Hah. That's tough. First off, it's really difficult to try to establish a mechanism for WHY calorie restriction slows down ageing when we are not even in agreement as to what the mechanism of ageing is. This was discovered in the 1930's (that CR extends lifespan in rodents) and since then the proposed mechanism has changed as research into ageing has progressed.
Although there are many proposed mechanisms for the effect of CR you can put most of the mechanism into two camps.
1) Metabolism is very bad.
This camp basically points to the fact that with less food, you invariably have a lower metabolic rate, less oxidative damage, less inflammation, less plasma glucose, lower body temperature. These results are pretty well established. The story most biologists will give you is the story of blood glucose metabolism. We know if blood glucose is high then mitochondria operate poorly and generate more superoxide (a free radical implicated in ageing). If blood glucose is too high for too long we know cells eventually become insulin resistant which results in a increased glycation of proteins, higher probability of infection (bugs eat sugar), and what we call metabolic syndrome or type II diabetes.
2) Metabolism is not necessarily bad, but the lack of metabolism "trains" the body and this adaptation increases lifespan.
This camp really turns the idea of "free radical damage" on it's head. Most of the research in this camp is less than 5 years old. It's been shown in nematode that small increases in oxidative stress lead to increased resistance to further oxidative stress. Basically, it's the idea that the free radical damage actually trains the cell to be robust to further damage. It's believed that the low-intensity biological stress of calorie restriction A) Causes changes in cell physiology which make it more robust in the face of stress B) Cause transcription of "longevity genes" which turn on protective proteins like heat-shock protein.
********************
In all likelihood, its probably a combination of factors from both camps and a few unmentioned factors. But....the reason why the studies about intermittent fasting are so encouraging is because they shift the likelihood that camp 2 is playing a major role in the increased longevity. If IF == CR as far as results, it's not simply the lack of calories. It's a poor analogy, but I would describe Camp 1 is sort of like saying you will live longer if you do less because by doing less you accumulate less damage. To effectively calorie restrict activity must be kept to a minimum which is not what people who love to train want to hear! Camp 2 can be summarized as saying "just like you train your your muscles (with progressive overload) and they supercompensate so to must you train your cells (by intermittently reducing the amount of energy you provide to them) so they will supercompensate and become efficient
at glucose metabolism and ready to withstand all the stresses of life!
Now that's something we can all get behind! Hope that made sense!
