If you’re not engaging in intermittent fasting yourself, chances are you know of someone who is. Time-restricted eating has become a popular approach to losing weight and improving metabolic health—but how effective is it really?
In a paper published in October in the journal Science Translational Medicine, a team of researchers investigated the effects of time-restricted eating (TRE, a form of intermittent fasting) in overweight women over a course of two weeks. Their results reveal that TRE without changing the amount of calories consumed doesn’t actually ameliorate metabolism or cardiovascular variables in a measurable way, but it does change our internal clocks.
Eating within a set time period
Time-restricted eating involves consuming all food within a daily window of no more than 10 hours, followed by a fast lasting at least 14 hours. People undertake TRE to control their weight and improve their metabolism (how we convert food and drink into energy and carry out essential chemical processes in our cells). In fact, human studies indicate that TRE leads to beneficial cardiometabolic results, including enhanced glucose, triglyceride, and cholesterol levels; insulin sensitivity; and some weight and fat loss. As such, TRE could be an effective tool for fighting diabetes and insulin resistance.
However, earlier TRE trials have not been completely consistent, nor have they revealed if the metabolic benefits come from people eating during shorter periods of time, naturally eating less, or both. Within this context, researchers carried out a two-week trial with 31 overweight or obese women in which they each ate their usual calorie and nutrient intake (isocaloric) either between 8 a.m. and 4 p.m. (eTRE) or between 1 p.m. and 9 p.m. (lTRE).
Throughout the two weeks, the researchers measured the women’s metabolic parameters, glucose levels, food intake, physical activity, and internal clock. Our bodies are governed by biological day-night rhythms (circadian clocks) driven by molecular processes. Most cells in our body have an internal clock that is sensitive to influences such as light, exercise, and nutrition.
No obvious metabolic improvements
The results could be bad news for time-restricted eaters. After the two-week period, researchers didn’t find any significant shifts in insulin sensitivity, blood sugar levels, blood fats, or inflammatory markers. In other words, they didn’t observe any clear metabolic benefits.
“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,” Olga Ramich, a co-author of the study and Head of the Department of Molecular Metabolism and Precision Nutrition at the German Institute of Human Nutrition Potsdam-Rehbruecke, said in a statement by the German Center for Diabetes Research.
Changes in internal clocks and sleep
However, Ramich and her colleagues did find TRE to have an impact on sleep timing and blood cells’ individual rhythms. On average, the lTRE schedule was followed by an internal clock delay of 40 minutes in comparison to the eTRE schedule. What’s more, the women who participated in the lTRE schedule went to bed later and woke up later.
“The timing of food intake acts as a cue for our biological rhythms—similar to light,” explained Beeke Peters, first author of the study and also from the Department of Molecular Metabolism and Precision Nutrition.
Ultimately, the study highlights that intermittent fasting’s health improvements are centered on simply eating less. “Those who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their energy balance,” said Ramich.