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A multi-isotopic study of several components of milk has been carried out on commercial samples and on milk produced in feeding experiments involving different kinds of diets originating from C(3) or C(4) photosynthetic metabolisms and exhibiting a relatively wide range of isotope ratios. The dispersion of the carbon, hydrogen, and nitrogen isotope parameters of dried matter and of the lactose, protein, and lipid components has been estimated. In addition, the carbohydrates were represented by the site specific isotope ratios (SNIF-NMR) of ethanol resulting from standardized fermentation of lactose. The rates of response of the isotopic parameters to changes in the feeding materials is slower for the minor components, proteins, and lipids than for lactose and ethanol. For similar diets, the nonexchangeable sites of lactose and the methyl site of ethanol, in particular, are relatively enriched in deuterium in the case of polygastric animals, cow, goat, and ewe, as compared to the monogastric species, sow and mare, and woman. From an analytical point of view, the carbon and hydrogen parameters of ethanol provide efficient criteria for identifying a whey origin with respect to other agricultural and fossil sources.

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