Some essential amino acids modulate lifespan; the effect of non-essential amino acids, such as tyrosine (Tyr), is less clear. In a recent study, Kosakamoto et al. examine the contribution of Tyr to Drosophila melanogaster physiology.

Using a synthetic diet, the researchers established that Tyr deprivation prolonged lifespan and negatively affected reproductive functions. However, higher dietary amino acid levels or loss of reproductive capacity, which is a nutrient-consuming process, abolished the effects of Tyr restriction on physiology, suggesting that Tyr may become limiting only when flies have a high demand for amino acids and a low supply. Mechanistically, Kosakamoto et al. showed that Tyr deprivation decreased internal Tyr levels and activated the transcription factor ATF4 through an unclear pathway, yet removing ATF4 was not enough to eliminate the increase in lifespan caused by the lack of Tyr.