Eating in Any 8-Hour Window Daily Yields Durable Weight Loss

Medscape

13-05-2025

Restricting eating to an 8-hour window for 3 months improved weight loss in patients with overweight or obesity for at least a year, regardless of the timing of the eating window, according to preliminary results of a randomized controlled trial.
The strategy, called time-restricted eating (TRE), limits when — but not what — an individual eats on a daily basis.
'A key point of this study is that the timing of the 8-hour eating window may not be as critical as previously thought, suggesting flexibility in implementation,' Jonatan Ruiz, PhD, of the University of Granada, Granada, Spain, and the Spanish Biomedical Research Centre in Physiopathology of Obesity and Nutrition in Madrid, Spain, told Medscape Medical News .
The study, presented at the European Congress on Obesity (ECO) 2025 in Málaga, Spain, is an extension of an earlier randomized controlled trial that showed benefits of TRE at three different schedules over the short term (12-week follow-up).
'Low-Cost and Sustainable'
Ruiz and colleagues conducted the 12-month follow-up in 99 adults with overweight or obesity (50% women; average age, 49 years; average BMI, 32) from Granada. Participants were randomly assigned to one of four groups for 12 weeks: Habitual eating window of ≥ 12 hours; early TRE (8-hour eating window starting before 10:00 AM); late TRE (8-hour eating window starting after 1 PM); or self-selected TRE (allowing participants to choose their own 8-hour eating window).
All groups also took part in a Mediterranean diet education program to encourage healthy eating.
Researchers measured body weight and waist and hip circumferences at the start of the trial, after the 12-week intervention, and 12 months after the intervention ended.
After the intervention, the habitual eating group experienced an average weight loss of
–1.4 kg (–1.5%). All TRE groups achieved significantly greater weight loss: An average of –4.2 kg (–4.5%) in the early TRE group, –3.1 kg (–3.5%) in the late group, and –3.8 kg (–3.9%) in the self-selected group.
In addition, the habitual eating group had a lower waist circumference (–1.1 cm) and hip circumference (–1.4 cm) after 12 weeks of intervention.
In comparison, the early TRE group experienced significantly greater reductions in waist and hip circumferences (average, –4.1 and –4.6 cm, respectively).
The late TRE group also achieved a significant reduction in waist circumference (average, –4.1 cm), but there was no significant reduction in hip circumference (average, –3.2 cm).
The self-selected TRE group also achieved reductions in waist and hip circumferences (average, –3.7 and –3.6 cm, respectively), although the reductions were not significant.
One possible explanation for the less robust response in the self-selected group, Ruiz said, 'is that allowing participants to choose their own eating window may have led to greater variability in timing, potentially reducing the consistency of the fasting window. This is just a hypothesis we plan to explore further in future studies.'
At 12 months after the intervention ended, preliminary results reveal that the habitual eating group had an average body weight increase of 0.4 kg (+0.5%). By contrast, both the early TRE and the late TRE groups maintained significantly greater weight loss (average, –2.1% [−2.2 kg] and –2.0% [−2.0 kg], respectively).
The self-selected TRE group also maintained greater body weight loss than the habitual eating group (average, –0.7% [−0.7 kg]), although the change was not statistically significant.
Furthermore, the habitual eating group showed an increase in waist circumference of +1.8 cm at 12 months and a slight increase of +0.03 cm in hip circumference.
In comparison, both waist and hip circumferences remained significantly lower in the late TRE group (average, –5.6 and –3.4 cm, respectively) than the habitual eating group. And, although not statistically significant, the early TRE group and self-selected TRE group also showed lower values in waist (average, –0.5 and –1.3 cm, respectively) and hip circumferences (average, –1.0 and –1.8 cm, respectively).
'This makes TRE a potentially attractive, low-cost, and sustainable strategy to support weight management — particularly for patients who struggle with strict calorie counting or rigid diet plans,' Ruiz said. 'However, longer and larger trials are needed to confirm these findings across different populations.'
The team's next steps include combining TRE with exercise, he noted. 'We are currently finalizing the last measurements, and we are eager to see the outcomes.'
Expert Reactions
'One general note of caution on TRE protocols is that altering the opportunities to eat and the potential for meal skipping may compromise the nutritional adequacy of the diet,' Adam Collins, PhD, associate professor of nutrition, University of Surrey, Surrey, England, said in expert commentary in a press release on the presentation.
'This may be an issue for those whose diet was marginally nutritionally replete to start with,' he said. 'To this end, we are midway through a study exploring the impact of TRE specifically on eating behavior and nutritional adequacy of people's diets.'
Regarding the finding that the self-selected group didn't manage to keep weight off as successfully, he noted, 'It is possible that following the more regimented early and late TRE created more sustainable changes in eating behavior and dietary habits that remained after the intervention.'
Maria Chondronikola, PhD, principal investigator and lead for Human Nutrition, University of Cambridge, Cambridge, England, commented, 'It is important to note this study did not include a caloric restriction group, and therefore, its results cannot be directly compared with other weight loss strategies that involve intentional caloric restriction.'
'Furthermore, additional information on participant adherence to the prescribed eating windows is crucial,' she said. 'Understanding how well participants adhered to the timing of their meals, the level of their caloric intake, and whether TRE changed any obesity-related metabolic outcomes would provide valuable insight into the true effectiveness of TRE.'
Ruiz told Medscape Medical News that 'adherence was high (85%-88%)' and that 'no serious adverse events were reported.'
Ruiz and Collins declared no relevant interests. Chondronikola is currently leading an intervention study on the effects of TRE on cardiometabolic health in the United Kingdom and globally.