The Part-Time Fast: Should You Try It?

Surveying the studies on part-time fasting.

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Have you seen "Eat, Fast and Live Longer?" The recent PBS special chronicled the quest of popular British journalist and TV personality Michael Mosley toward weight loss and better health. Even if you haven't seen it, there's a good chance you've heard about, or perhaps you've read, "The Fast Diet," Mosley's best-selling book, or one of the many available 5:2 Diet books. Yet another—"The 2-Day Diet"—will soon be added to the crop of books that prescribe some form of fasting for weight loss.

At first glance, the idea of going without food for more than a few hours at a time may have little appeal. After all, if eating gives you energy, it makes sense that going without food for any period of time can make you feel sluggish, not to mention extremely hungry. It also makes sense that if you skip meals or try to subsist on little or no food for hours on end, you're more likely to finish that fast with a feast—a recipe for overeating. (Seriously, don't burgers, fries, chips, cupcakes and ice cream look extra good when you haven't had food in a while?)

[See How to Conquer Food Cravings.]

Fasting is not something I have ever recommended for weight loss. The possible perils of fasting include lower nutrient intake and low blood sugar (fasting can be especially dangerous for pregnant or nursing women, those with diabetes, those who take certain medications and athletes). But if you're looking to trim down for summer, for an event or for a big birthday, promises of "quick and effortless weight loss" and "incredible physical and mental changes," even while eating "all the foods you love," may lure you to give fasting a try. Should you?

[See 7 Ways to Lose Fat Fast—And Fit Into Your Skinny Jeans!]

Emerging research, mainly in animals, suggests benefits of this on-again, off-again eating pattern. Several animal studies have found that intermittent fasting preserves memory, reduces disease risk and even promotes longevity. In a recent article in the Canadian Medical Association Journal, National Institute on Aging researcher Mark Mattson suggests that fasting may exert physiological benefits; because it leads to mild stress in body cells, cells "respond to the stress adaptively by enhancing their ability to cope with stress and, maybe, to resist disease."

Despite these benefits, a 2013 study published earlier this year in PLoS One found that while intermittent fasting and dietary restriction contributed to significant weight loss in rats, it impaired their ability to reproduce.

While the data on the safety and effectiveness of fasting diets in humans is relatively sparse, some recent studies suggest some benefits. In a study published this month in British Journal of Nutrition, researchers examined the effects of intermittent energy restriction vs. daily energy restriction on weight loss and metabolic disease risk markers in 115 overweight women over four months (the first three months was the weight loss phase, and the fourth month was the weight maintenance phase).

[See The No. 1 Skill for Weight Management.]

Those in the intermittent energy and carbohydrate restriction (IECR) group consumed about 25 percent less than their baseline energy needs and 40 grams of carbohydrates for two consecutive days each week; they ate a Mediterranean-style diet to meet their estimated baseline energy needs the rest of the week. Another group followed the same protocol but was allowed unlimited lean meat, fish, eggs, tofu, monounsaturated fatty acids and polyunsaturated fatty acids on the two restricted days. Those in the daily energy restriction (DER) group consumed a diet that was "high" in protein and "moderate" in carbohydrates and fat and provided 25 percent less energy than estimated baseline energy needs. Results showed that women in the first two groups lost significantly more body fat and had greater improvements in insulin sensitivity than those in the DER group.

A 2012 study published in Nutrition Journal looked at the effects of intermittent fasting plus calorie restriction (IFCR)—with or without a liquid diet—on body weight, body composition and the risk of coronary heart disease. Fifty-four obese women were randomly assigned to follow, for eight weeks, one of the following protocols: a six-day, calorie-restricted liquid diet followed by a one-day "fast" (the "fast" provided 120 calories) on day seven; or a calorie-restricted, food-based diet for six days followed by the same "fast" used in the liquid diet protocol on day seven. The study showed that combining calorie restriction with a one-day fast contributed to reduced body weight and body fat (including visceral or abdominal or "belly" fat) as well as lower coronary heart disease risk in obese women.

[See The Truth About Women's Heart Health.]

Finally, in a 2011 study published in The International Journal of Obesity, researchers looked at the effects of intermittent energy restriction (IER) or continuous energy restriction (CER) over six months in 107 overweight or obese premenopausal women. Women were randomly assigned to follow either group. In the IER group, women consumed a very low-calorie diet that provided 25 percent less than their estimated energy requirements for weight maintenance for two consecutive days per week; for the remaining five days per week, women ate a diet that met their estimated daily energy requirements. In the CER group, women consumed 25 percent less than their estimated daily energy requirements seven days per week. All subjects selected their own diets. Researchers found that the IER and CER interventions were equally effective in helping women lose weight. Subjects in both groups also experienced similar reductions in cholesterol, triglycerides, blood pressure and other disease risk markers.

[See New Ways to Lower Cholesterol with Diet.]

Most animal and human studies thus far suggest potential weight-loss and health benefits of fasting—not the kind of fasting where you don't eat, but the kind that includes some food (just a whole lot less than you're used to). However, it's way too soon to tell whether the purported benefits are due to eating less frequently, eating less or combining the two strategies. We need a lot more high-quality, long-term studies before we can conclude that any type of fast should be considered a viable long-term weight loss option.

[See Want to Maintain Your Weight? Get Moving.]

For most of us, skipping a meal here or there and ending your eating in early evening aren't likely to sabotage otherwise healthful habits or your overall nutrition status. But until more is known, why not do what's tried and true to achieve safe, sensible and sustainable long-term weight loss: Load up on produce, whole grains and lean sources of protein; reduce portions, especially of high-calorie foods; move more and sit less throughout the day; and fast only when it's most convenient for you—when you sleep!

Hungry for more? Write to eatandrun@usnews.com with your questions, concerns, and feedback.

Elisa Zied, MS, RD, CDN, is the founder and president of Zied Health Communications, LLC, based in New York City. She's an award-winning registered dietitian and author of three books including Nutrition At Your Fingertips. A graduate of the University of Pennsylvania and New York University, Zied inspires others to make more healthful food choices and find enjoyable ways to "move it or lose it" through writing, public speaking, and media appearances. You can connect with her on twitter (@elisazied) and through her new Stressipes forum on her website: www.elisazied.com.