Effects of Dietary Fiber and Protein on Immunocompetence in White-footed Mice (Peromyscus leucopus)

Diet quality is known to have an effect on immune responsiveness. To determine what aspects of diet affect immunity and what aspects of the immune system are most vulnerable, we tested the null hypothesis that diet quality has no effect on immunocompetence and stress levels in the adult male white-footed mouse (Peromyscus leucopus). Adult males were trapped live from random patches of forest in western Kentucky. The mice were put on four diets of differing levels of protein and fiber. Health was assessed through measurement of daily metabolic rate, white blood cell counts, hematocrit, serum corticosterone level, and body organ masses. Differences in the four diets were confirmed by differences in the masses of the gastrointestinal organs, kidneys, and liver. Protein was more influential on organ masses than fiber. Despite differences in digestive efficiencies, final daily metabolic rates on all four diets were similar, confirming that all mice used similar amounts of metabolic energy per day. The differences in diet quality among groups were not associated with differences in immunocompetence. Our results indicated that differences in diet quality, which mimicked variation that occurs seasonally in the field, did not have a direct effect on immunocompetence. We propose that the relationships between diet quality and immunocompetence that occurred in a previous field study were simply correlative and not causal. Alternatively, immunocompetence may be more influenced by environmental stressors that are directly or indirectly correlated with diet quality such as predation, parasitism, or habitat quality, than by dietary factors alone