Intermittent Fasting Does Not Improve Metabolic Health Without Calorie Reduction, Finds Study

A recent study found that time-restricted eating, popularly known as intermittent fasting, doesn't show measurable improvements in metabolic or cardiovascular health when calorie intake remains unchanged. The study was conducted by researchers at the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité -- Universitatsmedizin Berlin and published in Science Translational Medicine. The study, led by Prof. Olga Ramich, Head of the Department of Molecular Metabolism and Precision Nutrition at the DIfE and Professor at the Charité -- Universitatsmedizin Berlin, however, found that meal timings did affect the body's internal clock.

Study Challenges Intermittent Fasting Benefits

Time-restricted eating (TRE) limits your daily food intake to a window of no more than ten hours. This is followed by a fasting period of at least 14 hours. People have adopted this way of eating for weight management and metabolic health. Previously, animal studies have shown that TRE can help protect rodents from diet-related obesity and metabolic problems. In humans, there have been benefits such as improved insulin sensitivity, healthier blood sugar and cholesterol levels, and moderate reductions in body weight and fat. These results have led people to adopt TRE for preventing insulin resistance and diabetes.

The ChronoFast Trial

Prof. Olga Ramich designed this study, the ChronoFast trial, to test whether an eight-hour eating window could improve insulin sensitivity and other metabolic markers when calorie intake was kept constant. The study included 31 women with overweight or obesity. Each of them followed two different eating schedules for two weeks at a time.

One of the schedules included early time-restricted eating between 8 AM and 4 PM. The other followed a later schedule from 1 PM to 9 PM. Throughout both phases, participants ate nearly identical meals with the same calorie and nutrient content. Researchers then collected blood samples during four clinic visits and performed oral glucose tolerance tests to assess glucose and fat metabolism. They also conducted continuous glucose monitoring and tracked blood sugar levels over 24 hours while food intake was recorded in detail. Physical activity was monitored using a motion sensor. The researchers also examined changes in the body's internal clock using isolated blood cells.

Prof. Achim Kramer from the Charité -- Universitatsmedizin Berlin, developed the BodyTime assay to measure individual circadian phases. This test needs only one blood sample and gives you a summary of an individual's internal timing. The ChronoFast study, then used this method and confirmed that eating schedules can shift internal clocks in humans.

Findings On Metabolism And Internal Clocks

The study found no meaningful changes in insulin sensitivity, blood sugar, blood fats, or inflammatory markers. Ramich says, "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."

The researchers didn't find any metabolic changes, however, the timing of meals did affect circadian rhythms. Analysis of blood cells showed that the internal clock shifted by an average of 40 minutes during the late eating schedule compared to the early schedule. The authors said, "The timing of food intake acts as a cue for our biological rhythms -- similar to light."

The study highlights the need for calorie balance to get health benefits from intermittent fasting. Ramich said, "Those who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their energy balance."

Experts say that future studies will need to explore whether combining time-restricted eating with reduced calorie intake shows stronger benefits.

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