You'll recall the recent study on mortality and weight that I wasn't overly impressed with. I've seen one or two articles of late in which negative comments were made about calorie restriction in this context. In an e-mail to the Extropy Chat list, Brian Delaney of the Calorie Restriction Society notes the fallacies inherent in this sort of thing:
No conclusions about the benefits of a CR program can be drawn from studies showing a correlation between weight and mortality. People trying to draw such a conclusion are committing a serious logical error, one with potentially devastating health consequences for those who buy the error.
"CR leads to a reduction in weight." Yes.
"A reduction in weight is a sign of CR." No.
CR is not about being thin. Naturally obese mice (ob/ob) on severe CR are still chubby, but live much longer than naturally thin mice not on CR. Energy-restriction shifts resources away from growth and reproduction towards repair and maintenance. Doesn't matter what you weigh.
Indeed, the assumption (or false conclusion) that "accidental/unintentional CR" is more likely to be found among the underweight is not only wrong, it may even be backwards. People in the countries whre these mortality studies tend to be conducted who are naturally thin have LESS reason to restrict their food intake (and note: food restriction is not the same as Calorie restriction -- though that's a minor point), given societal pressures to be thin.
The way to determine whether or not CR reduces mortality is to look at people on CR and compare them to people not on CR. This is being done. Some initial results include those reported by Fontana .
It will take a long time before we can be certain that CR dramatically reduces mortality, but it seems extraordinarily like that it does so, and we can be certain that risks of diseases of aging (certainly, type 2 diabetes) is reduced significantly.
There are LOTS of sensible reasons not to be on CR. Believing that "it doesn't work" isn't one of them.
 Fontana L, Meyer TE, Klein S, Holloszy JO.
"Long-term calorie restriction is highly effective in reducing the risk for atherosclerosis in humans."
Proc Natl Acad Sci U S A. 2004 Apr 27;101(17):6659-63. Epub 2004 Apr 19.