Scientists who have reviewed existing evidence on intermittent fasting—where large gaps are left between periods of eating—could have a range of health benefits.
Mark Mattson, a Johns Hopkins Medicine neuroscientist who adopted intermittent fasting himself 20 years ago, and his colleagues looked at studies involving animals and humans.
Intermittent fasting involves prolonged periods where an individual doesn’t eat. The most common forms are alternate-day fasting, where food is cut out or heavily restricted every other day; 5:2 where one 500 to 700 calorie meal is eaten on two days per week; and daily-time restricted feeding, where a person will eat during a specific window, for instance eight hours.
In their review published in the New England Journal of Medicine, the authors showcase links between fasting and improvements in cognitive performance, cardiovascular health, physical performance, and symptoms of diabetes and obesity.
Many studies involving animals such as rats and some in humans indicate restricting eating times triggers what is known as metabolic switching, where the body’s source of energy changes from sugar to fat, which could improve health span.
When food is restricted to 500 to 700 calories one or more days per week, the levels of molecules called ketones in the body rise. The metabolic switch could make the metabolism more flexible and efficient use in using energy, they believe. Ketones also appear to regulate the expression and activity of proteins linked to health and aging, and genes associated with psychiatric and neurodegenerative disorders.
The authors believe the method could prevent the body from creating free radicals—atoms which can damage cells—as well as aid with weight loss. But it also appears to trigger cellular responses between and in organs which help with blood sugar regulation, and make the body more resistant to stress and inflammation. The practice is thought to activate pathways which boost the body’s defences against stresses, and helps to remove and repair damaged molecules.
Animals in various intermittent fasting studies saw improvements in different types of memory, they said, and alternate day fasting and calorie restriction reversed the the adverse effects of diabetes, neuroinflammation, and also helped with spatial learning and memory. Fasting has also been shown to improve running endurance in mice.
One study involving rats saw their average life span increase by 80 percent when they were fed alternate days from young adulthood. And a review of data from 1934 to 2012 showed cutting calories boosted the longevity of rats by 14 to 45 percent, but 4 to 27 percent in mice.
In humans, the regimes are associated with improvements in obesity, insulin resistance, and high blood pressure. And these effects were beyond what could be attributed to cutting calories alone. One study in older adults found improvements in verbal memory. Another led to improvements in thinking skills in overweight adults with mild cognitive impairment.
Research on people who live on the Japanese island of Okinawa who practice fasting reflect their low rates of obesity, diabetes, and longer lives. The authors also said there are indications it could help with heart disease, the outcomes of certain types of cancer, may prevent Alzheimer’s, and ease the symptoms of asthma.
However, most people don’t reap the apparent benefits of fasting as they eat three meals a day plus snacks. In contrast, our ancestors had more irregular eating patterns and weren’t sedentary, the authors wrote. Such habits are ingrained in culture and therefore “rarely be contemplated by patients or doctors.”
“The abundance of food and extensive marketing in developed nations are also major hurdles to be overcome,” they said.
People may also struggle with the hunger and irritability and problems concentration when food is restricting—but these side effects pass after one month, according to the authors. Doctors can advise patients to reduce their intake gradually to a fasting window of 18 hours a day, including sleep time, or transitioning to a 5:2 diet over a period of four months.
“Although we do not fully understand the specific mechanisms, the beneficial effects of intermittent fasting involve metabolic switching and cellular stress resistance,” they concluded.
The authors said more research needs to be done to see if fasting is safe and beneficial in populations that haven’t yet been studied, as most focus on overweight younger and middle-aged adults.
“We cannot generalize to other age groups the benefits and safety of intermittent fasting that have been observed in these studies,” they wrote.
Intermittent fasting may not be suitable for people with certain long-term health conditions or who have a history of eating disorders should not adopt this methods. Health experts recommend people speak to their doctor before trying the regime.
Mattson told Newsweek the NEJM asked him to write the article because many patients are asking physicians whether intermittent fasting would help them to lose weight or improve symptoms of chronic conditions, but many doctors aren’t familiar with the research on the topic.
He said both sedentary and overindulgent lifestyles are unhealthy, and people who are overweight and have or are at risk of diabetes, heart disease, and chronic inflammatory disorders like arthritis, asthma, multiple sclerosis “will likely benefit from intermittent fasting.”
Mattson said the easiest method for most people is limiting daily food intake to a 6 to 8 hour time window, which amounts to fasting for 16 to 18 hours each day.
Asked for tips on how to adopt the lifestyle, he said: “try starting intermittent fasting with a spouse, partner or friends at work. Just as with starting an exercise regimen it is easier to do with someone else than alone.”
Drinking plenty of water, as well as eating healthy foods such as vegetables, fruits, nuts, whole grains, fish and lean meat, and yogurt, and avoiding sugar, salt and fried foods is also advisable, he said.