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 [1-4]. 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. We find, however, that resource allocation to somatic tissue relative to investment to eggs is greatest in females on a restricted diet. To account for these patterns we propose that dietary restriction in Drosophila may extend lifespan through somatic investment relative to damage incurred from reproduction [5].