The Medicine Cabinet: Ask the Harvard Experts
Q: I am considering intermittent fasting. While I definitely need to shed a few pounds, I am also intrigued by the possibility of it extending life. What are your thoughts?
A: Intermittent fasting involves alternating intervals of extreme calorie reduction with periods of normal eating. Proponents suggest it offers a faster road to weight loss and perhaps a healthier and longer life.
All diets achieve weight loss through the same equation — you take in less food energy each day than your body burns. Intermittent fasting achieves this goal by severely limiting calories during certain days of the week or during specified hours during the day. The theory is that this type of diet will help decrease appetite by slowing the body’s metabolism.
The schedule for intermittent fasting varies. For instance, one schedule calls for eating your normal diet five out of seven days each week. On the two “fasting days” you restrict food intake to just 500 calories. Another variation calls for alternating “fast” days, in which you consume a quarter or less of your basic calorie requirement, with “feast” days, during which you eat whatever you choose.
Part of the fascination with intermittent fasting arises from animal research showing that fasting may reduce cancer risk and slow aging. But will that be true for humans?
So far, the limited evidence we have is not promising. In one recent study, 100 overweight people were assigned to one of three eating plans: restricting daily calorie intake by the same amount every day (similar to a traditional diet plan), fasting on alternate days, and continuing with normal eating habits. At the end of the 12-month study, both diet groups had lost weight compared with the normal eaters. However, the fasters didn’t fare any better than the conventional calorie cutters.
A notable aspect of this trial was a very high dropout rate (38 percent) among the people assigned to the fasting regimen. This may reflect a real-life pitfall of fasting as a weight-loss approach.
It’s human nature to want to reward yourself after if you do something good for your body, like fasting to lose weight. But there’s no gain if the reward means indulging in unhealthy foods on non-fasting days. In fact, there’s a strong biological push to overeat following fasting periods. Your appetite hormones and hunger center in your brain go into overdrive when you are deprived of food.
If you are considering intermittent fasting, make sure to discuss it with your doctor. Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes, and for those taking medications for high blood pressure or heart disease.
(Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)