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ABO blood group phenotype and levels of serum total cholesterol (TC), triglyceride (TG), β + pre-β lipoprotein (LP Index) and lipoprotein cholesterol fractions (pre- β-, β- and a-LPC) were determined for 656 White and 371 Black adolescent school children participating in a community-based screening program for cardiovascular risk factor variables (Bogalusa Heart Study). Following appropriate adjustment of lipid values for concomitant variables, mean TC levels of ABO phenotypes were found to differ significantly in both races (A > B in Whites, B > O) in Blacks). Differences between highest and lowest ranking phenotypic means amounted to approximately 8 mg/dl TC in both races. Slightly over 1% (White sample) or 2% (Black sample) of the total variation of adjusted TC values was accounted for by ABO phenotype. Although the difference between B and O phenotypic means in Blacks was not statistically significant when adjusted cholesterol values were log10 transformed, the transform had little effect on the results of F-tests for heterogeneity of means or r2 estimates in either race. Analysis of phenotypic ratios at different percentile levels of lipid and lipoprotein distributions indicated that phenotypic ratios for Black and White children ranking above the 85th percentile of their respective β-LPC distributions differed significantly from ratios prevailing in the remainder of the study population. Among children with high β-LPC levels 0:non-0 ratios were consistently reduced in all race-sex groups; concomitantly, A:non-A ratios were selectively increased in Whites and B:non-B ratios increased in Blacks. Although ratio changes in the TC and LP index distributions were concordant with those in the β-LPC distribution, the data strongly suggest that the most direct association between the ABO locus and serum lipids or lipoproteins is with the β-LPC fraction.