The goal of this protocol is to determine the vitamin A value (equivalence) of spirulina. The investigation will use intrinsically deuterium labeled spirulina and an isotope vitamin A reference dose, 13C10- retinyl acetate (13C10 RAc), in males (n=20). Up to 45 blood samples (10 ml/sample) will be collected from each subject over a two-month period to evaluate the bioavailability and bioconversion of spirulina ß-carotene to vitamin A.