Dear Editor:

We have read with great interest the article "Intermittent fasting scheme and reduction of anthropometric measurements, lipid profile, blood pressure and cardiovascular risk" published by Dr Javier Wong-Gonzáles et al¹, in number 1, volume 22 of your magazine; where the purpose of the research focuses on the assessment of the efficacy of intermittent fasting as a strategy for the modification of anthropometric parameters and cardiovascular risk variables; We would like to contribute the importance of defining the times of the day in which the periods of food intake and abstinence are framed during intermittent fasting, since the induced metabolic effects are highly dependent on circadian fluctuations².

Intermittent fasting is a dietary approach that implements periods of energy restriction, ranging from 12 hours to days on a regular basis³. Of the different types of methodologies for conducting intermittent fasting, time-restricted eating is the one that has gained the most popularity in recent years, which consists of limiting the daily eating window⁴. This feature makes the strategy highly dependent on circadian cycles and their impact on energy and nutrient metabolism. In favor of this argument, there are both studies in animals that have shown that food intake serves as a regulator of peripheral clocks such as those of the liver, brain, adipose tissue and muscle², as well as those carried out in humans that show that Circadian disruptions due to shift work and the systemic nocturnal feeding pattern generate metabolic alterations ^4,5.

A 16/8 intermittent fasting scheme like the one designed in the research could have notable differences in both anthropometric (weight, BMI, waist circumference) and metabolic results depending on the time at which fasting begins. For example, eating dinner at 10 PM has been shown to produce a postprandial response with higher plasma glucose, delayed triglyceride peaks, and less oxidation of dietary fatty acids and FFA, compared to dinner patterns. earliest⁶. Likewise, in patients with one or more cardiovascular risk factors, it has been found that systemic patterns of nocturnal feeding and nocturnal overeating are associated with subclinical vascular damage⁵. In contrast, limiting intake to the morning hours has been shown to lead to better responses in insulin sensitivity, pancreatic beta function, blood pressure, inflammation, and oxidative stress⁴.

On the other hand, not having had a comparison group that was subject to daily calorie restriction limits the interpretation of the results, which were favorable. In support of the above, a recently published meta-analysis comparing the short-term ( < 3 months), medium-term (3-12 months) and long-term ( > 12 months) effects of intermittent fasting with daily caloric restriction, found that there are no differences regarding BMI, waist circumference, or blood pressure between both dietary approaches, in the medium term. Likewise, although weight loss was greater for intermittent fasting compared to caloric restriction, it did not become clinically significant⁷. Considering that according to what was reported by the study published in this journal, 62.5% of the volunteers were in a hypocaloric regimen while performing the intermittent fasting scheme¹, it is difficult to analyze the impact of time-restricted feeding and that of the hypocaloric effect.

With all of the above, this letter aims to encourage the development of research taking into account the points described above, where the development of intermittent fasting protocols contemplates that the times of the day in which the intake is circumscribed have a direct impact on the rhythms circadian, with the consequent modification of the metabolic state and control of body weight. Likewise, the usefulness of including control groups in studies on time-restricted feeding will allow elucidating the independent effects that each of these dietary approaches generate.