Intermittent fasting has been a diet trend for years, but research on its potential benefits and risks have remained mixed. One new study suggests it may help reduce the risk of heart disease and diabetes.
Only eating within a consistent window of 8-10 hours a day is better for metabolic health, researchers found. Time-restricted eating is a type of intermittent fasting that limits eating to a certain number of hours a day.
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"People who are trying to lose weight and live a healthier lifestyle should pay more attention to when they eat as well as what they eat," said Dr. Satchidananda Panda, of the Salk Institute for Biological Studies. "Time-restricted eating is an easy-to-follow and effective dietary strategy that requires less mental math than counting calories."
The researchers found that restricting eating to less than 12 hours a day also improve sleep and reduces risk for liver disease and obesity. Overall quality of life also improved.
Previous research has shown that genes, hormones and metabolism fluctuate at different times of the day, suggesting that aligning meal timing to the body's internal clock could be helpful in the prevention and management of chronic metabolic diseases.
When people eat at random times throughout the day, they are breaking "the synchrony of our internal program," Panda said.
How does it work?
Intermittent fasting focuses more on when people eat than how much they eat. The diet includes periods of fasting or restricted calories. It is separate from the fasting many cultures practice for religious or spiritual purposes.
One of the most popular forms is the 16:8 method, in which people only eat during an eight-hour window and fast the remaining 16 hours of the day.
Other advocates prefer the 5:2 diet, in which people only consume 500 calories on two non-consecutive days each week. Some dieters prefer to do a 24-hour fast on those two days instead. Yet, others prefer fasting or restricted eating on alternate days or simply having one large meal a day.
Health experts say intermittent fasting is so effective because it sparks changes on the cellular level. When cells run out of the energy they have stored up, they convert more fat to energy.
This process — called ketosis — increases the production of ketones, an alternative fuel source made by the liver, according to the Mayo Clinic. Higher levels of ketones have been associated with better weight loss and other health benefits, including the reduction of epileptic seizures in children and improvements in cognitive, heart and tissue health.
"Fasting also affects metabolic processes in the body," the Mayo Clinic says. "These processes trigger a number of responses, including decreased inflammation, improved blood sugar regulation, and better response to physical stress. The research shows intermittent fasting could have other health benefits as well, but more study is needed."
Most studies so far have only offered a mixed bag of possible benefits and side effects. Some, like a Harvard Medical School review of 40 studies, found that dieters lose an average of 7-11 pounds over 10 weeks using intermittent fasting. It has also been found to help stabilize blood sugar and decrease blood pressure and blood lipid levels.
But other study results suggest this approach isn't as effective as some people tout. One study found it was no more effective than simple calorie-counting. Another more long-term study suggests that it has potentially adverse long-term effects such as muscle loss.
Is it safe for everyone?
Most experts agree that more research is needed on the long-term consequences of the different forms of intermittent fasting and that it is not a safe approach for everyone.
People with diabetes who are taking medication or insulin might be at higher risk for hypoglycemia — when blood sugar levels plummet — which can increase the risk of falls, some experts say. It also is not recommended for people with thyroid conditions, high blood pressure or heart disease.
Intermittent fasting can increase a person's risk for severe hydration and eating disorders. Nutritionists emphasize that people who decide to follow a type of intermittent fasting still need to be eating a very healthy diet of mostly fruits, vegetables, whole grains, nuts and beans, just in smaller portions.
The best strategy for overall health is still to adopt healthy eating and exercise habits that can be maintained consistently over time, experts say. People should talk to a health care provider and a nutritionist before using this approach to make sure they are practicing it in a healthy way.