Is anorexia during infection in animals affected by food composition

A reduction in voluntary food intake, referred to as anorexia, is a consistent feature of infection in both humans and animals. Its occurrence is considered a paradox and the magnitude and duration of anorexia appear to be unpredictable. The suggestion made here is that the phenomenon would be more predictable if the different characteristics of anorexia were separately considered to establish how each is affected by food composition. The anorexia characteristics considered are the lag time between infection and appearance of anorexia, the rate of decline in food intake, the extent and duration of anorexia, and the rate of recovery in intake. The food composition characteristics considered are nutrient content, energy content and source, and ‘forage quality', as the phenomenon has received particular attention in ruminant animals. It is concluded that at least the last three characteristics of anorexia are affected by food composition in a predictable manner: subjects on a higher nutrient content food are expected to suffer anorexia to a greater extent than subjects on a lower nutrient content food, but, at the same time, show anorexia for a shorter period of time, and recover their food intake at a faster rate. An explanation is offered to account for this apparent contradiction, which is linked to the mechanisms of appetite regulation and the relationship between anorexia and host immune response. Proposed experiments that will provide stronger support to these suggestions and enhance our understanding of the mechanisms that underlie regulation of food intake during infection are put forward. Two of the issues to be resolved by such experiments are how food energy content affects the extent of anorexia across species, and whether and how food nitrogen content affects the same anorexia characteristic in ruminant hosts. Knowledge of the relationship between anorexia during infection and food composition will enable us to deal with the phenomenon in a more effective manner and overcome to a certain extent its negative consequences on infected hosts.