Desaturation and interconversion of dietary stearic and palmitic acids in human plasma and lipoproteins.

ABSTRACT

Dietary saturated fatty acids are implicated as a risk factor for atherosclerosis. The conversion of the major dietary saturated fatty acids stearic acid (180) and palmitic acid (160) to monounsaturated fatty acids in whole plasma and lipoprotein fractions is reported for seven healthy adult humans over 6 d using [U-13C]stearic acid (180*) and [U-13C]palmitic acid (160*) and high-precision mass spectrometry. A tracer dose (28-32 mg) of 180* or 160* was loaded into an emulsion and orally administered before breakfast. Serial blood samples were collected on day 1 and fasting blood was drawn daily until day 7. Overall conversion of 180 to 181 was approximately 14%, whereas that of 180 to 160 was approximately 2% in plasma up to 144 h. Conversion of 160 to 161 was < 2%, whereas conversion of 160 to 180 was approximately 6%. No other fatty acid metabolites were detected for 180* or 160*. The conversion products were observed mainly in chylomicrons and very-low-density lipoproteins, indicating that the intestine and liver have comparable roles in desaturating 180 and 160. Overall, these data indicate that dietary 180 desaturation is severalfold greater than 160 desaturation. The low level (14%) of 180 desaturation in omnivorous adults may have little influence on blood lipid profiles relevant to atherosclerosis risk.