Eating is an inherently dangerous activity.
As carnivores, our immune system must provide protection from a wide range of potentially pathogenic micro-organisms and toxins that can contaminate meat, other foods and water, a task that must be achieved without at the same time provoking immune reactions to food itself. This difficult balancing act works well for most — but not all — people. The cost of immune hyper-vigilance is the propensity to develop food allergies, which can sometimes be fatal.
And as herbivores, we also need to protect ourselves from an enormous number of chemical substances — “secondary metabolites” — that plants produce. To help deal with the daily chemical onslaught in our diet, we have evolved a number of behavioural, metabolic and other non-immunological adaptations that allow most of us to choose plant foods we can eat safely. Some people, however, are more sensitive to the adverse effects of various natural and added food chemicals. Clinically, this problem presents as food intolerance. Though unpleasant, inconvenient, and at times debilitating, this propensity can also have survival advantages for affected individuals and their offspring.