The insulin/insulin-like growth factor-like signaling pathway, present in all multicellular organisms, regulates diverse functions including growth, development, fecundity, metabolic homeostasis, and lifespan. In flies, ligands of the insulin/insulin-like growth factor-like signaling pathway, the Drosophila insulin-like peptides, regulate growth and hemolymph carbohydrate homeostasis during development and are expressed in a stage- and tissue-specific manner. Here, we show that ablation of Drosophila insulin-like peptide-producing median neurosecretory cells in the brain leads to increased fasting glucose levels in the hemolymph of adults similar to that found in diabetic mammals. They also exhibit increased storage of lipid and carbohydrate, reduced fecundity, and reduced tolerance of heat and cold. However, the ablated flies show an extension of median and maximal lifespan and increased resistance to oxidative stress and starvation.

In a number of organisms, dietary restriction has been shown to extend lifespan at the expense of reduced fecundity. An example of competition for limited resources, the thinking generally goes, as nutrients are reallocated from reproduction to somatic maintenance. Grandison et al. now demonstrate that, in Drosophila, dietary restriction does not induce reallocation, because different amino acids limit lifespan and fecundity. They find that fecundity in long-life flies can be rescued by the addition of methionine alone. Long lifespan and high fecundity can thus occur simultaneously, without dietary restriction itself, by adjustment of the ratio of amino acids in the diet. These findings imply that humans might be able to enjoy the benefits of reduced food intake, without the downsides, by adjustment of dietary nutrients. Dietary restriction extends healthy lifespan in diverse organisms but reduces fecundity; this is thought to be because of an adaptive reallocation of nutrients from reproduction to somatic maintenance. Here, the nutrients producing the responses of lifespan and fecundity to dietary restriction in Drosophila are identified. Adding essential amino acids to the dietary restriction condition increased fecundity and decreased lifespan; furthermore, addition of methionine alone rescued fecundity. Dietary restriction extends healthy lifespan in diverse organisms and reduces fecundity1,2. It is widely assumed to induce adaptive reallocation of nutrients from reproduction to somatic maintenance, aiding survival of food shortages in nature3,4,5,6. If this were the case, long life under dietary restriction and high fecundity under full feeding would be mutually exclusive, through competition for the same limiting nutrients. Here we report a test of this idea in which we identified the nutrients producing the responses of lifespan and fecundity to dietary restriction in Drosophila. Adding essential amino acids to the dietary restriction condition increased fecundity and decreased lifespan, similar to the effects of full feeding, with other nutrients having little or no effect. However, methionine alone was necessary and sufficient to increase fecundity as much as did full feeding, but without reducing lifespan. Reallocation of nutrients therefore does not explain the responses to dietary restriction. Lifespan was decreased by the addition of amino acids, with an interaction between methionine and other essential amino acids having a key role. Hence, an imbalance in dietary amino acids away from the ratio optimal for reproduction shortens lifespan during full feeding and limits fecundity during dietary restriction. Reduced activity of the insulin/insulin-like growth factor signalling pathway extends lifespan in diverse organisms7, and we find that it also protects against the shortening of lifespan with full feeding. In other organisms, including mammals, it may be possible to obtain the benefits to lifespan of dietary restriction without incurring a reduction in fecundity, through a suitable balance of nutrients in the diet.

Overexpression of sirtuins has been reported to increase lifespan in Caenorhabditis elegans and Drosophila melanogaster. The effects of dietary restriction on the lifespan of flies have also been reported to be Sir2-dependent. Here it is shown that these findings are attributable to the confounding effects of genetic background. This work suggests that the life-extending effect of sirtuin overexpression may be limited to yeast, and also supports studies in yeast and C. elegans that show that the effects of dietary restriction are not mediated by sirtuins. Overexpression of sirtuins (NAD+-dependent protein deacetylases) has been reported to increase lifespan in budding yeast (Saccharomyces cerevisiae), Caenorhabditis elegans and Drosophila melanogaster1,2,3. Studies of the effects of genes on ageing are vulnerable to confounding effects of genetic background4. Here we re-examined the reported effects of sirtuin overexpression on ageing and found that standardization of genetic background and the use of appropriate controls abolished the apparent effects in both C. elegans and Drosophila. In C. elegans, outcrossing of a line with high-level sir-2.1 overexpression1 abrogated the longevity increase, but did not abrogate sir-2.1 overexpression. Instead, longevity co-segregated with a second-site mutation affecting sensory neurons. Outcrossing of a line with low-copy-number sir-2.1 overexpression2 also abrogated longevity. A Drosophila strain with ubiquitous overexpression of dSir2 using the UAS-GAL4 system was long-lived relative to wild-type controls, as previously reported3, but was not long-lived relative to the appropriate transgenic controls, and nor was a new line with stronger overexpression of dSir2. These findings underscore the importance of controlling for genetic background and for the mutagenic effects of transgene insertions in studies of genetic effects on lifespan. The life-extending effect of dietary restriction on ageing in Drosophila has also been reported to be dSir2 dependent3. We found that dietary restriction increased fly lifespan independently of dSir2. Our findings do not rule out a role for sirtuins in determination of metazoan lifespan, but they do cast doubt on the robustness of the previously reported effects of sirtuins on lifespan in C. elegans and Drosophila.

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.

A chemically defined diet for Drosophila melanogaster is described. It should enable a variety of behavioral, metabolic and fitness studies where controlled nutrition is important. A critical requirement for research using model organisms is a well-defined and consistent diet. There is currently no complete chemically defined (holidic) diet available for Drosophila melanogaster. We describe a holidic medium that is equal in performance to an oligidic diet optimized for adult fecundity and lifespan. This holidic diet supports development over multiple generations but at a reduced rate. Over 7 years of experiments, the holidic diet yielded more consistent experimental outcomes than did oligidic food for egg laying by females. Nutrients and drugs were more available to flies in holidic medium and, similar to dietary restriction on oligidic food, amino acid dilution increased fly lifespan. We used this holidic medium to investigate amino acid–specific effects on food-choice behavior and report that folic acid from the microbiota is sufficient for Drosophila development.