Some people say that losing weight is easy compared with keeping it off. Alternate-day fasting may offer a way around this problem. People who adopt this eating style eat whatever they want one day, but on the following day, they restrict their caloric intake to about 25 percent of what they require to maintain their weight.
For example, a sedentary 50-year-old man who weighs 200 pounds needs about 2,200 calories a day to maintain his body weight. On fasting days he would allow himself only about 500 to 550 calories, which is far from starvation rations. So if he started his 24-hour fast after dinner, he could have a plain omelet with one piece of toast for breakfast (250 calories), a salad with a generous tablespoon of an olive oil-based dressing for lunch (about 150 calories) and a big apple (100 calories) for an afternoon snack. Then he could revert to his normal eating habits for the next 24 hours, beginning at dinnertime.
Alternate-day fasting does not require fasting every other day. That man could "fast" every third or fourth or fifth day and still reduce his cumulative weekly caloric intake without suffering the gnawing hunger that tends to descend on traditional dieters who restrict their caloric intake every day.
Yes, but people who practice alternate-day fasting will make up for lost calories on the days they can eat normally, right?
Not so, according to Krista A. Varady, an assistant professor of kinesiology and nutrition at the University of Illinois in Chicago, who has been conducting research on alternate-day fasting.
"They ate between 100 and 125 percent of their energy needs on normal days," she said of people who have participated in her studies. "We assumed they would eat double their energy needs (on nonfast days), but that doesn't happen, possibly because their stomachs are shrinking. Even if they wanted to eat a lot they couldn't. A lot of them say they look forward to their feed day — they wake up and make a big breakfast — but then they can't finish it."
Varady suspects that alternate-day fasting triggers some sort of satiety response — the body learns to recognize when it has consumed enough calories.
"Because we're eating all the time in this society, we've kind of forgotten how to figure out when we feel full," she said.
In addition, she added, cutting calories one day is not so frustrating when you know you can eat normally the next. The diet is so congenial that some subjects, even after they reach their weight goals, continue to practice a modified form of alternate-day fasting.
"They continue doing alternate-day fasting by eating 50 percent of their calorie needs on fasting days instead of 25 percent, and they're able to maintain their weight loss," Varady said. "The structure seems to help them adhere to the diet."
Alternate-day fasting may also alleviate one of the most unfortunate aspects of traditional dieting: the loss of muscle mass along with fat. When the body is deprived of sufficient calories to meet its energy needs, it draws on fat stores, but it also breaks down the protein in muscle fibers. Dieters who don't exercise may lose almost as much muscle mass as fat.
"With alternate-day fasting, pretty much all the weight loss is fat mass," Varady said. "Lean mass is maintained, even in the absence of additional exercising. I'm not sure why that happens. We need to look into the mechanisms of protein turnover and why they maintain muscle mass with the fasting regimen."
Alternate-day fasting has been around for at least 20 years, according to Varady, who started to study it to see if it could prevent cancer and other diseases. She quickly noticed that lab mice on the diet, even though they were losing weight, weren't losing muscle mass.
"I started getting e-mails from doctors interested in trying alternate-day fasting themselves to lose weight," she said. "I get updates periodically from seven to 10 people who have been doing it for two or three years. It works for them. They feel they can incorporate it into their lifestyle."
Those who drop out of her studies complain that they find alternate-day fasting disrupts social eating, such as family meals and dinner parties.
"What's really difficult for them is, if you're allowed only lunch one day, what do you do if you want to sit down with your family for dinner?" Varady said. "And what if it's Thanksgiving or Christmas? We tell them they can eat on Christmas — one day out of a 12-week study doesn't matter — but food is so incorporated into the way we socialize."
Tom Valeo writes frequently about health matters. He can be reached at firstname.lastname@example.org.