Biochemical changes initiated by calorie restriction appear quite rapidly at the cellular level. The value to your health is in keeping that going over the long-term, but it is interesting to see the speed with which the controlling systems of metabolism react to dietary changes. Recent research into calorie restriction as an adjunct to chemotherapy illustrates the point:
Fasting before chemotherapy makes the toxic treatments less dangerous and more effective - a clever hack that could let doctors deliver drugs straight to cancer cells without actually targeting them.
Caloric restriction activates cell-rejuvenating genes and could explain why tumor-ridden mice starved for two days were able to handle chemotherapy doses that killed half of a control group and left the other half devastated.
Critically, while healthy cells were protected, cancer cells remained vulnerable to the treatment. Longo effectively sent drugs straight into the tumors - a much-anticipated technique that's so far eluded cancer researchers.
Though Longo duplicated the effect in human cells, it needs to be stressed that this is a very early finding. Nonetheless, scientists are enthusiastic: if healthy cells are exempted, chemotherapy doses could be upped and side effects lowered.
The paper can be found at PNAS for those who like to read the scientific literature.
I point this out largely out of interest value for most of us. I'd like to think that there won't be much bulk, untargeted chemotherapy taking place in the world two decades from now - a reasonable expectation given current trends and progress in cancer research. The replacement for chemotherapy will be engineered and targeted vector therapies, using new knowledge of the molecules and cell structures that mark cancer cells as different, destroying only those cancer cells while leaving the patient otherwise perfectly healthy and unharmed.
The practice of calorie restriction for health is still a smart idea, even an age of nanomedical cancer killers, of course. Why be more unhealthy than you have to be? Every extra year you can live in good health without the intervention of medical science is another year for that medical science to advance and improve for the time when you do need it.