A dietary sterol trade-off determines lifespan responses to dietary restriction in Drosophila melanogaster females.

For the past fifteen years, animal studies have consistently shown that a low-protein, high-carbohydrate (‘carbs’) diet can extend the lifespan of many organisms, but at the cost of the number of offspring an individual can produce. Yet, it is still unclear what the best dietary balance is, and how these effects arise. One potential explanation could be that reproduction damages the body: low levels of proteins would therefore prolong life by lowering the reproductive output. Here, Zanco et al. examined the possibility that protein intake in fruit flies could instead be acting indirectly by changing the levels of a fat-like molecule called cholesterol, which is used to maintain the body and to support reproduction. To test this idea, groups of fruit flies were fed high levels of proteins. This led to increased reproduction rates, in turn depleting the mothers’ reserves of cholesterol. Without enough of the molecule in their diet, the insects were less able to maintain their bodies, which reduced their lifespan. When Zanco et al. added cholesterol to a high-protein diet, the flies lived for the normal length of time. Longer lifespan therefore did not require restriction of the diet or any of its components. In fact, the flies that lived the longest ate protein rich diets, and reproduced the most. This study helps to better understand why changes in diet can influence how long an organism lives for, highlighting that the abundance of certain key molecules may be more important than restricting the levels of proteins, carbs or calories actually consumed.