Scientists have discovered that the optimal diet for male fruit flies may depend strongly on whether they are reproducing, challenging long-standing assumptions about nutrition and ageing.
Led by researchers from the University of Liverpool's Department of Evolution, Ecology and Behaviour, the international team used fruit flies to investigate how diet and reproductive activity influenced lifespan and fertility.
The team found that cholesterol, a key nutrient in the flies' diet, plays a surprising and context-dependent role. Male flies required cholesterol to maintain reproductive activity, but only when they had access to mating opportunities. When males were not reproducing, they often lived longer without cholesterol in their diet.
The findings suggest that reproduction places a significant nutritional demand on males. Researchers believe that mating males may lose cholesterol through sperm and seminal fluid, depleting their reserves and shortening lifespan.
These results challenge previous research which suggested that cholesterol didn't affect male lifespan. By allowing males to mate during experiments, the researchers were able to uncover differences that had previously gone unnoticed.
Lead author Dr Andy McCracken, University of Liverpool, said: "One of the big challenges is understanding what constitutes a healthy diet. It was previously thought that there was a single optimal diet for males. However, our work suggests that the best diet may depend on their reproductive state, much like what has already been shown in females.
"If we want to understand how diet shapes ageing, especially in males, we need to pay closer attention to reproduction and to whether our experiments actually allow animals to express the biology we are trying to measure."
The study also revealed a trade-off between longevity and reproductive health. Males lived longer on diets relatively high in protein and low in carbohydrates but showed healthier reproductive ageing when consuming lower-protein, higher-carbohydrate diets.
The work forms part of a broader effort to understand why ageing occurs. Fruit flies are a model organism highly responsive to nutritional changes and by manipulating their diet, the researchers aim to uncover the biological mechanisms that link reproduction, nutrition and lifespan.
However, the team cautions that the findings should not be directly applied to humans. Unlike fruit flies, humans can produce cholesterol internally, meaning dietary requirements differ significantly. The results are nevertheless important because they show how a single non-energy dietary component can alter the balance between survival and reproduction, and helps reveal why lifespan and reproductive success often appear to be in conflict within species.
The research team will now investigate how cholesterol is used during reproduction, including whether it is incorporated into sperm and seminal fluid or converted into hormones, to better understand the biological processes driving these effects.
The study 'Mating-dependent lifespan cost of sterol depletion in male Drosophila melanogaster' was published in PNAS (DOI:10.1073/pnas.2533735123)
