When it comes to losing weight, intermittent fasting has similar benefits to calorie-restricted diets. This is suggested by a meta-analysis published this Thursday in The British Medical Journal.
According to this review, which analyzed the results of 99 previous studies on the subject, of three types of intermittent fasting (alternate-day fasting, time-restricted feeding, and whole-day fasting), the first could have a slightly greater impact than other strategies for body weight reduction, although researchers emphasize the need for larger studies to confirm these data.
According to data from the World Health Organization (WHO), approximately 43% of the global population - 2.5 billion people - are overweight, and about 16% - around 890 million - are obese. This excess weight is related to various cardiometabolic risk factors, such as hypertension, hypercholesterolemia, or diabetes, among other conditions.
Intermittent fasting has become popular in recent years as an alternative to conventional diets that involve restricting the number of calories consumed. There are different modalities to carry out this type of eating pattern, ultimately involving alternating long fasting periods with times for eating.
Some methods suggest combining 16 hours of fasting with an eight-hour eating window; others advocate alternating full fasting days with conventional eating days, and others recommend combining five days of unrestricted eating with two days of fasting. Specialists recommend consulting with a professional who can analyze the specific characteristics of each case and recommend the best personalized alternative before starting this type of eating pattern.
The authors of this research, scientists from the University of Toronto (Canada), aimed to analyze the effects of these different intermittent fasting plans compared to a diet that restricted calorie consumption and set a maximum, as well as with unrestricted eating. The goal was to determine the final impact of these different habits on the participants' body weight and various cardiometabolic risk factors.
To do this, they analyzed the results of 99 randomized clinical trials with data from 6,582 adults with an average age of around 45 years, of whom 66% were women. The participants had, on average, a body mass index (BMI) of 31, indicating obesity, and 90% had pre-existing conditions.
The study results showed that both different methods of intermittent fasting and continuous calorie-restricted diets achieved small reductions in body weight compared to an unrestricted diet.
The only method of intermittent fasting that showed a small benefit over calorie restriction in terms of weight loss was the plan that alternates fasting days with conventional eating days, a strategy that showed a 1.29 kg greater weight loss.
In their work, the scientists point out that the research has limitations due to the heterogeneity of the analyzed studies, the small size of some of the included studies, and the levels of evidence presented in some of the research included in the study.
"Current evidence provides some indications that intermittent fasting has similar benefits to continuous energy restriction for weight loss and cardiometabolic factors," the scientists state in a statement, while also noting that "longer-term trials are needed to substantiate these findings."
The value of this work is not to establish a universally superior strategy but to position intermittent fasting that alternates days as an additional option within the repertoire, according to an editorial accompanying the study in the medical journal.
"This meta-analysis [published by] The BMJ rigorously and comprehensively analyzes the impact of different intermittent fasting strategies on body weight and cardiometabolic risk factors, comparing them with traditional caloric restriction. The authors have done an excellent methodological job, combining direct and indirect data through a network of clinical trials, allowing for more precise estimates," stated Ismael San Mauro Martín, a specialist at the Research Center on Nutrition and Health (CINUSA Group), in comments to SMC Spain.
"It is particularly noteworthy how clearly they present the study's limitations (relatively short duration of trials, high heterogeneity among strategies, low to moderate certainty of evidence), adding value and transparency to their conclusions. Although weight loss benefits are modest, this work positions alternate-day fasting as a valid additional tool within personalized dietary approaches, without aiming to replace other strategies. In clinical practice, it could be useful as long as it is tailored to the patient's needs and characteristics," he added.
"This meta-analysis, mainly composed of small clinical trials, provides a general idea of the benefits of intermittent fasting, even if some of the included trials were not optimal. Overall, the results are not surprising, as intermittent fasting is not magical for weight loss beyond being another way to maintain a total caloric intake lower than usual. This helps people maintain a lower weight than usual. Therefore, it becomes another lifestyle option for weight control. It is worth examining if it is sustainable in the long term, while for those who need to lose much more weight, there are clearly other available options," added Naveed Sattar, Professor of Cardiometabolic Medicine and Honorary Consultant at the University of Glasgow (UK), also in comments to SMC Spain.