Here is one research result among the many generated by scientists investigating the biochemistry of calorie restriction, seeking after a greater understanding of how it improves health and extends life:

[Researchers] examined the role of the compound β-hydroxybutyrate (βOHB), a so-called "ketone body" that is produced during a prolonged low-calorie or ketogenic diet. While ketone bodies such as βOHB can be toxic when present at very high concentrations in people with diseases such as Type I diabetes, Dr. Verdin and colleagues found that at lower concentrations, βOHB helps protect cells from "oxidative stress" - which occurs as certain molecules build to toxic levels in the body and contributes to the aging process.

"Over the years, studies have found that restricting calories slows aging and increases longevity - however the mechanism of this effect has remained elusive. Here, we find that βOHB - the body's major source of energy during exercise or fasting - blocks a class of enzymes that would otherwise promote oxidative stress, thus protecting cells from aging."

The researchers found that calorie restriction spurs βOHB production, which blocked the activity of a class of enzymes called histone deacetylases, or HDACs. Normally HDACs keep a pair of genes, called Foxo3a and Mt2, switched off. But increased levels of βOHB block the HDACs from doing so, which by default activates the two genes. Once activated, these genes kick-start a process that helps cells resist oxidative stress.

Link: http://www.sciencedaily.com/releases/2012/12/121206142025.htm