Fasting periodically throughout the day or week is becoming popular as a weight-loss technique, and studies show it may also be a strategy to preserve health and extend life.
Restricting food intake to a 6- to 8-hour period each day or eating lightly two days a week may have considerable benefit for overall health, according to a recent article in the New England Journal of Medicine.
“Fasting has become quite the thing to do for restricting calories,” says UCI Health Dr. Bavani Nadeswaran, a UCI Health expert in weight management and professor of internal medicine at the UCI School of Medicine.
“But it’s not new. In the 1900s, people tried fasting to cure epilepsy. Every culture and many religions have some tradition of fasting.”
Scientific evidence for fasting
“We now have more scientific evidence behind it,” Nadeswaran says. “Initially, many people did it for weight loss, but animal and human studies show it may have greater benefits.”
Research has found two intermittent fasting techniques to be beneficial:
- The 16:8 method, in which daily eating is restricted to an 8-hour window, fasting for the remaining 16 hours.
- The 5:2 schedule, in which eating is restricted to about 25% of normal (or about 500 to 600 calories) for two days each week, and normal food intake the other five days.
Normally, our bodies burn readily available glucose for energy. During periods of fasting, our bodies start to use stored ketones and fatty acids instead of glucose.
This metabolic switch from glucose-based to ketone-based energy, the authors of the journal review paper conclude, increases the body’s resistance to stress, increases longevity and decreases incidence of diseases, including cancer and obesity.
Benefits of intermittent fasting
Fasting has been shown to deliver a range of benefits, according to Nadeswaran. These include:
Getting started with fasting
If you want to try fasting, which method is best?
It’s mostly personal preference, however Nadeswaran says limiting food intake to an eight-hour period daily is better for people taking insulin or blood pressure medicine than two days a week of calorie restriction.
Regardless of method, she recommends people ease into it. Patients using the 16:8 method might start fasting only five days per week for 10 hours, then five days the next week for 12 hours, eventually working toward fasting 16 hours seven days a week.
Those trying the 5:2 schedule could begin by eating 600 or 700 calories on the two restricted days each week, with the aim of getting down to 500 calories.
Fasting is not for everyone
Fasting can be difficult — especially at first.
“In the first month, you might feel hungry, irritable or have trouble concentrating,” Nadeswaran says. “But the majority of my patients find it easier after three or four weeks, and they feel better, have more energy, increased concentration and improved memory.”
She acknowledges that fasting isn’t for everyone, but says it is an effective alternative strategy for some people.
Still, our abundant food choices and 24/7 availability of them, not to mention nonstop marketing, make fasting a tricky lifestyle choice.
Be flexible with fasting
In addition, fasters need to be flexible enough to make room for social, cultural and familial traditions, such as eating three meals a day with family members on the weekends or vacations.
“There is a benefit to fasting for six months, then doing it on and off every few weeks or months as much as possible for a while,” Nadeswaran says.
Fasting can be especially beneficial before major medical procedures, she adds. “People anticipating surgery should try fasting for a month or two beforehand to aid in their recovery.”
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