Their body temperature is down as well, by about half a degree. Metabolism seems to be reset after about 18 months on the diet, during which period the animals experience the biggest drop in weight. When they stabilize at a lower weight, there's a continued reduction in energy expenditure, which would have to be the case because they're eating less. The researchers now plan to compare how different organs respond to the calorie restriction to see whether some reduce their metabolism more than others. The lower the metabolism, the less oxygen is consumed and the fewer free radicals from oxygen are available to damage cells.
Many researchers suspect that damage by free radicals plays a role in the aging process.
To pinpoint this mechanism in the monkeys, the researchers plan to examine the membranes of mitochondria, where cellular energy and free radicals are produced. The basic fuel molecule that gets burned in metabolism is the sugar glucose. The calorie-restricted monkeys seem to be avoiding diabetes, one of the clearest health benefits of the primate study so far, says Kemnitz. At the start of the Wisconsin study, insulin concentrations after fasting indicated that four of the monkeys had a genetic risk of diabetes. Two of them were put on the restricted diet.