Broiler breeders, with their elevated metabolism and higher body temperatures, actually generate substantially more free radicals than a comparably sized mammal, yet our studies reveal that the concentration of glucose derived crosslinks is approximately a thousand-fold lower than in mammals. One clue to understanding this apparent anomaly is the finding that diet restriction, a normal feeding practice for broiler breeders, dramatically limits the crosslinking process in both birds and mammals and thus retards the rate of tissue aging. However, diet restriction in mammals is associated with a reduction in overall plasma glucose concentrations, whereas in birds, concentrations of plasma glucose may stay the same or increase slightly. Diet restriction is associated with a much lowered production of free radicals. Even so, this does not explain why the overall concentration of tissue crosslinks is markedly lower in birds compared to mammals.
Most mammals excrete urate as a minor end product of nitrogen metabolism.
In contrast, urate is excreted as the major end product of nitrogen metabolism by birds, most reptiles and some amphibians . Collaborative studies have shown that a reduction in uric acid concentrations in chickens dramatically increases the rate of tissue aging, actually to a rate found in diabetic humans. Birds maintained on a lowered blood concentration of uric acid for as little as two months show a highly significant increase in oxidative stress and tissue crosslink formation. The marked decrease in meat tenderness as measured by shear force values in these animals is consistent with an increase in tissue crosslinking. In mammals this results in a condition called gout.