Six dairy cows of two breeds were fed during three alternating periods with products from C,-and C,-plants to yield different natural 13C enrichments of the diet (6I3C range: -28.0 to -13.7%,,). The resulting changes in the I3C enrichment of breath carbon dioxide, serum and milk of the animals followed the 13C:1zC of the food, in agreement with the individual biological half-lives of those products, and established isotope discriminations. Breath CO, was more enriched in I3C than expected. This could be related to isotope discriminations during rumen fermentation. From these results an isotopic balance model for the breath CO, could be established. Carbon isotope discrimination: C,-and C,-plants: Cow

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In this study the fate of 100 mg orally administered (1-13C)- and (8-13C)triolein was traced in the serum lipids of four healthy human subjects. After an overnight fast the subjects consumed hourly meals of a liquid formula diet over 12 h. Ninety minutes after the first meal in the first study (1-13C)triolein was given and in the repeat study the same subject received (8-13C)triolein. Triacylglycerol (TG), phospholipid (PL) and cholesterol ester (CE) were isolated from serum sampled prior to and in intervals after isotope administration. Fatty acid composition of serum lipids was measured using gas chromatography/mass spectrometry. 13C enrichment in palmitic, stearic, oleic and linoleic acids of these fractions was determined by gas chromatography/isotope ratio mass spectrometry. With (8-13C)triolein a significantly higher enrichment (peak +17.1 +/- 14.3/1000 delta 13C) was found in the oleic acid of TG fraction than with (1-13C)triolein (peak -7.1 +/- 4.2/1000), which may be due to a faster elimination of (1-13C)oleic acid from serum TG. 13C enrichments in the other fatty acids of the TG fraction as well as of PL and CE fractions were in the range of natural 13C abundance (-25 to -32/1000).

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