Measurement systems provide a determination of relative concentrations of biological analytes based on transmission or reflection of near-infrared radiation by an in vivo specimen. Concentration and concentrations ratios associated with (.omega.-3, .omega.-6, and .omega.-9 fatty acids, lipids, glycosylated proteins, blood glucose, and cholesterol can be determined, and based on the determination an indication of subject health can be provided, or a dietary recommendation can be made. In one example, ratio of a concentration of .omega.-3 fatty acids to a combined concentration of .omega.-6 and .omega.-9 concentrations is determined. Dietary supplements can be recommended or ordered from a supplier based on the concentrations and concentration ratios.