Their study of 16 nonobese men and women looked at the effects of alternate-day fasting, an eating plan that interspersed fasting days with "feasting" days that allowed participants to eat as much as they wanted.

The researchers were interested in whether food deprivation every second day would be easier on people than counting calories on a daily basis, lead author Dr. Eric Ravussin told Reuters Health.

They found that the diet plan was indeed "feasible," at least for the duration of the 3-week study. Overall, the men and women lost an average of 5 pounds, while shedding some body fat.

On the other hand, Ravussin said, "most people were not happy" -- a major problem being the crankiness that erupted on the fasting days and did not abate over the 3 weeks.

Ravussin and his colleagues at the Pennington Biomedical Research Center in Baton Rouge, Louisiana, report the findings in the American Journal of Clinical Nutrition.

Uncovering the most tolerable ways for people to cut calories is not just a matter of trimming waistlines. A number of studies, Ravussin explained, have now shown that calorie restriction can extend the lives of everything from yeast and worms to rodents and, possibly, monkeys.

There are a number of theories on why limiting food might make for a longer, healthier life. One idea, Ravussin noted, is that slowing the rate of metabolism reduces the generation of oxygen free radicals, potentially cell-damaging molecules that are a normal byproduct of the metabolic process.

No one knows if calorie counting can extend human life as it does for some animals. Ravussin and his colleagues are currently conducting a trial, sponsored by the National Institutes of Health, which is examining how long-term dietary restriction affects people's health and longevity.

Studying dietary restriction in people is, of course, far more complicated than studying it in lab rats. The ongoing trial is investigating how calorie cutting affects "biomarkers of longevity" in people, such as levels of blood sugar and insulin, a hormone key in regulating blood sugar.

The trial is testing several methods of dietary restriction -- from pure calorie cutting to burning extra calories through exercise -- to see which are most viable.

In the current study, Ravussin's team evaluated the effects of alternate-day fasting, an approach not included in that trial. They wanted to see if the eating plan was feasible and whether it influenced biomarkers of longevity, as well as participants' weight and metabolism.

As mentioned, every other day for 3 weeks the men and women subsisted on calorie-free beverages and sugarless gum. On the days in between, they ate whatever they wanted.

According to Ravussin, participants were not able to "make up" for what they didn't eat on fasting days, and consequently, they generally lost a few pounds and some fat mass. In addition, their insulin levels declined by an average of about 57 percent.

However, the researchers also found that many participants said they were irritable on food-free days, and they did not appear to get used to having an empty stomach every other day.

Calorie restriction, Ravussin noted, is the only behavior that has so far been shown to prolong life. "There is a ton of data suggesting that this is the only way to the 'fountain of youth', if you want," he said. If the current findings are any indication, the researcher added, alternate-day fasting may not be the easiest path to that fountain.