A recent study conducted by the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité -- Universitätsmedizin Berlin has investigated the effects of time-restricted eating.
The research indicates that time-restricted eating, when calorie intake remains constant, does not lead to measurable improvements in metabolic or cardiovascular health. However, the timing of meals was observed to affect the body's internal clocks. These findings emerged from the ChronoFast study, led by Prof. Olga Ramich, and were published in Science Translational Medicine.
Understanding Time-Restricted Eating
Time-restricted eating (TRE) is a form of intermittent fasting that limits daily food consumption to a window of no more than ten hours, followed by a fasting period of at least 14 hours. This approach has gained attention as a strategy to support weight management and metabolic health. Animal studies have suggested that TRE can protect rodents from diet-related obesity and metabolic problems. Previous human studies have reported potential benefits such as improved insulin sensitivity, healthier blood sugar and cholesterol levels, and modest reductions in body weight and body fat.
Study Rationale and Design
Earlier research on TRE has yielded varied results, with many studies not able to definitively determine whether observed health improvements were due to shorter eating windows, unintentional calorie reduction, or a combination of both.
To address these inconsistencies, Prof. Olga Ramich designed the ChronoFast trial. The study aimed to evaluate whether an eight-hour eating window could enhance insulin sensitivity and other metabolic markers when calorie intake was kept constant.
The study employed a randomized crossover design and included 31 women who were overweight or obese. Each participant followed two distinct eating schedules for two weeks each:
- Early time-restricted eating (eTRE): Meals consumed between 8 a.m. and 4 p.m.
- Late time-restricted eating (lTRE): Meals consumed between 1 p.m. and 9 p.m.
Throughout both phases, participants consumed nearly identical meals with consistent calorie and nutrient content. 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, and food intake was meticulously recorded. Physical activity was monitored using a motion sensor. In collaboration with Prof. Achim Kramer from Charité -- Universitätsmedizin Berlin, changes in the body's internal clock were examined using isolated blood cells.
Measuring Circadian Rhythms
Human biology operates on internally generated rhythms, known as circadian clocks, which align with the length of a day. These rhythms regulate numerous physiological processes, including sleep and metabolism. Most cells in the body contain their own internal clock, influenced by factors such as light, physical activity, and food timing.
Prof. Dr. Achim Kramer developed the BodyTime assay, a test requiring a single blood sample to provide an objective snapshot of an individual's internal timing. The ChronoFast study utilized this method and confirmed that eating schedules can shift internal clocks in humans.
Study Findings
The ChronoFast study reported no clinically significant changes in insulin sensitivity, blood sugar, blood fats, or inflammatory markers after the two-week interventions. Prof. Ramich stated that these results suggest health benefits observed in earlier studies were likely attributable to unintentional calorie reduction rather than solely the shortened eating period.
While metabolic measures remained largely unchanged, 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. Participants following the later eating window also reported going to bed and waking up later. First author Beeke Peters commented that the timing of food intake acts as a cue for biological rhythms, similar to light.
Conclusion and Future Research
The findings emphasize the importance of calorie balance in achieving health benefits from intermittent fasting. Ramich concluded that individuals aiming for weight loss or metabolic improvement should consider both meal timing and energy balance.
Future research will investigate whether combining time-restricted eating with reduced calorie intake produces more pronounced benefits. Scientists also plan to explore how individual factors, including chronotype and genetics, may influence responses to different eating schedules.