Lessons from animal nutritionists: dietary amino acid requirement studies and considerations for healthy aging studies.

Dietary restriction (DR) increases median life span and protects against age-related disease. Improved longevity can be achieved by restriction of dietary energy, protein, or amino acids (AAs), such as methionine (Met). Met requirements have been defined using methodologies that measure the dose response to Met when all other dietary variables are held constant and with outcomes focused on protein turnover. Here, we cover protein and sulfur AA requirements and discuss the terms "deficient," "optimal," and "excess" and how these need to be considered. We additionally discuss the effect of methyl-donating compounds on sulfur AA metabolism and outcomes. We will discuss how the mechanistic target of rapamycin complex 1 (mTORC1) signaling network regulates protein turnover, lipogenesis and cell growth, proliferation, differentiation, and metabolism in response to hormones, AAs, and cellular energy status. Inhibition of mTORC1 signaling with rapamycin or genetic mutation increases median life span in model organisms, and mTORC1 inhibition may be responsible for some of the life span-extending effects of DR. Finally, we discuss how the sulfur AAs may regulate aspects of reactive oxygen species (ROS) mitigation. Overall, we suggest that approaches evaluating AA intake need to consider whole-body protein synthesis and measures related to tissue-specific and whole-body metabolism that have been associated with longevity.