“We’ve known for a long time that if you reduce the calorie intake of rats or mice, they live much longer,” says Mark Mattson, chief of the laboratory of neurosciences at the National Institute on Aging (NIA) in Baltimore. Do these intermittent fasting benefits carry over to humans?
What happens in species closer to humans is more complicated. Rhesus monkeys fed 30 percent fewer calories lived longer in a study at the University of Wisconsin, but not in a study at the NIA.
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Why the different results?
One possibility: The Wisconsin monkeys were fed fewer calories than monkeys fed as much high-sugar, high-fat food as they wanted. In contrast, the NIA monkeys were fed fewer calories than monkeys fed as much (low-sugar, low-fat) food as they needed to maintain their weight.
“One take-home message is that if you are an overweight monkey like those in Wisconsin, cutting back on calories will extend your lifespan,” says Mattson. “Whereas if you are eating a healthy diet and not overweight like the NIA monkeys, cutting back on calories may not extend your life, although you may experience some health benefits.”
What if you’re human?
In the first good study in normal-weight or slightly overweight (but not obese) people, researchers asked roughly 150 men and women to consume 25 percent fewer calories at each meal than they needed to maintain their weight, and 75 similar people to follow their normal diet, for two years.
The calorie cutters managed to eat 12 percent fewer calories, and they lost 10 percent of their body weight. That may explain why their blood pressure was lower and their insulin worked better than those who ate their normal diets.
“They lowered their risk factors for heart attack, stroke, and diabetes,” says Eric Ravussin, director of the Nutrition Obesity Research Center at the Pennington Biomedical Research Center in Baton Rouge, Louisiana.
Why didn’t the calorie cutters cut more? “Eating less than you would like to every day is a struggle,” says Ravussin. “Some people can do it, but many cannot.”
But cutting calories only on some days (or intermittent fasting) may be easier.
Mon, Tues, Weds, Thurs, Fri, Sat, Sun
“There’s the alternate-day modified fast,” says Ravussin. “Every other day you eat only one meal with maybe 30 percent of your normal daily calorie intake.”
And there’s the 5:2 diet, in which you eat a normal diet five days of the week, and only 30 percent of what you would normally eat on the other two days.
“Whichever it is, you have to make sure you don’t overeat on the normal days,” cautions Ravussin.
Intermittent fasting benefits seem promising in both animals and people
“Laboratory animals that get no food at all on alternate days live about 30 percent longer than animals that eat their regular diets every day,” says Mattson.
In humans, the 5:2 plan seems to hold an edge over fasting every day.
In two of the best studies, Mattson and colleagues divided 166 overweight middle-aged women into two groups. Both were told to cut calories by 25 percent—one by trimming the calories in each meal, the other by following a 5:2 plan.
In both studies, the women were told to eat a high-protein “Mediterranean-type” diet with fruits, vegetables, whole grains, nuts, seafood, and olive oil, and only moderate amounts of dairy, poultry, eggs, and lean red meat.
On the two fasting days of the 5:2 diet, one study prescribed just four cups of low-fat milk, four servings of vegetables, and one serving of fruit. The other study prescribed about 9 oz. of lean protein, 3 servings of low-fat dairy, 4 servings of low-carb vegetables, and a low-carb fruit. Both also recommended low-calorie drinks and a multivitamin-and-mineral.
In each study, both calorie-cutting groups lost about the same weight. “But insulin resistance declined more in the 5:2 groups than in those who cut calories daily,” says Mattson. And in the three-month trial (the other trial lasted six months) the women on the 5:2 regimen lost more body fat.
Why did the 5:2 dieters do better? They were more likely to stick to their plan. “And on the two days that they ate only 500 to 600 calories, their metabolism shifted to burning fat,” says Mattson.
Does intermittent fasting benefit the brain?
Middle-aged rats went on a brain diet, after being deprived of all food every other day for three months, lost 23 percent of their body weight and had better motor coordination and cognitive skills than similar rats who could eat all they wanted.
One possible reason: “Intermittent fasting benefits include increased brain levels of a protein that stimulates the growth of new brain cells and the connections between them,” says Mattson.
“We think what’s happening is that when you’re hungry, your brain cells are more active so you can figure out how to find food,” he explains. “During evolution, those who were able to figure out how to get food were the ones who survived.”
Today, our brain cells may respond in a similar way when we’re hungry.
Other intermittent fasting benefits include the possibility of postponing dementia, at least in animals. In mice bred to show signs of Alzheimer’s disease by middle age, eating only every other day delays the onset of dementia by the human equivalent of about 10 years.
“That’s a big effect,” says Mattson. “But we’re nowhere near being able to say the same about humans.”
The bottom line: Cutting calories may not prolong your life, but it may lower your blood pressure and make your insulin work better.
Do you have experience with intermittent fasting? What results did you get? Let us know in the comments.
Sources: Science 325: 201, 2009; Nature 489: 318, 2012; Mech. Ageing Dev. 55: 69, 1990; Int. J. Obes. 35: 714, 2011; Br. J. Nutr. 110: 1534, 2013; Age 34: 917, 2012; Endocrinology 144: 2446, 2003; Neurobiol. Dis. 26: 212, 2007.
This article was originally published in 2014 and is updated regularly.