Every diet has its day in the sun, but emerging evidence is now casting a shadow on the hype of intermittent fasting.
According to a sweeping new review of clinical studies, various patterns of fasting are no better than conventional diets for weight loss – at least in the short term.
In fact, intermittent fasting produced barely any clinically meaningful changes over the course of a year. The popular practice appears to be no better at cutting 5 percent body weight than making no diet changes at all.
"Compared to traditional dietary advice (like restricting calories or eating different types of foods), intermittent fasting may make little to no difference to weight loss and quality of life in adults living with overweight or obesity," conclude the authors of the review, led by Luis Garegnani from the University Institute of the Italian Hospital in Buenos Aires, Argentina.
The international research team says they are "moderately confident" in their findings on weight loss; however, for other diet outcomes, they consider the evidence "very uncertain."
Their global review covered 22 randomized controlled trials on intermittent fasting published between 2016 and 2024. As many as 75 studies were excluded, some because their follow-up period was less than 6 months.
Altogether, the analysis included nearly 2,000 participants spread across North America, Australia, China, Denmark, Germany, Norway, and Brazil.
The trials covered various approaches, including time-restricted eating (limiting meals to a daily window, usually 10 hours or less), periodic fasting for one or two days a week, alternate-day fasting, and modified alternate-day fasting (such as two days of fasting and five days of typical eating).
Many of these trials produced inconclusive results, and none reported participant satisfaction, diabetes status, or other health problems. Among studies comparing intermittent fasting with no diet changes, only one considered quality of life, and only two considered adverse events, such as fatigue, headache, or feeling sick.
"Although a recent Cochrane review addressed the effects of intermittent fasting in preventing and reducing the risk of cardiovascular disease, most included studies recruited participants without overweight or obesity, limiting their findings for this population," note the review authors.
Clearly, there is still a lot of work to be done before intermittent fasting is recommended by physicians as a way for people living with overweight or obesity to lose weight or improve their metabolic health.
Despite the lack of robust evidence, the diet has grown in popularity in recent years. It is based on the idea that, in a 'fasted' state, the body's metabolism switches to burn fat stores for energy.
Initial evidence suggests that this may confer health benefits for the brain, the gut, and the metabolism. But there are many different versions of intermittent fasting, and some evidence suggests they may not all alter markers of metabolic or cardiovascular health.
Even if you stick to one fasting diet perfectly, studies suggest the benefits vary widely from person to person, and there are potential downsides to consider.
Related: Study Raises Serious Questions About The Benefits of Intermittent Fasting
Given how little we know, Garegnani and colleagues are calling for new research on how intermittent fasting impacts various cohorts, including different genders and socioeconomic groups, where they say "the potential impact of intermittent fasting may exacerbate any nutritional inadequacies."
Maik Pietzner, a health data modeler at the Berlin Institute of Health at Charité, told The Guardian that his own research has found that several days of fasting are needed for metabolic benefits to appear in the blood.
"If people feel better on such diet regimens, I wouldn't stop them, but this work, along with others in the field, clearly shows that there's no robust evidence for positive effects beyond a possible moderate weight loss," Pietzner told reporter Ian Sample.
"Our bodies have evolved under constant scarcity of food, and can deal really well with prolonged periods without it, but that does not mean that we perform any better once these [evolutionarily] conserved programs kick in."
The study was published in the Cochrane Database of Systematic Reviews.