We all (at least secretly if not openly) want to latch onto a strategy that will allow us to eat whatever we want but still somehow stay trim. One such strategy is intermittent fasting. It’s a fast-growing diet trend that people practice in different ways. Some completely fast or limit food intake for certain days of the week…some fast for specific hours of the day. For the rest of the time, they may eat and drink all they want without care. Many people on such a diets think they’re great. They can do the trick—help you shed pounds. But how do they work—and how safe and effective are these diets in the long run?


Popular intermittent diets include the so-called “Warrior Diet,” which instructs you to lightly snack on fluids, fruits and vegetables during 20 hours of the day and overeat during the remaining four hours…and diets that encourage restricting calories to only 500 to 600 for two days per week. Research in animals and humans shows that, at least in the short-term, intermittent fasting may be useful for weight loss and improved health, but no thorough scientific studies on the long-term effects of intermittent fasting in humans have yet been done, and researchers and dietitians are now saying that intermittent fasting is probably counterproductive in the long run. If done wrong, it can, in fact, wreak havoc on your metabolism.

We reached out to Michael J. Nusbaum, MD, surgical director of the Metabolic Medicine and Weight Control Center at Morristown Medical Center in Morristown, New Jersey, for his views on intermittent fasting and what you need to know to balance the cons with the pros of this popular diet trend.


Although reduced-calorie diets have been associated with weight control, longevity and even fertility in animals and humans, here’s the downside: When you deprive your body of food on a regular basis, it can go into “starvation mode,” recalibrating your metabolic rate (the rate at which you burn calories for energy). When your body thinks it’s starving, it slows the metabolic rate, storing more food as fat. This means that, if you keep up with intermittent fasting for several weeks, you may find that you are not only hungry all the time, but your weight-loss progress levels off to where it becomes increasingly difficult to lose weight and keep it off. What’s more, once your body becomes convinced that it’s in continual starvation mode, getting your body to not feel as if it’s in starvation mode even when you are back to eating regular three square meals a day can become a long, hard climb, lasting several months.

Side effects of intermittent fasting that Dr. Nusbaum sees in his practice include fatigue, lethargy and the inability to lose weight. No surprise. Anyone who has ever gone for most of a day without food knows that famished, fatigued and short-tempered are how you can expect to feel during the fasts. And so, intermittent fasters ironically find themselves obsessed with food, Dr. Nusbaum told me. They also may have trouble sleeping because they’re so hungry. And on their nonfasting days, they will typically crave high-calorie empty carbohydrates, such as pasta, cakes, bread and cookies. Like anorexics and bulimics, they can wind up binge eating and malnourished.


That said, fasting for not more than 24 hours periodically—say, once per month—whether for health or religious reasons, will not have a detrimental impact on metabolism, but it won’t make a dent in weight loss, Dr. Nusbaum said. For people who occasionally fast or who insist on trying intermittent fasting as a quick fix for weight loss, Dr. Nusbaum suggests that when you do take food, you resist empty carbs and instead eat protein (lean meat or chicken, fish, legumes and dairy) to quell hunger and ensure that your diet is nutrient-rich.

He also stresses that we should always eat more than 500 calories per day despite what intermittent-fasting gurus say. “Your body is a machine,” said Dr. Nusbaum. “It takes calories in and burns them as fuel. Reducing calories too much will trigger your body to shut down certain functions in an attempt to survive.”

Finally, he urges that you do not starve yourself in hopes of losing weight. Although calorie reduction is important to weight loss, it needs to be done within limits that do not set off the body’s ancient survival mechanisms, he said. Rather than excessively reducing calorie intake to achieve weight loss, a person should work toward increasing the body’s use of calories through both weight-bearing and aerobic exercise and maintaining a healthy metabolism.