A new study has cast doubt on the widely held belief in the benefits of intermittent fasting, showing that limiting eating to certain hours without reducing calorie intake does not improve metabolic or cardiovascular health.

ChronoFast Study: Goals and Features

Researchers from the German Institute of Human Nutrition (DIfE) and Charité University Hospital in Berlin conducted the ChronoFast study under the leadership of Professor Olga Ramich. The results were published in the journal Science Translational Medicine. The aim was to determine whether an eight-hour eating window could positively affect insulin sensitivity and other metabolic markers when calorie intake remains unchanged.

What is Time-Restricted Eating (TRE)?

Time-Restricted Eating (TRE) is a form of intermittent fasting where daily food intake is limited to a specific window (usually no more than 10 hours), and the rest of the day, no food is consumed. This approach has become popular as a simple strategy for weight control and maintaining metabolic health. Animal studies have shown that TRE can protect rodents from obesity and diet-related metabolic disorders. Early human studies also reported benefits such as improved insulin sensitivity, better blood sugar and cholesterol levels, and moderate reductions in body weight and fat. As a result, TRE has been considered a promising tool for preventing insulin resistance and diabetes.

Conflicting Data and Research Gaps

Despite its popularity, previous TRE studies have produced mixed results. In many cases, it was difficult to determine whether observed improvements were due to the shortened eating window, unintentional calorie reduction, or a combination of both. Additionally, most early studies did not track calorie intake or control for other factors that could influence metabolic outcomes.

Study Design and Procedure

The ChronoFast study involved 31 women with overweight or obesity. Each participant followed two different eating schedules for two weeks each: an early eating window from 8:00 a.m. to 4:00 p.m. (eTRE) and a late window from 1:00 p.m. to 9:00 p.m. (lTRE). In both cases, the diet was isocaloric, meaning calorie content and nutrient composition were the same.

During the experiment, blood samples were taken, oral glucose tolerance tests were performed to assess glucose and fat metabolism, and continuous blood sugar monitoring was conducted. Physical activity was tracked using motion sensors. Together with Professor Achim Kramer, the team also studied changes in internal biological clocks by analyzing isolated blood cells.

The Impact of Meal Timing on Circadian Rhythms

Human biology is governed by internal rhythms close to the 24-hour cycle, which regulate most physiological processes, including sleep and metabolism. Nearly all cells in the body have their own internal clocks, which can be influenced by light, physical activity, and meal timing.

To measure individual circadian phases, Professor Kramer developed the BodyTime test, which allows for an objective assessment of the state of internal biological clocks from a single blood sample. In the ChronoFast study, this method confirmed that eating schedules can indeed shift people’s internal clocks.

Main Findings

Contrary to expectations based on previous research, ChronoFast did not find clinically significant changes in insulin sensitivity, blood sugar, blood lipids, or inflammatory markers after the two-week interventions. According to Professor Ramich, the health benefits observed in earlier studies were likely due to unintentional calorie reduction, not the shortened eating window itself.

Although overall metabolic markers did not change, meal timing did affect circadian rhythms. Blood cell analysis showed that internal clocks shifted by an average of 40 minutes with the later eating schedule compared to the earlier one. Participants who followed the late eating window also went to bed and woke up later. As first author Beke Peters notes, meal timing acts as a signal for our biological rhythms, much like light does.

The Importance of Energy Balance

The results highlight the importance of energy balance for achieving the benefits of intermittent fasting. Those looking to lose weight or improve metabolism should pay attention not only to meal timing but also to overall energy balance, concludes Ramich.

Future Research Directions

In the future, scientists aim to find out whether combining time-restricted eating with calorie reduction leads to more pronounced effects. They also plan to study how individual factors, including chronotype and genetics, may influence the body’s response to different eating patterns.