Over the past 10 years, a dieting practice known as intermittent fasting has gained some attention. It involves fasting either during certain hours each day or all day two to four days a week and has been promoted as an effective and simple way to lose weight. The diet is meant to mimic the natural food-scarcity conditions that prehistoric humans likely endured for tens of thousands of years, says Mark P. Mattson, PhD, retired adjunct professor of neuroscience at Johns Hopkins University in Baltimore.
The three varieties are 5:2, alternate day, and time restricted, says Krista Varady, PhD, professor of nutrition at the University of Illinois in Chicago. Those on the 5:2 version eat a low-calorie diet (generally 500 to 1,000 calories per day) two days a week and follow a healthy diet without counting calories five days a week; the fasting days can be consecutive or not. An alternate-day plan involves fasting—which means either drinking fluids only or eating 25 percent of a normal caloric intake (about 500 calories)—every other day; on eating days, people can consume what they want. In time-restricted eating, people limit meals, snacks, and caloric beverages to a specific window of time each day, generally four to eight hours, and have only water, tea, black coffee, or other zero-calorie beverages for the other 16 to 20 hours each day.
In terms of weight loss, a review of studies published in Nature Reviews: Endocrinology in May 2022 found that intermittent fasting helps people lose about 3 to 8 percent of their body weight over eight to 12 weeks, which is on par with a conventional reduced-calorie diet. Some studies show improvements in blood pressure, LDL cholesterol, and blood sugar and insulin sensitivity, but others have found no benefits, says Dr. Varady, co-author with Bill Gottlieb of The Every-Other-Day Diet: The Diet That Lets You Eat All You Want (Half the Time) and Keep the Weight Off.
Scientists continue to examine the eating plan to understand its effects, if any, on the brain and certain neurologic diseases such as dementia, epilepsy, multiple sclerosis, Parkinson's disease, and stroke. Research so far has been mostly confined to studies of mice and small pilot studies of humans, but investigators hope that more concrete results will be available in a few years. For now, here's what we know.
A National Institute on Aging study of overweight women ages 55 to 70 is currently looking at the effects of the diet for eight weeks on memory, thinking, and risk of Alzheimer's disease. Researchers will use MRIs as well as blood work and spinal fluid tests to look for early signs of Alzheimer's-related brain changes.
Animal studies suggest that fasting protects brain cells by providing ketones for fuel instead of glucose. Ketones appear to help the brain produce brain-derived neurotrophic factor (BDNF), a compound that promotes the growth of new brain cells and new connections between them. BDNF also protects cells from stress, allowing them to live longer and work more effectively. Lab studies have shown that fasting slows the development of the plaques and tangles characteristic of Alzheimer's disease, says Dr. Mattson.
In subjects with Parkinson's disease, lab results indicate that fasting helps mitochondria function better and protects key neurons that produce dopamine, says Rodolfo Savica, MD, PhD, FAAN, professor of neurology at Mayo Clinic in Rochester, MN, who studies the condition. (Mitochondria are tiny organs in the cells that generate most of the chemical energy needed to power cells' biochemical reactions.) Dopamine-producing cells are damaged and destroyed in people with Parkinson's. When the mitochondria stop working properly, they become hyperactive; intermittent fasting may help reduce that hyperactivity, Dr. Savica says.
An animal study published in Stroke in February 2020 showed that the ketones produced during intermittent fasting may lessen brain damage and improve recovery by reducing inflammation and levels of free radicals that kill vulnerable brain cells in the minutes and hours after a stroke. The first human clinical trials exploring the effects of fasting on stroke recovery are ramping up at the National Taiwan University Hospital and China's Ulsan University Hospital.
Ketones also may reduce epileptic seizures, according to a study in Frontiers in Cellular Neuroscience published in January 2016. Researchers suspect that ketones may discourage—by reducing inflammation or changing signaling between cells—the bursts of overactivity in neurons that cause seizures.
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