This article in PLoS caught my eye today. It’s entitled, “Calories Do Not Explain Extension of Life Span by Dietary Restriction in Drosophila”, and is an extension of the body of science showing that caloric restriction in a variety of animals, from fruit flies to non-human primates, may dramatically extend life-span.
Currently the mechanism is not well understood, but this surprising new result suggests that rather than absolute calorie restriction, decreased protein intake may be more critical for this beneficial effect.
In this study, fruit flies were fed a mixture of carbohydrate and protein, and their life span was measured with restriction of the carbohydrate, the protein, or both. Their results were surprising:
Flies fed food media with very similar caloric content showed marked differences in their life spans (see Figure 3). This finding is in direct contrast to what would be predicted if ingested calories were the key mediator of life span in D. melanogaster and demonstrates that the nutritional composition of the diet affects life-span extension by DR in this species. Reduction in the concentration of either sugar or yeast levels increased life span (see Figures 2 and 3). However, the magnitude of the effects on life span when the caloric content of the food was changed via altering yeast concentration far exceeded that seen when calories were changed to the same extent via manipulation of sugar levels, suggesting that protein/lipid levels have a greater effect on Drosophila survival than does carbohydrate. The differing effect of sugar and yeast on mortality in Drosophila could occur if different pathways sense nutrients during DR, possibly with different outputs affecting life span.
Here’s a table summarizing some of the data
DR SY corresponds to restriction of both carbohydrate and protein (sugar and yeast), DR Y is dietary restriction of protein, DR S is dietary restriction of carbohydrate, and the control SY is normal diet. As you can see, both the restriction of both protein and carbohydrate together, as well as restriction of just protein, had a dramatic effect on the median and maximum life span of the fruit flies.
Now, this study has only been completed in fruit flies, an animal very distant from humans, but I still think it’s interesting for a couple reasons. One, the extension of lifespan through restriction of caloric ntake has been consistent across a variety of animals, and appears to be a very real effect. It seems likely, to me at least, that this effect will be replicated in mammals especially considering a brief statement from the introduction suggesting studies in mice in rats are already underway showing a similar effect, “However, in other experiments, rats fed isocaloric diets with altered nutritional composition [17,18] or reduced protein  showed life-span extension. Furthermore, reducing just one amino acid (methionine) increases life span in both mice (R. Miller, personal communication) and rats .”
This doesn’t mean anyone should make drastic changes in the composition of their diets, but this sure will be interesting to follow. In the future it may become apparent that protein-rich diets are not as beneficial as some would currently like us to believe. I look forward to the extension of these studies to more relevant animal models.