Maintaining a healthy weight depends not only on what you put on your plate, but also on when you eat. A new study published in the International Journal of Behavioral Nutrition and Physical Activity identifies two habits associated with a lower body mass index (BMI) over time: extending the overnight fast and eating an early breakfast.
The study, led by the Barcelona Institute for Global Health (ISGlobal), is based on data from more than 7,000 adults ages 40 to 65. In 2018, participants provided details about their height, weight, meal timing, lifestyle habits and socioeconomic background. Five years later, in 2023, more than 3,000 of them returned for follow-up assessments. Researchers collected updated data that enabled them to track changes and identify trends over time.
"Our results, which are consistent with other recent studies, suggest that extending the overnight fast may help maintain a healthy weight, provided it is accompanied by an early dinner and an early breakfast," explained researcher Luciana Pons-Muzzo. "We believe that eating earlier in the day is better aligned with our biological clock. This allows for more efficient calorie burning and improved appetite regulation. However, it is still too early to draw definitive conclusions, and we will need to wait for stronger evidence before making formal recommendations."
Women ate healthier, men who skipped breakfast ate less
When the data were analyzed by gender, researchers found notable differences. Overall, women had a lower BMI, adhered more closely to a Mediterranean diet and were less likely to consume alcohol. On the other hand, they reported poorer mental health and said they carried a greater share of the burden of managing the household and family.
Researchers also identified a distinct group of men who stood out: they tended to eat their first meal only after 2 p.m. and fasted for about 17 consecutive hours. Compared with other participants, men in this group smoked more, drank more alcohol, were less physically active and were less likely to follow a Mediterranean diet. They also tended to have lower levels of education and higher unemployment rates. Among women, by contrast, no similar pattern was observed.
"There are various ways to practice 'intermittent fasting,' and our study focuses on the overnight fast," noted senior researcher Camille Lassale. "What we saw in that group of men who skip breakfast in order to fast is that this practice simply has no effect on body weight. Other studies have already shown that this method is no more effective in the long term than simply reducing calorie intake."
It’s all about timing: When does the body digest best?
The current study is part of a growing field known as "chrononutrition" — which examines not only what we eat, but also at what times and how frequently. Another ISGlobal researcher, Anna Palomar-Cros, explained: "The basis of this research is the understanding that irregular eating patterns may conflict with our biological clock — the internal system that regulates day-night cycles and the physiological processes associated with them."
These findings build on previous research by the institute, which showed that earlier breakfast and dinner times are also associated with a lower risk of cardiovascular disease and type 2 diabetes. In short, the data continue to reinforce the understanding that meal timing plays a significant role in long-term health.
This trend is also supported by other recent studies. A comprehensive review and meta-analysis published in BMJ Medicine — a leading international medical journal from the British Medical Journal group — found that intermittent fasting can have a positive effect on metabolic health, but only when the eating window is set at the right hours. Researchers found that participants who limited their eating to the morning or midday hours achieved significantly better results in weight loss, fat reduction and lower blood sugar and blood pressure levels, compared with those who delayed their eating window into the evening.
The scientific explanation lies in the fact that the body’s systems — particularly insulin sensitivity and metabolic processes — function most efficiently during daytime hours. When calorie intake is aligned with the biological clock’s active phase, a concept known as "chronobiology," the body processes food more effectively. By contrast, the study emphasized that late eating, especially when combined with a prolonged eating window extending into the night, is the least effective dietary pattern for improving health markers and maintaining weight.