Obesity is a growing challenge for global health, since it is closely linked to a greater risk of chronic diseases such as diabetes, cardiovascular diseases and cancer. Since maintaining a long -term caloric restriction can be difficult, new nutritional strategies are emerging that put the focus not so much how you eat, but also when. A promising strategy in this regard is the fast -time fasting, a type of fast in which people eat within a limited daily window (usually 4 to 10 hours) and fast for the rest of the time. This type of fast has shown to be generally effective to reduce body weight and to produce slight improvements in cardiometabolic health without having calories, simply taking into account the number of hours eaten. Despite these promising results, questions are to be resolved.
Returning to obesity, one of the main problems derives from the accumulation of fat in the abdomen area. The abdominal fat comprises two main types: the visceral adipose tissue, located around the internal organs and related to greater cardiometabolic risk, and the subcutaneous adipose tissue, which is found just below the skin. What is the effect of fasting in time both on visceral and subcutaneous fat?
It matters not only how much, also when we eat
One of the important points of this type of fast is that not only does it matter what we eat, but also when we do it, since our circadian system regulates many metabolic processes throughout the day. Therefore, different fasting fasting protocols-early feeding vents (EG from 8:00 a.m. to 4:00 p.m.), late (EG from 1:00 p.m. to 9:00 p.m.) or self-selected-may have Different effects on heart health.
Does it produce the same effects on weight loss, fat or improvement of cardiomethabolic markers concentrate meals in the early hours of the day as in the last? Impact of fasting fasting on time on cardiometabolic health
With these questions in mind, our research group of the University of Granada together with that of Idoia Labayen, from the Public University of Navarra, we have recently published a study in Nature Medicine ( you can read it by clicking here ) 1 in which we investigate the effects of Three different schedules of restricted fasting: an 8 -hour feed window in the first part of the day (the participants began to eat before 10:00), another 8 hours concentrated in the last hours of the day (ate from 1:00 p.m.), and a self-selected feed window, in which participants chose their own 8-hour feeding window.
In addition, each group followed dietary guidelines that consisted of bimonnsual sessions of a group nutritional education program based on the Mediterranean diet . These three groups were compared to a control group that maintained their usual feeding window exceeding 12 hours and that only received the traditional advice based on Mediterranean diet. The duration of the study was 12 weeks and the changes in the visceral adipose tissue and the cardiometabolic health of men and women were investigated.
In addition, this randomized controlled trial was multicentric, developed in Granada and Pamplona, with a total of 197 participants (50% women) between 30 and 60 years overweight or obesity (body mass index ≥25.0 and <40.0 kg/m²) .
Weight and fat loss Is the fast restricted to the Mediterranean diet superior?
After 12 weeks we discovered that performing restricted fasting, regardless of the food window schedule, did not offer higher benefits in the loss of visceral fat with respect to the group that followed only the Mediterranean diet guidelines . However, early fasting produced a greater reduction in subcutaneous fat in the abdominal region compared to the only Mediterranean diet group.
Regarding body weight, all types of fasting foster greater weight loss compared to the control group. For example, the early fasting group lost 2.9 kg more than the control, that of late fasting 2.4 kg and that of self-selected fasting 3.1 kg ( Figure 1 ). These data show us that reducing the food window of more than 12 hours to only 8, regardless of the time of the day, is effective in losing weight . This was partly due to the fact that fasting groups reduced their caloric intake between 300-500 kcal, that is, between 20 and 25% their caloric intake very easily.
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It should be noted that there were no differences in fat or fat -free mass between the groups after the intervention of 12 weeks. These findings emphasize that restricted fasting can be a promising nutritional strategy for body weight management in overweight or obesity.
Figure 1 . Change in body weight and visceral fat. Adapted from dowry-montero et al. 1
Impact of restricted fasting on glucose homeostasis
Glycemia control is a parameter closely related to cardiometabolic health. In this research we also measure the fasting glucose levels before and after the intervention of 12 weeks and monitor glucose levels for 24 hours 2 weeks before and 2 weeks after finishing the study. This helped us to have a deeper vision about daily glycemic homeostasis.
What we discovered was that following an early fasting schedule (8 -hour window in the first part of the day) reduced fasting glucose levels as well as night glucose levels compared to the rest of the groups. It is important to note that the variability of 24-hour and daytime blood glucose, evaluated by variation coefficients, was greater in early fasting hours than in late and self-selected schedules, while the night glycemic variability was lower compared to comparison Traditional intervention. Consuming most of the calories in the early hours of the day after a 16 -hour fast, as occurs in early fasting, could cause more pronounced glucose peaks, however, the recorded values are within normality . More studies are needed to better understand the impact of intake schedules on glucose variability on overweight or obesity.
Figure 2. 24 hours of glucose before and during the last two weeks of the intervention. Adapted from dowry-montero et al. 1
Adverse effects
During the 12 weeks of intervention, the adhesion rates to the fast -time fast ranging between 85 and 88%, and no serious adverse events were reported, which suggests that restricted fasting could be a safe and well tolerated strategy for people overweight or obesity.
Conclusions and perspectives
These results suggest that reducing the food window, especially concentrating meals in the early hours of the day, is a safe strategy, easy to implement and with the potential to improve weight loss and glycemia regulation without having calories .
References
1. dow-doto, M. et al. Effects of Early, Late and self-selected time-recreated eating on visceral adipose tissue and cardiometabolic health in participants with overweight or obesity: a randomized controlled trial. Nat Med (2025) DOI: 10.1038/S41591-024-03375-Y.