Intermittent fasting does not seem to work for overweight or obese adults

Intermittent fasting is unlikely to lead to greater weight loss in overweight or obese adults than traditional dietary advice or doing nothing, a new Cochrane review finds.

Obesity is a significant public health problem that has become a leading cause of death in high-income countries. Worldwide adult obesity has more than tripled since 1975, according to the WHO. In 2022, 2.5 billion adults were overweight. Of these, 890 million were living with obesity.

Intermittent fasting has surged in popularity in recent years, fuelled by social media, lifestyle influencers, and claims of rapid weight loss and metabolic benefits.

No meaningful difference in weight loss

Researchers analyzed evidence from 22 randomized clinical trials involving 1,995 adults across North America, Europe, China, Australia, and South America. Trials examined multiple forms of intermittent fasting, including alternate-day fasting, periodic fasting, and time-restricted feeding. Most studies followed participants for up to 12 months.

The review compared intermittent fasting with traditional dietary advice and with no intervention. Intermittent fasting did not appear to have a clinically meaningful effect on weight loss compared to standard dietary advice or doing nothing.

Reporting of side effects was inconsistent across trials, making it difficult to draw firm conclusions. The evidence base remains limited, with only 22 trials, many with small sample sizes and inconsistent reporting.

Intermittent fasting just doesn't seem to work for overweight or obese adults trying to lose weight."

Luis Garegnani, lead author of the review, Universidad Hospital Italiano de Buenos Aires Cochrane Associate Centre

Hype outpaces the evidence

Garegnani also cautioned against the hype surrounding fasting online. "Intermittent fasting may be a reasonable option for some people, but the current evidence doesn't justify the enthusiasm we see on social media."

Few trials have looked at the long-term results of intermittent fasting. "Obesity is a chronic condition. Short-term trials make it difficult to guide long-term decision-making for patients and clinicians," Garegnani added.

The majority of the included studies enrolled predominantly white populations in high-income countries. As obesity is a rapidly growing crisis in low- and middle-income countries, further research is needed in these populations.

The authors therefore warn that these results may provide clues, but cannot be extrapolated to the entire population, as they may vary depending on sex, age, ethnic origin, disease status, or underlying eating disorders or behaviours.

"With the current evidence available, it's hard to make a general recommendation," said Eva Madrid, senior author from Cochrane Evidence Synthesis Unit Iberoamerica. "Doctors will need to take a case-by-case approach when advising an overweight adult on losing weight."