As more research groups pay attention to the underlying genetics and biochemistry of calorie restriction - the only diet that has been proven to extend healthy life span in mammals such as mice and primates, and looks very promising for humans too - the basic science is becoming more solid. See this piece at Betterhumans:
The exact molecular and cellular pathways through which caloric restriction works, however, have proven unclear. It is generally thought that caloric restriction works by triggering a family of enzymes called sirtuins that protect cells and delay cell death.
This hypothesis is based on studies in yeast and worms that have shown that a sirtuin called Sir2 and its homologues are a key lifespan regulator. But while the Sir2 molecular pathway has been linked to calorie availability in yeast, write Rogina and Helfand, it has not been shown to function in the calorie restriction pathway in multicellular organisms.
For their study, the researchers created strains of flies that either overexpressed or underexpressed Sir2. They found that in four strains in which Sir2 expression was significantly increased, lifespan was extended by as much as 57%.
Conversely, in two other strains in which Sir2 expression was not elevated or was only marginally elevated, lifespan was not extended. The researchers also showed that lifespan cannot be extended by caloric restriction in flies that lack Sir2, providing evidence for a primary role of Sir2 activity in determining longevity.
"Together, these observations make a strong case that calorie restriction extends lifespan in flies by increasing Sir2 activity," write the researchers.
It's still possible that calorie restriction operates differently in yeast, but with more people looking at the problem now, we should know for sure within a few years. Financial backers are sensing a comparatively near term business opportunity in calorie restriction mimetic drugs - substances that reproduce the benefits to health and longevity without the dieting - and the money is flowing.