Eating in a 6-hour window and fasting for 18 hours might help you live longer

Source: (CNN) — Abstaining from food for 16 to 18 hours a day could be key to treating a variety of health conditions -- even if you've got to train yourself to push past the hunger.

A review of past animal and human studies in The New England Journal of Medicine suggests that intermittent fasting can reduce blood pressure, aid in weight loss and improve longevity. The report functions as a road map of sorts for physicians to prescribe fasting as a method of prevention or treatment for obesity, cancer, diabetes and heart disease. 

Study author Mark Mattson, a professor of neuroscience at Johns Hopkins University, hones in on two types: Daily time-restricted feeding (eating 6-8 hours a day and fasting for 16-18 hours) and 5:2 intermittent fasting (fasting two days a week, usually capping a fasting day at 500 calories).The catch? Most Americans don't intermittently fast (the norm is three meals a day plus snacks) and thus physicians are less inclined to consider fasting a solution to a broad range of health conditions, according to the review. Because the research is relatively new, the report advises physicians to monitor their patients throughout intermittent fasting and gradually increase the duration and frequency of fasting to guide their transition. How it worksIntermittent fasting has been studied in rodents and overweight adults to improve health across the spectrum, though it's not clear if those benefits are the result of weight loss. Alternating between fasting and eating can improve cellular health, Mattson said, most likely by triggering metabolic switching. In metabolic switching, cells use up their fuel stores and convert fat to energy -- "flipping a switch" from fat-storing to fat-saving. The benefits findings on intermittent fasting range in the diet's effectiveness, but some studies in animals and humans have linked the practice to longer lives, healthier hearts and improved cognition. The article points to the residents of Okinawa, known for their extreme longevity and low-calorie, nutrient-rich diet. Their intermittent fasting might contribute to their life spans and keep obesity at bay, the authors posited.

Intermittent fasting is thought to improve insulin resistance, which can stabilize blood sugar levels. Findings from a small 2018 study found that three men with type 2 diabetes, also known as adult-onset diabetes, were able to stop taking insulin after losing weight from intermittent fasting -- findings that clash with the widely-held belief that diabetes is incurable.A previous study (that Mattson co-authored) showed the switch can increase resistance to stress by optimizing brain function and neuroplasticity, or the brain's ability to adapt to develop throughout one's life. And older adults who were put on a restricted-calorie diet showed improved verbal memory compared to two other groups who hadn't fasted, a 2009 study found. Physical function even improved for some patients. A study of young men who fasted every day for 16 hours lost fat and retained muscle while resistance training for two months.The limitations long-term effects of intermittent fasting require more research that isn't available yet, and the studies that do exist are narrow. The clinical trials focused on overweight young and middle-aged adults, so the benefits and safety can't be generalized to other groups, the authors said.

Another thing: It's a difficult diet to stick to, particularly in the United States, where the concept of three meals a day is "so ingrained in our culture" that a change in eating pattern often doesn't cross doctors' or patients' minds, Mattson wrote. It'll almost definitely leave participants hungry, irritable and less able to concentrate, the study said. Almost 40% of people who were assigned to a fasting diet dropped out of a 2017 JAMA study -- one of the drawbacks of intermittent fasting as a weight-loss regimen, Dr Frank Hu, chairman of the Department of Nutrition at Harvard University's T.H. Chan School of Public Health, noted to Harvard Health Publishing."It's human nature for people to want to reward themselves after doing very hard work, such as exercise or fasting for a long period of time," he said. "So there is a danger of indulging in unhealthy dietary habits on non-fasting days."Get CNN Health's weekly newsletter When the brain is deprived of food, appetite hormones in the hypothalamus, the brain's "hunger center," are released in a flurry and can trigger overeating. But Mattson said the pain is temporary."Patients should be advised that feeling hungry and irritable is common initially and usually passes after two weeks to a month as the body and brain become accustomed to the new habit," he said.