Keeping a longer overnight fast and eating an early breakfast may be associated with a lower body mass index

To keep weight in check, it is not only important to consider what we eat, but also the times at which we eat. According to a study published in the International Journal of Behavioral Nutrition and Physical Activity, there are two specific habits that are associated with a lower body mass index (BMI) in the long term: keeping a longer overnight fast and eating breakfast early. This research was led by the Barcelona Institute for Global Health (ISGlobal).

The study involved more than 7,000 volunteers aged 40-65 from the GCAT | Genomes for Life cohort, a project led by the Germans Trias i Pujol Research Institute (IGTP). In 2018, participants answered questionnaires about their weight and height, eating habits including meal times, other lifestyle habits and socioeconomic status. In 2023, after five years, more than 3,000 participants made a follow-up visit to the research team, where their measures were registered again and new questionnaires were completed.

“Our results, in line with other recent studies, suggest that extending the overnight fast could help maintain a healthy weight if accompanied by an early dinner and an early breakfast. We think this may be because eating earlier in the day is more in line with circadian rhythms and allows for better calorie burning and appetite regulation, which can help maintain a healthy weight. However, it is too soon to draw definitive conclusions, so recommendations will have to wait for more robust evidence,” explains Luciana Pons-Muzzo, researcher at ISGlobal at the time of the study and currently at IESE Business School.

Gender differences

Analysis of the data by gender showed that, compared to men, women generally showed lower BMI, higher adherence to the Mediterranean diet, lower propensity to consume alcohol, poorer mental health, and were more likely to be responsible for household or family supervision.

The team used a statistical technique called ‘cluster analysis’ to group individuals with similar characteristics. From the results of this analysis, the authors were struck by a small group of men whose first meal of the day was after 14:00 and who, on average, fasted for 17 hours.

Compared to the rest, this group of men tended to have less healthy lifestyles (more likely to smoke, drink alcohol, less physical activity, less adherence to the Mediterranean diet), and had lower levels of educational attainment and were more likely to be unemployed. These patterns were not observed in any group of women.

On intermittent fasting

“There are different ways of practicing what is known as ‘intermittent fasting’ and our study relates to one of them, which is overnight fasting. What we observed in a subgroup of men who do intermittent fasting by skipping breakfast is that this practice has no effect on body weight. Other intervention studies in participants with obesity have shown that this tactic is no more effective than reducing calorie intake in reducing body weight in the long term,” says Camille Lassale, ISGlobal researcher and senior co-author of the study.

“Our research is part of an emerging field of research known as ‘chrononutrition,” which focuses not only on analyzing what we eat, but also the times of day and the number of times we eat,” says Anna Palomar-Cros, researcher at ISGlobal at the time of the study and currently at IDIAP Jordi Gol.

“At the basis of this research is the knowledge that unusual food intake patterns can conflict with the circadian system, the set of internal clocks that regulate the cycles of night and day and the physiological processes that must accompany them,” she adds.

This study provides continuity to a line of ISGlobal research on chrononutrition, which in recent years has published two other studies with results in the same direction. In these studies, it was observed that eating dinner and breakfast early was associated, respectively, with a lower risk of cardiovascular disease and type 2 diabetes.