General and histopathological results of a two-year study of rats fed semi-purified diets containing casein and soya protein.

Two semi-purified diets, identical except for protein (soya or casein), and a Purina chow diet were fed to groups of Sprague-Dawley rats and compared over a 22-month period for effects on body weight, organ weight, feed consumption, feed efficiency, protein efficiency, organ-to-body-weight ratios, certain organ mineral levels, gross pathology and histopathology. Feed efficiency, protein efficiency, body weight, and most organ weights were higher in the groups fed soya and casein, while feed consumption and most organ-to-body-weight ratios were relatively lower. Significant increases in the final pathology findings in the groups fed soya and casein included nephrocalcinosis (in females), hepatocellular vacuolization, gastric trichobezoars (hairballs), ulcerative pododermatitis (hock ulcers) and splenomegaly. This study demonstrates that the long-term feeding of semi-purified diets is feasible. Moreover, the differences observed between the protein sources, that is, soya and casein, were minimal, which suggests that either protein source is acceptable for long-term feeding studies.