Does fasting earlier in the day improve metabolism? This study says no

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Eating within an eight-hour window altered participants’ internal circadian timing but did not improve glucose control, cholesterol levels, or inflammation, suggesting that meal timing alone may not be sufficient to enhance metabolic health.

Study: Intended isocaloric time-restricted eating shifts circadian clocks but does not improve cardiometabolic health in women with overweight. Image Credit: goffkein.pro / Shutterstock

In a recent article in Science Translational Medicine, researchers investigated whether Time-Restricted Eating (TRE) enhances circadian rhythms, cardiometabolic health, and insulin sensitivity in women with obesity or overweight status.

Their findings indicate that neither late TRE (lTRE) nor early TRE (eTRE) improved insulin sensitivity or other metabolic markers after controlling for caloric intake, although the end-of-intervention circadian phase was 40 minutes later after lTRE, and sleep timing was 15 minutes later compared to eTRE.

Defining Time-Restricted Eating and Its Rationale

TRE is a type of intermittent fasting where daily food intake is confined to a period of 10 hours or less, with at least 14 hours of fasting each day. It has gained popularity as a simple method to manage body weight and improve metabolic health.

Animal studies indicate that TRE can prevent obesity and related metabolic issues, while human trials have reported benefits, including improved glucose control, lipid levels, blood pressure, and reduced body fat. However, these findings are inconsistent across studies.

Uncertain Mechanisms Behind Reported Benefits

It remains uncertain whether these metabolic benefits arise from the restricted eating window itself, from reduced calorie intake and weight loss, or a combination of both. Many previous studies did not tightly monitor calorie intake or physical activity.

This trial, therefore, aimed to test whether an 8-hour TRE schedule could enhance insulin sensitivity and other cardiometabolic measures under controlled, isocaloric conditions. Additionally, the researchers compared eTRE and lTRE eating schedules, as earlier eating may align better with circadian rhythms and yield greater benefits.

Because previous research often excluded women, this study focused exclusively on women with overweight or obesity to assess TRE’s metabolic and circadian effects more accurately.

Study Design: Comparing Early and Late TRE

Researchers conducted a 10-week randomized crossover trial that compared early (8 am–4 pm) and late (1 pm–9 pm) TRE in overweight or obese women. Each two-week intervention was separated by a 2-week washout and preceded by a baseline period.

Thirty-one participants were randomized based on age and body mass index (BMI) and attended study visits for blood tests, body measurements, and oral glucose tolerance tests (OGTT). Throughout the baseline and intervention periods, participants wore glucose monitors and activity trackers and recorded data on food, sleep, and weight.

Controlled Conditions and Measured Outcomes

Participants were instructed to maintain their normal diets, activities, and sleep patterns, consuming food only within the assigned eight-hour window. Insulin sensitivity (Matsuda index) from OGTT was the primary outcome, while glucose, lipids, blood pressure, inflammatory markers, and circadian gene expression were secondary outcomes.

Blood and CGM data were processed and analyzed under standardized laboratory conditions by investigators who were blinded to the intervention assignment. Physical activity and sleep were objectively tracked and assessed through questionnaires. Statistical analyses used linear mixed models and paired tests.

Participant Profile and Adherence Rates

The trial examined the effects of early and late TRE in 31 women with a mean BMI of 30.5 kg/m² and a median age of 62 years. All participants completed the study with high adherence (>96%) to the eating schedules.

Energy intake declined modestly during eTRE (−167 kcal) but remained essentially unchanged in lTRE (−97 kcal/day, P = 0.06), and physical activity did not differ between phases.

Weight, Fat, and Glucose Responses to TRE

Both interventions resulted in small but significant weight loss, which was greater in eTRE (−1.08 kg) than in lTRE (−0.44 kg), accompanied by corresponding decreases in BMI. Fat and lean mass decreased only during eTRE.

Fasting glucose dropped slightly in eTRE but rose marginally in lTRE, though insulin sensitivity and mean 24-hour glucose levels remained unchanged. Intraday glucose variability increased in eTRE, whereas interday variability decreased in lTRE, while overall mean glucose remained stable.

Minor impairments in oral glucose tolerance and insulin secretion were observed during eTRE due to the longer fasting duration before testing; however, these did not affect the continuous glucose monitoring results.

Lipid Profiles, Liver Enzymes, and Inflammatory Markers

Neither eating schedule altered blood pressure, low-density lipoprotein (LDL) cholesterol, or triglycerides; however, high-density lipoprotein (HDL) cholesterol decreased significantly in both interventions.

Liver enzyme GGT declined in both conditions, while AST and ALT remained unchanged, indicating possible but unconfirmed hepatic effects.

Inflammatory and oxidative stress markers remained unchanged, although leptin levels decreased in both interventions, while adiponectin levels decreased only during eTRE.

Appetite Hormones and Hunger Perception

Participants reported reduced hunger and eating desire during lTRE mornings, with similar satiety across conditions. PYY levels rose in lTRE and fell in eTRE, whereas ghrelin remained unchanged.

These hormonal shifts suggest that the timing of eating may influence appetite regulation independently of caloric intake.

Gene Expression and Circadian Regulation

Gene expression analyses indicated downregulation of circadian genes PER1 and NR1D1 during eTRE, supporting a link between eating timing and peripheral clock regulation.

Both eTRE and lTRE shifted internal circadian timing, highlighting meal timing as a modulator of biological rhythms.

Interpretation and Clinical Implications

This randomized crossover trial found that neither eTRE nor lTRE improved insulin sensitivity or cardiometabolic and inflammatory markers when calorie intake was kept nearly constant.

Despite high adherence and precise control of diet and activity, the metabolic benefits observed in previous studies were not seen, likely reflecting that calorie restriction, rather than meal timing alone, drives improvements in metabolic health.

Both interventions, however, shifted circadian timing, confirming that food intake acts as a cue for internal clocks.

Study Strengths, Limitations, and Future Directions

Participants in eTRE reported greater morning hunger, possibly linked to hormonal changes in PYY levels.

Strengths include the tightly controlled isocaloric design and high compliance, while limitations include the short two-week duration, all-female sample, and indirect body composition measures.

Including sleep-timing data (10 minutes earlier in eTRE and 15 minutes later in lTRE) provided further support for meal timing as a circadian cue.

Overall, the study suggests that altering meal timing without calorie reduction may not yield metabolic benefits, and future research should investigate longer, calorie-restricted trials and individual responses to different eating schedules.