Dietary restriction extends adult Drosophila melanogaster life span when the concentration of dietary yeast is diluted in a media with abundant carbohydrates. Here we vary the concentration of casein as a source of amino acids in adult diet to uncover a quality of nutrient yeast responsible for longevity control. Longevity is maximized upon diet with intermediary levels of casein. Differences in survival are not caused by elevated age-independent mortality; the longevity maximum at intermediate casein does not arise because casein is non-specifically harmful at higher concentrations. Furthermore, fecundity increases when the level of dietary casein is elevated. The demographic phenotypes of adult Drosophila maintained on intermediate levels of casein resemble their response to limited dietary yeast. Dietary restriction through dilution of yeast may extend longevity because this limits the intake of amino acids.