Intermittent fasting has had a good run. Fewer meals. Clear eating windows. Big promises around metabolism and long-term health. Many people swear by it, even as the science around it has often felt mixed.
A new study now adds an important layer to that conversation. Researchers from the German Institute of Human Nutrition Potsdam-Rehbruecke and Charité Universitätsmedizin Berlin found that when calorie intake stays the same, time-restricted eating does not lead to measurable improvements in metabolic or cardiovascular health. The findings were published in Science Translational Medicine.
What the study set out to test about intermittent fasting
The researchers focused on time-restricted eating, commonly referred to as TRE. This way of eating limits daily food intake to a window of eight to ten hours, followed by a fasting period of at least 14 hours. It has become popular for weight management and metabolic health.
Earlier studies, particularly in animals, showed that this pattern could protect against obesity and metabolic problems. Human research also suggested benefits like better insulin sensitivity, improved blood sugar control, healthier cholesterol levels, and modest weight loss. The aim here was to find out whether these effects came from meal timing itself or from people unintentionally eating fewer calories.
How the ChronoFast trial was designed and conducted
The study, known as the ChronoFast trial, was led by Prof Olga Ramich, Head of Molecular Metabolism and Precision Nutrition at DIfE and Professor at Charité. It included 31 women who were either overweight or obese.
Each participant followed two eating schedules for two weeks at a time. One involved early time-restricted eating between 8 am and 4 pm. The second schedule shifted meals later, from 1 pm to 9 pm. Across both phases, meals were kept nearly identical, with the same calorie intake and nutrient composition.
To capture detailed changes, researchers carried out several assessments. Blood samples were collected during clinic visits. Oral glucose tolerance tests were performed to evaluate glucose handling. Continuous glucose monitoring tracked blood sugar levels over 24 hours. Physical activity was measured using motion sensors, and food intake was recorded carefully throughout the study.
Why calorie intake mattered more than eating windows
By keeping calories constant, the researchers removed a key variable. According to Prof Ramich, “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.”
In simple terms, timing alone did not drive improvements. Energy balance played the bigger role.
What the study found about metabolism and heart health markers
When the data was analysed, the results were consistent. There were no meaningful differences in insulin sensitivity, blood sugar levels, blood fats, or inflammatory markers between the early and late eating schedules. Cardiometabolic health markers remained largely unchanged, regardless of when meals were eaten.
How meal timing influenced the body’s internal clock
While metabolism stayed the same, meal timing did affect the body’s internal clock. Using the BodyTime assay developed by Prof Achim Kramer, researchers found that the later eating schedule shifted circadian rhythms by an average of about 40 minutes compared to early eating.
The authors noted that the timing of food intake acts as a cue for biological rhythms, much like light.
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