To keep weight under control, it is not only crucial to pay attention to what we eat, but also when 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: a longer fasting period overnight and an early breakfast. This study was conducted by the Barcelona Institute for Global Health (ISGlobal), a center supported by the “la Caixa” Foundation.

Links Between Eating Habits and Body Weight

The study involved more than 7,000 volunteers aged 40 to 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 returned to the research team for a follow-up visit, where their measurements were taken again and new questionnaires were completed.

“Our findings are consistent with other recent studies and suggest that extending the nighttime fasting period in conjunction with an early dinner and early breakfast may help maintain a healthy weight. We suspect that this is because an earlier meal is more in line with the circadian rhythm and allows for better calorie burning and appetite regulation, which can contribute to maintaining a healthy weight. However, it is still too early to draw definitive conclusions, so recommendations can only be made once more solid evidence is available,” explains Luciana Pons-Muzzo, who was working at ISGlobal at the time of the study and is currently working at the IESE Business School.

Gender Differences

Analysis of the data by gender revealed that, compared to men, women generally had a lower BMI, adhered more strictly to the Mediterranean diet, were less likely to consume alcohol, had poorer mental health, and were more likely to be responsible for household or family care.

The team used a statistical technique called “cluster analysis” to group individuals with similar characteristics. The results of this analysis revealed a small group of men whose first meal of the day was after 2 p.m. and who fasted for an average of 17 hours. Compared to the others, these men tended to have a less healthy lifestyle (more likely to smoke, drink alcohol, engage in less physical activity, and adhere less to the Mediterranean diet), had a lower level of education, and were more likely to be unemployed. These patterns were not observed in any of the women’s groups.

Intermittent Fasting

Camille Lassale, ISGlobal researcher and lead co-author of the study, noted that there are different ways to practice so-called “intermittent fasting,” and this study refers to one of them, namely fasting overnight. What the researchers observed in a subgroup of men who fast intermittently by skipping breakfast is that this practice has no effect on body weight. Other intervention studies with overweight participants have shown that this method is no more effective than reducing calorie intake in the long term for reducing body weight.

“Our research is part of a new field of research called ‘chrononutrition’, which looks not only at what we eat, but also at the times of day and frequency of our meals,” explains Anna Palomar-Cros, who was working at ISGlobal at the time of the study and is currently at IDIAP Jordi Gol. “This research is based on the finding that unusual eating habits can conflict with the circadian system, i.e., the internal clocks that regulate the day-night rhythm and the associated physiological processes,” she adds. This study follows on from a series of research projects on chrononutrition conducted by ISGlobal, which have resulted in two other studies with similar findings being published in recent years. These studies observed that eating an early dinner and an early breakfast was associated with a lower risk of cardiovascular disease and type 2 diabetes, respectively.