At first glance, it would appear that the research is mixed — some studies find that intermittent fasting is no more effective for weight loss than other diets, while others report a small additional benefit. But many of the latter are based on small, low-quality studies, Dr. Betts said.
He and his colleagues have reviewed many studies published over the past few years and found that much of the pro-fasting research is corrupted by studies that make massive errors, such as counting study subjects twice or misinterpreting their own data.
“There are only probably about 20 or 30 studies out there that are good,” Dr. Varady said. “It’s not bad, but it’s like all diets,” she added. “It can’t produce more than 5 percent weight loss.”
The diet’s major selling point is that it’s easier to stick to than simple calorie restriction, but even this may be overstated, according to two recent papers. In other words, Dr. Harvie said, people seem to get bored with intermittent fasting just as they get bored with other diets.
How does it affect metabolic health?
Proponents of intermittent fasting say it improves your metabolic health in addition to helping you shed pounds. Research showed that people with diabetes who tried it were able to take less of their medicine because of improved insulin sensitivity, and that people with fatty liver disease reversed the fat accumulation. A study at Heidelberg University Hospital in Germany found that people with diabetes-related kidney problems saw improvements after a five-day fast.
But these metabolic benefits are about the same as what you would expect from any calorie-restriction diet that leads to weight loss, said Stephan Herzig, the researcher who ran the Heidelberg study.
He started his research hoping to find some special chemical or cellular changes that result from intermittent fasting, which might serve as the basis for new drugs — but he didn’t find any. He concluded that the metabolic benefits of intermittent fasting likely end the moment you start eating again.
Several studies even suggest that some forms of fasting may be bad for your metabolism. In a 2021 paper comparing alternate-day fasting to normal calorie restriction, people in both groups lost equivalent amounts of weight, but those who fasted lost far more muscle, which negatively affects metabolism.
Can it have cognitive benefits?
Some claim that the benefits of fasting extend beyond the body to the mind. The health influencer Andrew Huberman has said that intermittent fasting can be “great for focus and concentration.” In lab studies, mice that have been starved are faster to find food and less prone to learning and memory deficits associated with Alzheimer’s disease than well-fed counterparts.
Some of these claims may be true for long intermittent fasts. Only a few such experiments have been done, but they suggest that some cognitive enhancement is reported after about four to five days without food, as cells start metabolizing fats, said Mirjam Bloemendaal, a neuroscientist at Goethe University in Frankfurt.
But shorter, less intense fasting regimens seem to have no effect on cognition and are unlikely to provide a boost to those with cognitive decline, Dr. Bloemendaal said. A brutal four-day fast is completely different from the popular varieties, Dr. Varady added.
What about cancer?
There is some intriguing preliminary research suggesting that fasting might improve cancer prognoses, experts said, though not for the reason once theorized.
Around 2008, the gerontologist Valter Longo at the University of Southern California proposed that intense four- or five-day fasts could delay tumour growth.
The theory, suggested by mouse trials, was that fasting would make healthy cells more resilient while pushing cancerous cells into a weakened state, making them more vulnerable to treatments.
In 2020, a major study failed to find this effect but supported another idea: Intermittent fasting reduced the side effects of treatment, like nausea, bone marrow loss, and nerve damage.
Such effects make some patients reluctant to choose chemotherapy and stay with it, said Tanya Dorff, a medical oncologist at City of Hope in Duarte, California, who has worked with Dr. Longo. Fewer symptoms, she said, might simply “allow them to stay on their chemo at full dose and on schedule.”
A small study further suggested that a version of the 5:2 fast might let women with metastatic breast cancer live longer. For the women on the diet, it took on average 42 weeks for their cancer to progress, Dr. Harvie said, compared to 28 weeks for the women who were not on the diet, though this still needs to be confirmed by larger studies.
For those with advanced breast cancer, “it won’t do you any harm,” Dr. Harvie said. “And maybe it will keep your tumour smaller for longer.”
As for all the other still-contested claims around intermittent fasting — longevity, improved heart health, and lower risk of Parkinson’s disease — a clear trend is developing, said Dr. Varady: The better the study, the less it finds.
“I just don’t think we ever should have made these promises to begin with based on mouse trials,” she said.
