There are three types of laboratory animal diets in biomedical researches, including natural- ingredient diets, purified diets and chemically defined diets. There are variations in the composition of diets from different manufactures and different batches of one brand. The variation of the diet composition and the feeding regimes can affect the health status and metabolism of experiment animals, and enhance or mask the response of animals to a given stimulus, which can adversely affect the accuracy and precision of experiment results. As part of an experiment food intake may differ between control and experiment groups. This can create biased results and lead to a false interpretation of the data.