Intermittent fasting may not improve metabolism without calorie reduction

A two-week controlled trial in overweight women found that time-restricted eating did not change metabolic or cardiovascular markers when calorie intake was kept constant; it did, however, shift participants' sleep timing and internal clocks.

A recent two-week trial tested time-restricted eating (TRE) in 31 overweight or obese women while keeping calorie and nutrient intake unchanged. TRE confines daily eating to a roughly 10-hour window followed by a fast of at least 14 hours and has been proposed as a way to improve weight and metabolic health. Earlier human studies reported cardiometabolic benefits, but it was unclear whether those effects resulted from shorter eating windows or from eating fewer calories. This trial compared an early TRE window (8 a.m.–4 p.m.) with a late TRE window (1 p.m.–9 p.m.) under isocaloric conditions to isolate the role of timing. Key findings: - 31 women participated in a two-week isocaloric trial comparing an early TRE window (8 a.m.–4 p.m.) with a late TRE window (1 p.m.–9 p.m.). - No significant changes were observed in insulin sensitivity, blood glucose, blood fats, or inflammatory markers over the two weeks. - The late TRE schedule was associated with an average internal clock delay of about 40 minutes compared with early TRE, and participants on late TRE tended to go to bed and wake up later. - Researchers measured metabolic parameters, glucose levels, food intake, physical activity, and markers of cellular circadian rhythms. - The authors suggested that metabolic benefits reported in earlier TRE studies were likely driven by unintended calorie reduction rather than by shortening the daily eating window itself. Summary: The study indicates that when calorie intake is held constant, time-restricted eating did not produce measurable metabolic or cardiovascular improvements over two weeks, though it did alter sleep timing and some cellular rhythms. The researchers conclude earlier benefits may have reflected reduced calorie intake rather than meal-timing effects. Undetermined at this time.