A new study from Germany 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
What is time-restricted eating?
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, the researchers designed the ChronoFast trial. Its goal was to test whether an eight-hour eating window could improve insulin sensitivity and other metabolic markers when calorie intake was kept constant.
About the ChronoFast study
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.
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.
The team also examined changes in the body's internal clock using isolated blood cells.
Measuring the body's internal clock
Human biology follows internally generated rhythms that roughly align with the length of a day, which is why they are known as circadian clocks.
These rhythms help regulate nearly every physiological process, including sleep, digestion and metabolism.
Almost all cells in the body contain their own internal clock, which can be influenced by light, physical activity, and food timing.
To measure individual circadian phases, the research team developed the BodyTime assay, a test that requires only a single blood sample and provides an objective snapshot of a person's internal timing.
The ChronoFast study used this method and confirmed that eating schedules can shift internal clocks in humans.
No metabolic improvements found
The ChronoFast study found no clinically meaningful changes in insulin sensitivity, blood sugar, blood fats, or inflammatory markers after the two-week interventions.
"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," explained the lead researcher.
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 went to bed and woke up later. "The timing of food intake acts as a cue for our biological rhythms -- similar to light," said one of the study’s authors.
Calories and individual timing may matter most
The findings highlight the importance of calorie balance in achieving health benefits from intermittent fasting. People who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their calorie intake.
Future research is needed to explore whether combining time-restricted eating with reduced calorie intake produces stronger benefits.
Scientists also aim to better understand how individual factors, including chronotype and genetics, may influence how people respond to different eating schedules.
Source: Science Translational Medicine, October 29, 2025.
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