Ratios of nonhydrolyzed versus hydrolyzed nitrogenous constituents (NHP:HP) and medium-chain versus long-chain triglycerides (MCT:LCT) were modified in five formula-defined diets fed to male rats. We measured the effects of these dietary modifications on the incorporation of${}^{3}{\rm H}-{\rm TdR}$ in the thymus, the spleen, the lymph nodes, and the bone marrow of nonirradiated animals as well as on the survival of animals given total-body irradiation of 700 rad. It was found that both${}^{3}{\rm H}-{\rm TdR}$ incorporation in hematopoietic tissues (especially the bone marrow) and the ability to withstand the effects of 700 rad were inversely proportional to the NHP content of the diets. In the case of the lipidic constituents, a ratio of 1 MCT:4 LCT (providing that nitrogenous materials were present in a hydrolyzed form) gave the best results both in terms of a higher uptake of${}^{3}{\rm H}-{\rm TdR}$ by hematopoietic tissues and a greater resistance to radiation.

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