Acute bacterial challenge in Drosophila reveals age and sex dependent feeding and macronutrient choice without generalised anorexia

Sickness behaviours are often interpreted as adaptive host responses that reallocate resources from performance to defence. Anorexia - a reduction in food intake - is one of the most frequently cited examples, yet evidence across insects is variable and rarely separates the effects of wounding, immune stimulation, and live infection. Here we use a factorial design in Drosophila melanogaster to disentangle these components and quantify both feeding amount and macronutrient choice during the first four hours following acute bacterial challenge. Using the FlyPAD system, we recorded cumulative sips from protein and carbohydrate rich diets in young (2-4 days old) and older (22-24 days old) male and female flies. We compared naïve flies to those injured, immune stimulated with heat-killed Pseudomonas entomophila, or infected with live P. entomophila. Across sexes and ages, neither live infection nor sterile immune stimulation reduced total food intake during the first 4 h; notably, in old females and young males, live infection increased feeding. In contrast, immune stimulation alone elicited sex and age dependent shifts in macronutrient choice: in old females, injury and immune stimulation reduced protein preference relative to naïve, whereas in old males they increased it. Thus, ‘sickness feeding’ in Drosophila manifests primarily as an increase in feeing rate or a host driven macronutrient rebalancing rather than anorexia.