Knowledge of the normal nutrient requirements of a species is an important factor in selection of an appropriate model. Potential effects of diet on experimental variables can be evaluated for common laboratory species whose nutrient requirements are well known but this may not be possible for rarely used, or seldom studied, species whose normal diet may be unknown. Use of poorly nourished animal subjects, or subjects which lack some critical dietary component, may introduce extraneous, uncontrolled variability into experiments.