I’m on the 5:2 diet,” actor Benedict Cumberbatch told the (London) Times. “You have to, for Sherlock.” That’s his hit BBC and PBS series. Cumberbatch, and most other celebrities who have used the 5:2 diet, do so to lose (or not gain) weight.
Why? “For many people, it’s easier to not eat much on two days of the week and eat normally– but not overeat — on five days, versus counting calories at every meal,” says Mark Mattson, chief of the laboratory of neurosciences at the National Institute on Aging.
As it turns out, the 5:2 diet—also called the 2-day diet or intermittent fasting—may do more than trim your waistline.
“In animal models, intermittent fasting increases the resistance of cells to various types of stress and disease,” says Mattson. But the human evidence in favor of on-and-off fasting is just emerging.
On-and-off fasting may help lower the risk of type 2 diabetes not just by shrinking waistlines but by keeping the body’s insulin in good working order. That’s what happened in two studies, each on roughly 100 overweight or obese women.
“Half cut 25 percent of their calories every day, and half ate only 650 calories a day for two days per week” and didn’t cut calories on the other five days, says co-author Mattson.
After three to six months, “each group had lost about the same amount of weight,” he notes. “But the women on the 5:2 diet had better insulin function.”
And insulin matters.
Poor insulin function is “at the root of many weight-related diseases, such as type 2 diabetes, heart disease, some cancers, and possibly dementia,” wrote Michelle Harvie and Tony Howell in The 2 Day Diet.
The researchers, both at the Manchester Breast Centre in England, led the two studies in women.
“When you inject cancer cells under the skin of mice, alternate-day fasting slows the growth of the tumor cells,” says Mattson.
It’s not clear why. One possibility: “Almost all cancer cells use glucose as their energy source, so the fasting state is not ideal for cancer cells because the glucose levels are low,” explains Mattson. But studies in people are just starting.
“British researchers are putting women who have breast cancer on intermittent fasting diets throughout the entire five-month course of their chemotherapy treatments,” says Mattson. “The prediction is that intermittent fasting will enhance the effectiveness of the chemotherapeutic drugs by making cancer cells more vulnerable. And fasting may protect normal cells from the adverse effects of chemotherapy.”
In a study on mice engineered to get an Alzheimer’s-like disease, the animals performed better on memory tests—how to find a hidden platform in water, for example—when put on diets that cut calories daily or every other day than when eating an unlimited diet.
And healthy mice that were fed few calories for four days twice a month did better on memory tests than mice that never fasted.
“Animals on intermittent fasting are more alert than animals that have constant access to food,” notes Mattson. “And they have increased activity in a region of the brain called the hippocampus, which is important for learning and memory.”
That makes sense, given that animals in the wild are hungry most of the time. “It’s normal for cougars to go a week without eating anything, so they’re burning fat,” explains Mattson. “And their brains have to work well, so they can figure out how to track down their prey.”
What about evidence in humans?
“We’re about halfway through a study on people aged 55 to 70 who are at risk for cognitive impairment and Alzheimer’s disease because of their age and because they are obese and insulin resistant,” says Mattson. Half are on the 5:2 diet, while half are getting the usual advice for healthy eating.
“Before they start and two months later, we do a battery of tests to look at learning and memory,” says Mattson.
If the 5:2 dieters do better, it will take another study to know if that’s due to fasting or just having lost weight.
What to do until we know more?
“The first thing is to make sure you’re not overweight or obese,” says Tufts’ Susan Roberts, “because there’s a legion of studies that show that carrying excess body fat is unhealthy for all kinds of reasons.”
And if you find it easier to slash calories two days a week than to make smaller cuts every day, why not try it?
“To my knowledge, there is no evidence that intermittent fasting has adverse effects on healthy people unless they’re a young child or a frail older person,” says Mattson. (Granted, on-and-off fasting hasn’t been tested in studies lasting longer than six months.)
What if you’re not overweight or obese?
“The research so far indicates that some caloric restriction is very likely to be healthy even if you’re already normal weight,” says Roberts. That is, unless you end up too thin.
Sources: Int. J. Obes. 35: 714, 2011; Br. J. Nutr. 110: 1534, 2013; Sci. Transl. Med. 2012. doi:10.1126/scitranslmed.3003293; Neurobiol. Dis. 26: 212, 2007; Cell Metab. 22: 86, 2015.
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