DR is often called calorie restriction or "CR" because calorie intake has been so often implicated as the primary factor, regardless of where the calories come from but it is far from clear that calories is always the key and increased life-span has been shown is isocaloric diets with things like protein restriction etc. So like Tom says in the article, things are far from clear.

1. 20. Zimmerman JA, Malloy V, Krajcik R, Orentreich N (2003) Nutritional control of aging. Exp Gerontol 38: 47–52

2. Calories Do Not Explain Extension of Life Span by Dietary Restriction in Drosophila." PLoS Biology : Publishing Science, Accelerating Research. Web. 24 Nov. 2010. <http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030223>.

I'll provide the absract, conclusion and then a link to the full article.

Calories Do Not Explain Extension of Life Span by Dietary Restriction in Drosophila

William Mair, Matthew D. W. Piper, Linda Partridge

Centre for Research on Ageing, University College London, Department of Biology, London, United Kingdom

Abstract

Dietary restriction (DR) extends life span in diverse organisms, including mammals, and common mechanisms may be at work. DR is often known as calorie restriction, because it has been suggested that reduction of calories, rather than of particular nutrients in the diet, mediates extension of life span in rodents. We here demonstrate that extension of life span by DR in Drosophila is not attributable to the reduction in calorie intake. Reduction of either dietary yeast or sugar can reduce mortality and extend life span, but by an amount that is unrelated to the calorie content of the food, and with yeast having a much greater effect per calorie than does sugar. Calorie intake is therefore not the key factor in the reduction of mortality rate by DR in this species.

Conclusions

The response of Drosophila life span to nutrition is not governed by calories, but rather by specific nutritional components of the food. This finding represents a departure from the generally accepted model in rodents, where it has been suggested that the level of calorie intake per se, not the source of calories, is critical for life-span extension [1]. The apparent disparity between the factors in the diet that affect life span in fruit flies and rodents leads to two possible conclusions. First, the mechanisms by which these organisms respond to food shortage could be different. Second, the long-held view that calorie intake is the critical variable in the response of mammalian life span to DR may require further evaluation.

Despite some reports in the literature that DR did not extend life span [38,41,42], the overwhelming majority of data support the idea that DR in some form extends life span across diverse taxa. However, it is still unknown if life-span extension under DR is achieved through common mechanisms in different species. A case for conservation of the mechanisms by which DR extends life span can be made from evolutionary considerations. It has been suggested that, during times of famine, diversion of resources away from reproduction towards somatic maintenance will increase the chances of an organism surviving to more plentiful times and thus increase long-term reproductive success [43–46]. The selective advantage of shifting resources from reproduction to maintenance when food is restricted could be the “public” factor shared between diverse organisms. However, the mechanisms by which extension of life span is achieved could be an example of convergent evolution, producing the same plasticity of life span in response to food shortage through mechanisms at least to some extent specific to different organisms, dependent upon their diet, experience of food shortages, and life history. More work is needed to elucidate the precise relationship between the composition of the diet and life span in different organisms, including mammals. Our results suggest that it may be possible to obtain the full extension of life span by DR by reducing critical nutrients in the food without any reduction in overall calorie intake.

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0030223