Intermittent fasting - the practice of restricting eating to certain hours of the day or certain days of the week - is promoted across social media and embraced by millions of Americans.
But a Cochrane systematic review of the clinical evidence finds it produces nearly identical weight loss, quality of life and adverse events as conventional diets like calorie-counting.
"The differences we found between the diets were statistically indistinguishable from zero," said Diane Rigassio Radler, a professor of clinical nutrition at the Rutgers School of Health Professions and a co-author of the review.
Cochrane reviews are widely considered the gold standard for evaluating medical evidence. These independent, evidence-based health care studies follow strict protocols registered in advance and employ rigorous methods for searching, selecting and analyzing clinical trials.
This review of 22 randomized controlled trials with a combined 1,995 participants examined several forms of intermittent fasting: time-restricted eating (limiting meals to an eight- or 10-hour window), alternate-day fasting and the "5:2" approach of eating normally five days a week and drastically cutting calories on two.
It compared these fasting techniques with both standard dietary advice and no intervention.
Across the 21 studies involving 1,430 people that compared intermittent fasting and conventional diets, the difference in weight loss between intermittent fasting and standard dietary advice was 0.33 percentage points of body weight. Among the four studies that tracked whether participants achieved a 5% weight reduction, the rate was again virtually identical across groups.
In the six studies that compared intermittent fasting with no intervention, intermittent fasting produced more weight loss - about 3.4% of body weight - but less than the 5% threshold generally considered necessary for meaningful health benefits.
However, the underlying studies lacked sufficient information to enable the review team to answer several vital questions.
For example, the review says nothing about which approach is easier to sustain. Ten of the 22 studies tracked whether participants adhered to their assigned protocol, and those relied on self-report.
"Nutrition and diet studies are really difficult," Radler said. "We typically measure patient adherence with food journals and 24-hour dietary recalls, which are inherently imprecise."
The evidence base had other gaps. None of the 22 studies assessed participants' satisfaction with their assigned diet. None reported on diabetes outcomes. None lasted longer than a year. And 19 of the studies were conducted in high-income countries with predominantly white participants, limiting the findings' applicability elsewhere.
Most clinical practice guidelines for managing excess weight and obesity don't specifically recommend intermittent fasting, and, although the review found no significant safety concerns, its authors noted the evidence for such recommendations remains limited by short follow-up periods and low certainty.
In clinical practice, Radler said she takes a patient-centered approach. If a client is interested in intermittent fasting, she provides them with the facts and lets them decide.
"I tell them that, according to this systematic review of 22 studies, it's no better than any other kind of weight reduction intervention," she said. "But if that's something that they really want to try, we'll work with them so that they do it in a safe and healthy way."
The review authors, who hailed from the Universidad Hospital Italiano de Buenos Aires, with collaborators in Chile, Spain and the United States, called for future studies to extend follow-up beyond 12 months, include more diverse populations, and measure outcomes that matter to patients, such as whether they found the diet tolerable enough to sustain.
