Obesity Epidemiology

92 STUDY DESIGNS AND MEASUREMENTS(CEs), plasma triglycerides, and phospholipids from dietary metabolic trials. There wasa clear dose-response relationship between increasing dietary linoleic acid intake andobserved changes in serum CE or triglyceride linoleic acid concentrations. However, thedose-response relationship between serum phospholipids and dietary linoleic acid wasmuch weaker, suggesting that more tightly regulated tissues (in particular, phospholipidsof membranes) may not adequately reflect long-term intake.Sun et al. 43 compared fatty acid content of erythrocytes to that of plasma with respectto their abilities to reflect usual dietary fatty acid intake as measured by a FFQ. Docosahexaenoicacid (DHA, 22:6n-3) in plasma and erythrocytes provided the strongestcorrelations with its dietary intake, but erythrocytes (r = .56) were better than plasma(r = .48) as a biomarker. Similarly, total trans fatty acids (r = .43) and total 18:1 transisomers (r = .42) in erythrocytes were more strongly correlated with dietary intake thanplasma markers (r = .30 and r = .29, respectively). In addition, use of repeated measuresof diet further improved these correlation coefficients.Baylin et al. 44 evaluated whole blood as a biomarker of intake. The diet-whole bloodcorrelations were 0.43 for linoleic acid, 0.38 for alpha-linolenic acid, and 0.26 for 18:2trans fatty acids. These results show that whole blood was a reasonable alternative forplasma for the assessment of fatty acid intake.Adipose tissue is considered the best choice to assess long-term fatty acid intakebecause of its slow turnover rate. In a secondary prevention trial of coronary disease bysubstituting unsaturated fat for saturated fat, Dayton et al. 45 observed that adipose tissuelinoleic acid increased from 11% in the first year to 32% in year 5, suggesting excellentcompliance with the intervention. In epidemiologic studies, intakes of linoleic and transfatty acids estimated by FFQs are reasonably correlated with corresponding lipids in adiposetissue (with correlations in the range of 0.40 to 0.50). 22 However, these correlationsare only modestly higher than those for plasma markers. 46Adipose tissue levels of pentadecanoic acid (15:0) (PDA) and heptadecanoic acid (17:0)(HAD) can be used to reflect average long-term dairy fat consumption in free-livingsubjects. In a study of 81 healthy women aged 30 to 77 years in Sweden, Wolk et al. 47found a Pearson correlation coefficient of 0.63 between the 15:0 content in adipose tissueand intake from dairy foods from diet records, with a somewhat lower correlation for17:0 (r = .42). Baylin et al. 48 reported a correlation of 0.31 between adipose tissue contentof 15:0 (also 17:0) and dairy product intake in Costa Rican men and women. Sunet al. 49 reported correlation coefficients between 15:0 content and average dairy fat intakein 1986-1990 were 0.36 for plasma and 0.30 for erythrocytes. Trans 16:1n-7 in plasma(r = .30) and erythrocytes (r = .32) were also correlated with dairy fat intake.Because of their high cost, laboratory measurements of fatty acids are most suitable foruse in nested case- control or case-cohort studies, or as a reference method in validationstudies. In addition, because they are usually expressed as a percentage of total fattyacids, they only reflect relative intake, with no measure of absolute fatty acid intake. 40Thus, changes in one fatty acid affect the distributions of the others.Carbohydrates and Quality of CarbohydratesAs with total fat, there is no specific biomarker for total carbohydrate intake. However,plasma triglycerides rise in response to increasing carbohydrate intake (and decreasingfat intake), 38 and can be used as a sensitive but nonspecific marker of carbohydrate intake.Because both the amount and quality of carbohydrates are important determinants offasting plasma triacylglycerol concentrations, glycemic load has been used as a measurethat incorporates the quantity as well as the quality of dietary carbohydrates consumed. 50