There's an interesting article and synopsis at the public access journal PLOS Biology about the effect of diet on fly longevity. Many organisms live longer if they are fed a restricted diet; in the case of mice the optimum seems to be cutting intake by about a third of their free intake. However, some recent data from the centers for disease control suggests that the link between human intake and longevity may be complex.
The current effort studied the mix in the diet on fly lifespan, by varying yeast (a source of protein and fat) and sugars. The authors found that reducing yeast honly as a more pronounced effect than reducing sugars only; although the best effect was seen when both were reduced.
Dietary studies always remind me of Woody Allen's Sleeper. I eagerly await news that steak is the secret for long life.