Intermittent Fasting: What the Largest Clinical Study Says About Its Weight Loss Benefits

Overweight and obesity affect almost half of the world's adult population. In 2022, according to data from the World Health Organization (WHO), more than 2.5 billion people were overweight and around 890 million were living with obesity.

In this context, intermittent fasting was positioned as an alternative to traditional low-calorie diets.

Now, a new meta-analysis published in The BMJ analyzed 99 clinical trials and concluded that intermittent fasting provides similar results to continuous calorie-restricted diets, with some additional benefits in the alternate-day format.

What is intermittent fasting and how is it classified

Intermittent fasting is an eating pattern that alternates periods of eating with periods of abstinence, following schedules that vary according to the frequency, duration, and timing of meals. Unlike traditional diets that focus on reducing calories, intermittent fasting modifies the timing of food intake.

The most popular strategies can be grouped into three main categories:

Alternate-day fasting: involves fasting completely for 24 hours, every other day.

Full-day fasting: such as the 5:2 diet, where you fast two non-consecutive days per week and eat without restriction the other five days.

Time-restricted eating: limits the daily eating window, as in the 16:8 diet (16 hours of fasting, 8 hours for eating).

These variations have gained popularity due to their apparent flexibility and the perception that they can be more sustainable over time, compared to traditional low-calorie diets.

The new evidence: review of 99 clinical trials

To scientifically compare these strategies with other forms of dietary control, an international team led by nutritionist John Sievenpiper of the University of Toronto conducted a systematic review and network meta-analysis.

The study, published in The BMJ, examined 99 randomized clinical trials that included 6,582 adults from 24 countries.

Participants had an average age of 45 years, a mean BMI of 31, and 90% of them had preexisting health conditions.

The trials ranged in duration from 3 to 52 weeks, with a median of 12 weeks.

The objective was to evaluate the impact of intermittent fasting in its three forms versus continuous calorie restriction or unrestricted (ad libitum) diets on both body weight and cardiometabolic risk factors: blood pressure, blood glucose, lipid profile, and inflammatory markers.

Among the notable results, it was observed that:

All forms of fasting and continuous calorie restriction reduced body weight compared to an ad libitum diet.

Alternate-day fasting was the only modality that showed an additional difference compared to continuous calorie restriction, with an average weight loss of 1.29 kg.

Compared to other forms of fasting, alternate-day fasting also achieved better results: 1.69 kg more weight loss than time-restricted eating, and 1.05 kg more than full-day fasting.

Alternate-day fasting also achieved greater reductions in total and LDL cholesterol compared to the other strategies.

These differences, however, did not reach the clinically significant threshold of 2 kg that the study authors themselves defined for people with obesity. Even so, the data support the potential of intermittent fasting as a valid strategy for short-term weight loss.

Limitations noted by the authors

The study also cautioned about several factors that should be considered before generalizing the results. The high heterogeneity among the included trials—with differences in duration, type of participants, interventions, and methodologies—limits the ability to draw definitive conclusions.

Furthermore, many of the analyzed studies were small and had an average follow-up of only 12 weeks.

In the group of 17 trials lasting 24 weeks or longer, the benefits were observed only when compared to uncontrolled diets, but there were no sustained advantages compared to calorie restriction or between different fasting modalities.

The researchers assessed the certainty of the evidence using the GRADE system, which determined low to moderate quality for most outcomes. They also emphasized that the lack of uniformity in diet quality during free-feeding days could have influenced the observed effects.

What the experts say

For Jordi Salas Salvadó, a physician and professor of nutrition at the Rovira i Virgili University in Catalonia, Spain, and one of the study's co-authors, the findings should not be interpreted as a direct indication for the indiscriminate application of intermittent fasting. "There's no need to follow any of these methods if you already have a healthy weight. And these results don't mean that all obese people should start intermittent fasting," he explained. Instead, he maintained that it is another tool to consider, especially useful for people who tire of conventional diets.

In an editorial related to the study, a group of researchers from Colombia emphasized that the work does not attempt to establish a definitive hierarchy among methods, but rather to expand the available therapeutic repertoire. The true contribution of the study, they noted, is to position alternate-day fasting as an effective option, as long as it is integrated into a comprehensive and personalized nutritional care model.

Furthermore, they emphasized that the benefits obtained are not only due to the dietary pattern itself, but also to the professional support, meal planning, and dietary education that are often part of structured interventions.

“The focus should be on promoting sustainable changes over time ” they maintained. “Intermittent fasting is not intended to replace other dietary strategies, but rather to integrate and complement them within a patient-centered model.”