Body macronutrient composition is predicted by lipid and not protein content of the diet

1. Diet is an important determinant of fitness related traits including growth, reproduction and survival. Recent work suggests that variation in protein : lipid ratio, particularly the amount of protein, in the diet is a key nutritional parameter. However, the traits that mediate the link between dietary macronutrient ratio and fitness related traits are less well understood. An obvious candidate is body composition, given its well-known link to health.
2. Here we investigate the relationship between dietary and body macronutrient composition using a freshwater fish, the three-spine stickleback (Gasterosteus aculeatus). Carbohydrate is relatively unimportant in the diet of predatory fish, facilitating the exploration of how dietary protein to lipid ratio affects their relative deposition in the body.
3. We find a significant effect of lipid intake, rather than protein, on body protein : lipid ratio. This was not a result of absorbing macronutrients in relation to their relative abund...