A new study calls into question the effectiveness of time-restricted eating (TRE) in producing measurable improvements in metabolic or cardiovascular health when caloric intake is not reduced. However, the approach did have an impact on participants’ circadian rhythms.
TRE is a type of intermittent fasting in which the individual limits food consumption to 10 hours per day or less and fasts for at least 14 hours. It has become a popular way to control body weight and improve metabolic health, Olga Pivovarova-Ramich of the German Institute of Human Nutrition Potsdam-Rehbruecke, Nuthetal, and the Charité-Universitätsmedizin Berlin, Berlin, both in Germany, and colleagues wrote.
Previous research suggested that TRE may improve glucose concentrations, insulin resistance/insulin sensitivity, lipid concentrations, and blood pressure (BP) and reduce body weight and body fat. However, the findings have been “inconsistent,” and it’s unclear whether metabolic improvements result from restricting daily eating to specific times, from caloric reduction, or from a combination of both, the researchers noted. Moreover, previous TRE trials haven’t carefully monitored energy intake or other potential confounders.
Researcher, therefore, decided to conduct the ChronoFast trial to see whether 8-hour TRE with precisely controlled calorie intake, dietary composition, and physical activity would improve insulin sensitivity and cardiometabolic health.
Participants engaged in two different TRE schedules: early TRE (eTRE), in which they consumed their meals between 8 AM and 4 PM, and late TRE (lTRE), in which meals were consumed between 1 PM and 9 PM.
“Overall, in an intended isocaloric setting, neither eTRE nor lTRE improves insulin sensitivity or other cardiometabolic traits, despite a shift of internal circadian clocks,” the researchers concluded. “Our findings suggest the importance of calorie restriction for metabolic improvements in TRE.”
The study was published online in Science Translational Medicine.
Study Design
The trial included 31 women (median age, 62 years) with overweight or obesity (mean BMI, 30.5). All participants were White individuals, and 18 had normal glucose tolerance at baseline. Patients were excluded if they had type 1 or 2 diabetes or other endocrinologic disease.
Participants were randomly assigned to either begin with 2 weeks of eTRE followed by 2 weeks of lTRE with a 2-week washout period in between or vice versa. They were asked to maintain their usual diets, physical activity, and sleep patterns during the trial, which began with a 2- to 4-week baseline period.
Patients documented their food selection, amount, and consumption time and kept sleep and weight diaries. They provided blood samples and completed an oral glucose tolerance test before and after each intervention.
The primary outcome was insulin sensitivity, and secondary outcomes included glucose concentrations, glucose metabolism hormones, and mean 24-hour glucose, BP, lipids, adipokines and cytokines, anthropometric parameters, hunger and satiety scores, intervention adherence, physical activity, sleep, and the internal circadian phase, as measured by the BodyTime assay.
Cardiometabolic Parameters Unchanged
Patient adherence to the regimens was high. Energy intake remained unchanged in the lTRE and fell minimally in the eTRE (-167 calories/d) compared with baseline. Mean weight loss was -1.08 kg in eTRE and -0.44 kg in lTRE.
The researchers found no difference in insulin sensitivity between or within the interventions, nor were there clinically meaningful between- or within-intervention differences in 24-hour glucose, lipid, inflammatory, and oxidative stress markers.
Neither intervention affected systolic and diastolic blood pressure, total cholesterol, low density lipoprotein cholesterol, or triglyceride concentrations.
“The beneficial cardiometabolic effects described previously might be induced by TRE-mediated calorie restriction and not by the shortening of the eating window itself,” the authors wrote.
The circadian phase after lTRE was 40 minutes later than that after eTRE. Self-reported sleep timing parameters occurred later in lTRE than in eTRE. In fact, sleep timing advanced in eTRE, with a 12-minute earlier sleep offset and a 10-minute earlier sleep midpoint, compared with baseline. Sleep duration was not altered by either intervention.
Further studies should investigate whether time-based eating strategies can “resynchronize or restore circadian rhythms in individuals with circadian rhythm disorders,” the researchers wrote.
Significant Flaws
Satchidananda Panda, PhD, a professor at the Salk Institute for Biological Studies, San Diego, expressed concern about the study’s design, noting that participants’ baseline cardiometabolic parameters were normal. For example, the mean fasting glucose was < 100, mean A1c was < 5.6, median systolic BP was 117, median diastolic BP was 75, and mean total cholesterol was 5.63 mM.
“This is equivalent to taking healthy people without any glucose intolerance, high blood pressure or cholesterol, putting them on only 2 weeks of an intervention and expecting to see a clinically significant outcome,” said Panda, who was not involved in the study. “It would be like giving a headache medication to someone without a headache and then determining that the painkiller doesn’t work.”
Elizabeth Thomas, MD, associate professor, Division of Endocrinology, Metabolism and Diabetes, University of Colorado Anschutz in Aurora, Colorado, said that the study’s homogenous population limits its generalizability.
However, she said, the finding that meal timing induced changes in circadian phase is “novel and certainly deserves further investigation.”
Regarding the weight-loss finding in the eTRE group, Thomas noted that although it was modest, it occurred over just a 2-week period.
“If this trend had continued over a longer duration of time, it could certainly translate into a more significant amount of weight loss,” said Thomas, who wasn’t involved in the study. “This study shows that even when asking participants to maintain their usual food intake and physical activity, participants inadvertently reduced their energy intake and lost weight while following eTRE, suggesting eTRE may be an effective strategy to reduce energy intake and lose weight.”
This study was supported by grants from the German Research Foundation, the German Diabetes Association, and the European Association for the Study of Diabetes. The German Center for Diabetes Research is funded by the German Federal Ministry for Education and Research. Pivovarova-Ramich reported receiving lecture honoraria from Novo Nordisk. The other authors’ disclosures are listed on the original paper. Panda is the author of two books: The Circadian Code and The Circadian Diabetes Code. Thomas is conducting a randomized trial of early vs late TRE within the context of a behavioral weight loss intervention over a duration of 52 weeks.
Batya Swift Yasgur MA, LSW is a freelance writer with a counseling practice in Teaneck, New Jersey. She is a regular contributor to numerous medical publications, including Medscape and WebMD, and is the author of several consumer-oriented health books, as well as Behind the Burqa: Our Lives in Afghanistan and How We Escaped to Freedom (the memoir of two brave Afghan sisters who told her their story).