A clever experiment: "The ability of dietary restriction to increase animal life span is often thought to arise from differential allocation of resources between somatic investment and reproduction. In this theory, reproduction is repressed upon dietary restriction to make scarce nutrients available to somatic functions that increase survival. Here, we label nitrogen and carbon in the dietary yeast of Drosophila melanogaster with stable isotopes to determine whether resources are invested to somatic tissues at the expense of reproduction. We find that females on a full diet acquire and allocate more dietary carbon, nitrogen and essential amino acids (EAA) to eggs than females on a restricted diet. Full-diet females also invest more carbon, nitrogen and EAA into somatic tissue than those on a restricted diet. Thus, the longer lifespan of flies on a restricted diet relative to those on a full diet cannot be explained by greater absolute somatic investment, and high somatic investment does not ensure longevity." Alternative explanations: calorie restriction leads to changes in the efficiency with which resources are used, or in the efficiency with which cellular damage is repaired, either of which will reduce the rate at which damage accumulates - and thus tend to lengthen life.