Ask the Diet Doctor: The Truth About Intermittent Fasting

Q: Is intermittent fasting a safe weight-loss strategy?

A: Intermittent fasting has become increasingly popular over the past several years, especially in the fitness world. But it’s not a new trend—fasting has been around for centuries. Fasts have typically been used for religious reasons (Ramadan and Lent are the two most prominent examples), but even Hippocrates used fasting as a means of promoting weight loss:

Obese people and those desiring to lose weight should perform hard work before food. Meals should be taken after exertion and while still panting from fatigue. They should, moreover, only eat once per day and take no baths and sleep on a hard bed and walk naked as long as possible.

It is estimated that 14 percent of adults in the U.S. have used fasting as a method to lose weight. That is pretty significant considering at any given point 44 percent of Americans are dieting.

Recently there has been an explosion of new fasting methods for weight loss. One of the most popular strategies is to fast for 24 hours twice a week (or every seven days) and eat normal amounts of food the rest of the time.

RELATED: Could Intermittent Fasting Help You Lose Weight?

By not eating for an entire day, you aren’t going to develop any nutritional deficiencies or hormonal imbalances—you’re just going to be hungry. One of the lynchpins to fasting success is that when you stop your fast, you just go back to eating a regular healthy diet. One study published in Appetite found no changes in body weight during Ramadan (fasting from sun up to sun down). The simple explanation: Total calories matter, so if you eat all the calories you “fasted off” in the subsequent days (or hours), you won’t lose weight.

Same goes if you fast for 16 hours and limit your eating to the remaining 8 hours of the day, as one new book recommends. Fasting is not the “golden ticket” to uncontrollably indulge in two steaks with potatoes, finish it off with chocolate cake, and then expect to be dropping 1 percent of your body weight per week. It isn’t going to happen.

Won’t Fasting Crash My Metabolism?
I am often asked this question when the topic of fasting comes up. For years “cheat meals” during periods of dieting were touted for their ability to keep your metabolism elevated due to some fuzzy logic between calorie intake and thyroid function. Just as one high-calorie cheat day isn’t going to fix weight-loss-induced thyroid issues, not eating for 24 or 16 hours isn’t going to crash your metabolism and render weight loss impossible.

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Safe, Yes—but More Effective?
Intermittent fasting has been around for a long time but isn’t for everyone. I would recommend it only for people who have a good grasp on healthy eating and are looking to break a fat-loss plateau. I don’t use fasting with my clients because I choose to focus on developing other nutritional habits that I believe are crucial for long-term diet success.

While fasting can be a safe option for weight loss when employed correctly, the jury is still out on if it is a more effective method of weight loss. Many of the metabolic benefits that you see as a result of fasting are also experienced on a very low-carbohydrate diet (<10% of calories from carbs). It seems that the absence of carbs while fasting and also while on a very low-carbohydrate diet is the key factor, so while, yes, intermittent fasting is safe and effective, it most likely isn’t more effective than a very low-carb plan.