Mattison said this could be a limitation. "Certainly quality of the food and the nutrient composition/ratios could factor into the equation. Because calorie restriction is causing a metabolic stress, it is reasonable to speculate that a nutritionally complete and balanced diet would be better for the organism, regardless of the quantity," she said.
Another difference: The NIA monkeys were given two meals a day on a schedule while the Wisconsin monkeys ate whenever they pleased. Both groups were also genetically quite diverse; since each study included a relatively small number of individuals – 70 divided between calorie restriction and control groups – the genetic variations might have further skewed results.
Whatever the mechanisms may be, Chilton says the two primate studies are heroic and should be respected for being the first long-term investigations to provide clues about how humans might respond to eating a sparse diet. Nevertheless, the debate will certainly continue.
"We didn't see the miracle change in longevity we've seen in rodents and other primitive animals, but questions about the effects of calorie restriction on aging are far from answered," he said.
As for Averill and Paul McGothin, they say these recent findings don't shake their faith in calorie restriction in the least. They plan on continuing with the diet and spreading the gospel through their organization, CR Society International.
"At my recent physical exam my doctor told me I am in remarkable shape," McGothin says. "That's all the proof I need."