A number of studies have shown that putting rats on a diet severely restricted in calories increases their life span, and a few people have picked up on the notion and tried to do the same, eating only one small meal a day in hopes of living longer.
But there hasn't been much research on the effects of extremely low-calorie diets on humans. A new study published today in the Journal of the American Medical Association sheds some light on the subject, although it also raises new questions.
In the study, 48 overweight people were assigned to four groups. One group ate normally, while the other three were put on calorie-restricted diets that ranged from a 12.5 percent reduction in calories to no more than 890 calories a day.
The study was conducted by the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge.
Over the six-month study, researchers measured important changes, such as hormone levels, body temperature, insulin levels, fat mass and weight loss, among other things. They also examined energy expenditure and DNA damage.
At the end of the study, people in the three calorie-restricted groups had lowered their fat mass and body weight. In addition, they decreased their insulin level at fasting, body temperature, energy expenditure and DNA damage.
Scientists say that the decreased body temperature and insulin levels are particularly important in this study because they are good indicators of increased longevity and are often referred to as the biomarkers of longevity.
There are many scientific theories that try to explain why people age. According to some theories, aging is caused by damaged DNA, the genetic material of humans. As the DNA damage accumulates, disease and aging set in.
DNA can be damaged as a result of expending a large amount of energy, scientists say, and the fact that many participants in this study actually lowered their energy expenditure over the six-month period leads scientists to correlate this with decreased DNA damage and increased longevity.
Doctors, however, remain skeptical about whether this study has proved that calorie restriction increases longevity.
"Unfortunately, this study looked at biomarkers, not at actual longevity. ... Therefore, it is hard to attach any particular significance to it," said Dr. Darin Deen, a professor of clinical family and social medicine at Albert Einstein College of Medicine in New York.
Some physicians also questioned whether results based on this study's participants could be generalized to a larger population.
"Scientifically, this is not a significant study," said Dr. Eleftheria Maratos-Flier, an associate professor of medicine at Beth Israel Deaconess Medical Center in Boston "The group here was overweight. We already know that diet and weight loss are good for overweight people."
The six-month study also wasn't long enough, said Dr. Keith-Thomas Ayoob, an associate pediatrics professor at Albert Einstein.
The study's authors agreed that longer-term research is needed to determine whether their results can be sustained and whether calorie restriction does affect aging.
But even with long-term studies, many wonder if extreme calorie restriction is practical.
"The cynic might say, 'Sure you may live longer, but why bother if I can't eat?'" said Dr. John Messmer, an associate professor of family and community medicine at Penn State College of Medicine.
Dr. Jennifer Caudle is an intern at Delaware County Memorial Hospital in Drexel Hill, Pa. Caudle plans to specialize in family practice.