit

A German ChronoFast study with 31 overweight or obese women found that time‑restricted eating shifted circadian timing but produced no clinically meaningful changes in insulin sensitivity, blood sugar, blood fats or inflammatory markers when meals were matched for calories and nutrients. Researchers say metabolic benefits seen in earlier work may have come from unintended calorie reduction rather than the eating window itself.

A controlled study from the German Institute of Human Nutrition Potsdam‑Rehbruecke and Charité tested whether time‑restricted eating affects metabolism independent of calorie intake. The ChronoFast trial enrolled 31 women who were overweight or obese and compared two two‑week eating windows while providing meals matched for calories and nutrients. Researchers collected blood samples across four clinical visits and examined circadian changes in isolated cells. The study aimed to separate the effects of eating timing from changes in energy intake. Key findings: - Time‑restricted eating shifted participants' internal clocks (circadian rhythm). - There were no clinically meaningful changes in insulin sensitivity, blood glucose, blood lipids, or inflammatory markers over the study periods. - Meals were nearly identical in calorie and nutritional content for the different eating windows, so physiological results were not driven by changed meal composition. - The research team reported that observed benefits in earlier studies were likely linked to unintended calorie reduction rather than the shortened eating period itself. - The authors called for further study of individual factors such as chronotype and genetics to understand differing responses. Summary: The ChronoFast study indicates that altering eating windows can shift circadian timing but does not by itself produce measurable improvements in common metabolic or cardiovascular markers when calorie intake is held constant. Researchers recommend further investigation into how personal factors like chronotype and genetics influence responses to eating schedules.