Lifespan
Rats fed a low tryptophan diet showed reduced blood levels of triiodothyronine, which was suggested to retard the aging process. Rats on tryptophan-reduced diets have shown increased maximum life span and improved biomarkers of aging (although the rate of initial deaths was higher than in controls). The result was attributed to harmful effects of the age-related increase in brain serotonin. Cancers upregulate the liver enzyme tryptophan dioxygenase because its primary product, kynurenine, increases tumor growth.. That would be especially relevant to lifespan studies in rodents, insofar as most rodents die of cancer. Additionally, excess dietary tryptophan has been shown to induce insulin resistance in pigs, a physiological condition that has a deleterious effect on many organ systems.
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