I’ve long thought the human body was not meant to run on empty, that fasting was done primarily for religious reasons or political protest. Otherwise we needed a reliably renewed source of fuel to function optimally, mentally and emotionally as well as physically.
Personal experience reinforced that concept; I’m not pleasant to be around when I’m hungry. There’s even an official name for that state of mind, confirmed by research: Hangry!
But prompted by recent enthusiasm for fasting among people concerned about their health, weight or longevity, I looked into the evidence for possible benefits — and risks — of what researchers call intermittent fasting. Popular regimens range from ingesting few if any calories all day every other day or several times a week to fasting for 16 hours or more every day.
A man I know in his early 50s said he had lost 12 pounds in about two months on what he calls the 7-11 diet: He eats nothing from 7 p.m. until 11 a.m. the next morning, every day.
I was skeptical, but it turns out there is something to be said for practicing a rather prolonged diurnal fast, preferably one lasting at least 16 hours. Mark P. Mattson, neuroscientist at the National Institute on Aging and Johns Hopkins University School of Medicine, explained that the liver stores glucose, which the body uses preferentially for energy before it turns to burning body fat.
“It takes 10 to 12 hours to use up the calories in the liver before a metabolic shift occurs to using stored fat,” Dr. Mattson told me. After meals, glucose is used for energy and fat is stored in fat tissue, but during fasts, once glucose is depleted, fat is broken down and used for energy.
Most people trying to lose weight should strive for 16 calorie-free hours, he said, adding that “the easiest way to do this is to stop eating by 8 p.m., skip breakfast the next morning and then eat again at noon the next day.” (Caffeine-dependent people can have sugar- free black coffee or tea before lunch.) But don’t expect to see results immediately; it can take up to four weeks to notice an effect, he said.
Dr. Mattson and his colleague Rafael de Cabo at the aging institute recently reviewed the effects of intermittent fasting on health, aging and disease in The New England Journal of Medicine.
Their article was prompted by frequent questions patients are asking their doctors about the health effects of fasting. Given their limited knowledge of nutrition, doctors are often unable to advise their patients, Dr. Mattson said.