VANCOUVER, CANADA--A looming question in the calorie restriction field is what the drastic diet does to humans--and whether, as in many species, it extends life. Preliminary results from human tests can't answer that second question yet. However, they show that short-term calorie restriction prevents some oxygen damage to cells--but then, so do a milder combination of diet and exercise.
No one is quite sure how calorie restriction works or what it will do to people. But in nearly every species tested, from yeast to mice, the regimen extends life, sometimes by up to 50%. A popular theory is that the diet reduces oxidative stress, a common part of aging in which cells all over the body are damaged by free oxygen radicals. Long-term studies in primates are finding that cutting calories may prevent age-related diseases such as atherosclerosis, but it's not yet clear whether such a diet helps them live longer.
In 2002, the National Institute on Aging began funding studies called CALERIE, which included up to one year of calorie restriction in humans at three centers around the country. The first to finish, the Pennington Biomedical Research Center in Baton Rouge, Louisiana, reported results from its 6-month trial yesterday at the annual meeting of NAASO, the Obesity Society.
Run by physiologist Eric Ravussin, graduate student Madlyn Frisard, and others, the Pennington study recruited 48 sedentary, healthy, somewhat overweight individuals aged 25 to 50. The researchers assigned the subjects to one of four groups: calorie restriction by 25%; calorie restriction by 12.5% combined with aerobic exercise; cutting calories until volunteers had lost 15% of their body weight, and then maintaining that weight; and a healthy diet recommended by the American Heart Association that wasn't intended to induce weight loss. Volunteers were occasionally put in special chambers that measured their body's energy expenditure.
After 6 months, Frisard reported, the first three groups had similar results. Energy expenditure decreased by around 5%, meaning metabolism was lower, and blood tests showed that oxidative damage dropped to levels 10-15% lower than the control group on the heart-healthy diet. In other words, in this brief study, strict calorie restriction didn't seem to afford special benefits compared to other approaches. However, Frisard said she didn't know whether differences among these groups would show up in the longer term. And it's far too soon to say whether severe diets can extend human life, she added.
"It's very early," agreed James Hill, director of the Center for Human Nutrition at the University of Colorado in Denver. Still, he says, he found the results "optimistic"--because they hint that long-term strict dieting may not be the only way to protect cells from oxygen damage.