Does Time-Restricted Eating Make a Difference?
Have your friends or family asked you about intermittent fasting, AKA time-restricted eating? This popular method of dieting limits your “feeding window”, and has been advised as a way to lose weight and improve blood sugar.
However, new research suggests that if calories aren’t reduced during the fasting state, metabolic or heart health will not improve. The study was done at the German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE) and Charité – Universitätsmedizin, Berlin.
Oddly enough, the timing of meals did affect the body's internal clocks. These results come from the ChronoFast study led by Prof. Olga Ramich and were published in Science Translational Medicine.
Time-restricted eating (TRE) is a type of intermittent fasting. It advises limiting food intake to a 10-hour timeframe or less, followed by a 14-hour fasting window. Research on animals finds that TRE can prevent diet-related obesity and metabolic problems in rodents.
Previous studies in humans found benefits, including improved insulin sensitivity, lower blood sugar and cholesterol, and small reductions in weight and body fat. TRE has been seen as a potential tool for staving off insulin resistance and diabetes.
Conflicting Evidence from Previous Studies
Although it’s popular, older research on TRE had mixed results. Several studies weren’t able to find whether observed improvements in health parameters were from shorter eating windows, calorie reduction, or a combined effect. Additionally, most previous studies didn’t track calories carefully or control other factors that could impact metabolic outcomes.
Prof. Olga Ramich, Head of the Department of Molecular Metabolism and Precision Nutrition at the DIfE and Professor at the Charité -- Universitätsmedizin Berlin, designed the ChronoFast trial to focus on the gaps. The goal was to evaluate whether an eight-hour eating window could improve insulin sensitivity and other metabolic markers when calorie intake was kept consistent.
How the ChronoFast Study Was Done
The small study utilized a randomized crossover design with 31 women who were overweight or obese. Subjects adhered to two different eating schedules for two weeks at a time. One schedule included an early time-restricted eating pattern from 8 a.m. and 4 p.m.(eTRE). The other used a later timeframe from 1 p.m. to 9 p.m. (lTRE). During both phases, subjects consumed similar meals with the same calorie and nutrient content (isocaloric).
Blood samples were collected by the researchers during four clinic visits. They performed oral glucose tolerance tests to evaluate glucose and fat metabolism. Continuous glucose monitoring monitored blood sugar levels within 24 hours, while food intake was tracked in detail. Physical activity was tracked with a motion sensor. Along with Prof. Achim Kramer from the Charité -- Universitätsmedizin Berlin, the team also evaluated changes in the body's internal clock using isolated blood cells.
Measuring the Body's Internal Clock
Human physiology follows internally created rhythms that nearly align with the length of a day. This is why they’re known as circadian clocks (Latin: circa and dia). These rhythms help control almost every biological process, such as sleep and metabolism. Nearly all cells in the body house their own internal clock, which can be affected by light, exercise, and food timing.
Prof. Dr. Achim Kramer developed the BodyTime assay to measure subjects’ circadian phases. This test needs one single blood sample and provides an objective glimpse of a person's internal timing. The ChronoFast study used this method and discovered that eating schedules can move human internal clocks.
No Metabolic Improvements Observed
Despite expectations based on previous studies, the ChronoFast study did not see clinically significant changes in insulin sensitivity, blood sugar, blood fats, or inflammatory markers after the two-week trials. "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," explains Ramich.
Although metabolic measures did not change much, the timing of meals did affect circadian rhythms. Blood cell analysis showed that the internal clock shifted by roughly 40 minutes during the late-eating schedule, compared to the earlier schedule. In addition, subjects following the later eating schedule went to bed and woke up later. First author Beeke Peters notes, "The timing of food intake acts as a cue for our biological rhythms -- similar to light.”
Calories and Individual Timing Could Matter Most
The results highlight the necessity of caloric balance to gain health benefits from intermittent fasting. Ramich concludes, “Those who want to lose weight or improve their metabolism should pay attention not only to the clock, but also to their energy balance”.
Future studies should evaluate whether combining time-restricted eating with calorie reduction ends in improved health parameters. Researchers also aim to evaluate how individual traits, such as chronotype and genetics, could impact how individuals respond to different eating times.
If your clients are considering time-restricted eating, the following tips could help:
· Look at the quality of your diet and not just the quantity or timing of meals. Limit or avoid ultra-processed foods such as frozen dinners, fast food, and high-calorie snacks.
· Don’t skip ANY meals. Meal skipping is associated with increased risk of all-cause mortality. 2
· Eat breakfast. Breakfast eaters have less of an appetite by dinner, and a lower risk for cardiovascular disease. 3,4
· Cut out after-dinner snacking. This may reduce your risk of acid reflux and improve sleep.
· Follow your hunger cues, not just the clock. Pay attention to a growling stomach, fatigue, or headache.
Lisa Andrews, MEd, RD, LD
References:
Beeke Peters, Julia Schwarz, Bettina Schuppelius, Agnieszka Ottawa, Daniela A. Koppold, Daniela Weber, Nico Steckhan, Knut Mai, Tilman Grune, Andreas F. H. Pfeiffer, Andreas Michalsen, Achim Kramer, Olga Pivovarova-Ramich. Intended isocaloric time-restricted eating shifts circadian clocks but does not improve cardiometabolic health in women with overweight. Science Translational Medicine, 2025; 17 (822) DOI: 10.1126/scitranslmed.adv6787
Sun Y, Rong S, Liu B, Du Y, Wu Y, Chen L, Xiao Q, Snetselaar L, Wallace R, Bao W. Meal Skipping and Shorter Meal Intervals Are Associated with Increased Risk of All-Cause and Cardiovascular Disease Mortality among US Adults. J Acad Nutr Diet. 2023 Mar;123(3):417-426.e3. doi: 10.1016/j.jand.2022.08.119. Epub 2022 Aug 11. PMID: 35964910.
Leonie C. Ruddick-Collins, Peter J. Morgan, Claire L. Fyfe, Joao A.N. Filipe, Graham W. Horgan, Klaas R. Westerterp, Jonathan D. Johnston, Alexandra M. Johnstone. Timing of daily calorie loading affects appetite and hunger responses without changes in energy metabolism in healthy subjects with obesity. Cell Metabolism, 2022; DOI: 10.1016/j.cmet.2022.08.001
Palomar-Cros A, Andreeva VA, Fezeu LK, Julia C, Bellicha A, Kesse-Guyot E, Hercberg S, Romaguera D, Kogevinas M, Touvier M, Srour B. Dietary circadian rhythms and cardiovascular disease risk in the prospective NutriNet-Santé cohort. Nat Commun. 2023 Dec 14;14(1):7899. doi: 10.1038/s41467-023-43444-3. PMID: 38097547; PMCID: PMC10721609.