Research cautions that ignoring calorie reductions on intermittent fasting reduces benefits

Without changing a person’s caloric intake, intermittent fasting does not lead to measurable metabolic or cardiovascular improvements, according to new research. The study focused on time-restricted eating (TRE), a form of intermittent fasting that trims a person’s daily eating timeframe to no longer than ten hours and maintains a fasting period of at least 14 hours.

Contrary to previous studies suggesting the benefits of TRE, the ChronoFast study revealed no clinically relevant changes in insulin sensitivity, blood sugar levels, blood fats, or inflammatory markers, following a two-week intervention.

“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,” says study lead Olga Ramich. She is the head of the Department of Molecular Metabolism and Precision Nutrition at the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) as well as professor at the Charité – Universitätsmedizin Berlin.

Conflicting research

There is no clear definition of intermittent fasting, as it takes on different forms of time-based schedules for cycling between periods of eating and periods of fasting. In one example from a previous paper, a four-day fast and calorie restriction on three non-consecutive days outperformed caloric restriction for weight loss.

TRE offers a promising approach to insulin resistance and diabetes, according to previous research. In rodent studies, the intervention protects against diet-induced obesity and related metabolic dysfunctions.

Similarly, TRE studies in humans have supported numerous positive cardiometabolic effects, including improved insulin sensitivity, glucose, triglyceride, and cholesterol levels.

Research findings suggest that TRE benefits on weight management may be diminished when caloric intake remains the same.They have also led to moderate reductions in body weight and fat. But contrary to common assumptions, the new study suggests that TRE benefits on weight management may be diminished when caloric intake remains the same.

The authors say previous TRE trials have been partly contradictory and have not yet clarified if the metabolic improvements are because of restricted daily eating, spontaneous calorie restriction, or a combination of the two.

“In fact, most previous studies have not carefully monitored energy intake or other potential confounding factors,” they underscore.

Eight-hour eating period

The team investigated if an eight-hour eating period could improve insulin sensitivity and other cardiometabolic parameters.

The scientists completed a randomized crossover design involving a total of 31 women with overweight or obesity. Over two weeks, the participants consumed their usual meals either early, between 8 a.m. and 4 p.m., or late, between 1 p.m. and 9 p.m..

The women’s calorie and nutrient composition remained nearly identical.

Over four examinations, the participants submitted blood samples and completed an oral glucose tolerance test that examined the influence of TRE on glucose and fat metabolism, alongside other metabolic markers.

The study authors used continuous glucose monitoring to observe 24-hour glucose levels while simultaneously documenting food intake. They controlled physical activity using a motion sensor, while the researchers also studied the participants’ internal body clock in isolated blood cells.

No significant improvements

Even while the participants showed no marked metabolic improvements, the analysis of their internal clock in blood cells revealed that TRE influenced the circadian phase in blood cells and their sleep timing.

The internal clock was, on average, shifted back by 40 minutes after the late intervention compared to the early intervention, and participants who followed the late intervention went to bed and awoke later. 

“The timing of food intake acts as a cue for our biological rhythms — similar to light,” says first author Beeke Peters from DIfE.

The results support that calorie reduction plays a central role in the health benefits of intermittent fasting. “Those who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their energy balance,” Ramich concludes.

Future studies should clarify whether a specific timing of TRE, in combination with a reduced calorie intake, provides additional benefits and how individual factors, such as chronotype or genetics, influence these effects.

The results were published in the journal Science Translational Medicine.