Intermittent fasting may outpace traditional calorie counting, diet research suggests

Intermittent fasting may offer similar benefits to conventional calorie-restricted weight loss diets, according to Colombia (US) researchers in an analysis of trial evidence published in The BMJ.

The eating pattern cycles between periods of eating and fasting on a regular schedule and is becoming a popular alternative to traditional calorie-restricted diets, which the researchers say are often unsustainable in the long term.

The researchers add that alternate-day fasting also demonstrates “greater benefits” compared with both calorie restriction and other intermittent fasting approaches, but note longer trials are needed.

“The value of this study is not in establishing a universally superior strategy but in positioning alternate-day fasting as an additional option within the therapeutic repertoire,” say researchers from Colombia in a linked editorial.

They point out that any structured intervention — including continuous energy restriction — could show benefits derived not only from the dietary pattern but also from professional support, planning, and nutritional education, while diet quality during free eating days could also affect alternate-day fasting outcomes.

The focus should be on fostering sustainable changes over time, the authors explain. “Intermittent fasting does not aim to replace other dietary strategies but to integrate and complement them within a comprehensive, patient-centred nutritional care model.”

Research methodology

While there is no clear definition for intermittent fasting, its various methods can fall under three broad categories: time-restricted eating (such as the 16:8 diet involving a 16-hour fasting period followed by an eight-hour eating window), alternate-day fasting (a 24-hour fast on alternate days), and whole-day fasting (such as a 5:2 diet involving five days of unrestricted eating and two days of fasting).

Intermittent fasting lacks a precise definition, but it generally encompasses three main approaches: time-restricted eating (e.g., the 16:8 diet with 16 hours of fasting and an eight-hour eating window), alternate-day fasting (24-hour fasts every other day), and whole-day fasting (e.g., the 5:2 diet with two fasting days and five unrestricted eating days).

Despite these methods, the health effects of intermittent fasting compared to continuous calorie restriction or an unrestricted diet are not well understood, according to Columbia researchers.

To investigate this, they analyzed data from 99 randomized clinical trials. These trials involved 6,582 adults (average age 45, 66% female) with an average BMI of 31; nearly 90% had pre-existing health conditions. The trials varied in length (three to 52 weeks, average 12 weeks) and quality, but researchers used established tools to assess bias and evidence certainty.

Alternate-day fasting also demonstrates “greater benefits” compared with both calorie restriction and other intermittent fasting approaches, but note longer trials are needed.The findings indicate that all intermittent fasting methods and continuous calorie restriction can lead to modest weight loss when compared to an unrestricted diet. Alternate-day fasting was the only intermittent fasting strategy that showed a slight advantage in weight reduction compared to continuous calorie restriction (an average of 1.29 kg more weight loss).

It also resulted in slightly more weight loss than both time-restricted eating (1.69 kg more) and whole-day fasting (1.05 kg more). However, these differences were not significant enough to meet the researchers’ definition of clinically meaningful weight loss (at least 2 kg) for individuals with obesity.

Regarding other health markers, alternate-day fasting was associated with lower levels of total and “bad” cholesterol when compared to time-restricted eating. Conversely, time-restricted eating led to a small increase in cholesterol levels when compared to whole-day fasting.

No benefits were observed for blood sugar or “good” cholesterol levels across any of the diet comparisons. While estimates were consistent in trials shorter than 24 weeks, longer trials (24 weeks or more) only showed weight loss benefits for all diet strategies when compared to an unrestricted diet.

Limitations to diet strategy comparisons

The researchers point out several limitations, such as high variation (heterogeneity) among the diet strategy comparisons, small sample sizes of many included trials, and low to moderate certainty of evidence in most of the investigated outcomes.

Even so, they highlight this study as one of the first systematic reviews to combine direct and indirect comparisons across all dietary strategies, allowing for more precise estimates.

“The current evidence provides some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors. Longer duration trials are needed to further substantiate these findings,” they conclude.

According to the World Health Organization in 2022, approximately 2.5 billion adults, 43% of the global adult population, were overweight, and about 890 million (16%) lived with obesity.