Obesity Epidemiology

DIETARY ASSESSMENT METHODS 93The glycemic load of a specific food—calculated as the product of that food’s carbohydratecontent and its glycemic index value—has direct physiologic meaning in that eachunit can be interpreted as the equivalent of 1 g of carbohydrate from white bread (orglucose depending on the reference used in determining the glycemic index). Liu et al. 50found a positive relationship between serum fasting triglycerides and total carbohydrateintake, the overall dietary glycemic index, and the dietary glycemic load among postmenopausalwomen, with the strongest relationship for glycemic load. The associationbetween triglycerides and glycemic load appears to be stronger for overweight womenthan those who are not overweight, implying a biological interaction between underlyinginsulin resistance and carbohydrate metabolism.Biomarkers of One-Carbon (Methyl) MetabolismFolate, B 12, B 6, B 2, choline, methionine, and betaine play key roles in one-carbonmetabolism that involve transfer and utilization of one-carbon groups from one compoundto another. 51,52 One of folate’s main biological functions is remethylation of homocysteineto methionine. Folate status can be measured in serum (or plasma) and red cells. Serumfolate reflects short-term folate status (within the past few days), while a concentrationof red cell folate represents longer term and integrated folate intake because the half-lifeof red cells is approximately 4 months. 53 Thus, red cell folate more closely reflects tissuefolate status. Vitamin B 12status is measured by the serum cobalamin assay, and vitaminB 6status is indicated by the circulating concentration of pyridoxial-5′-phosphate (PLP).Elevated serum or plasma homocysteine is a sensitive and nonspecific marker of bothfolate and vitamin B 12deficiency, but it is not a reliable indicator of vitamin B 6status. 53In a recent study, Cho et al. 54 demonstrated a significant association between dietaryintake of choline plus betaine (assessed by FFQ in the Framingham Offspring Study)and lower homocysteine concentrations. Main sources of choline in the diet includedred meat, poultry, milk, eggs, and fish, while main sources of betaine included spinach,pasta, white bread, cold breakfast cereal, and English muffins, bagels, or rolls.Dietary folate assessed by FFQ is well correlated with biomarkers of folate status,including serum or plasma folate: r = .56 among 385 participants in the FraminghamHeart Study, 55 and r = .63 among 139 Boston-area participants. 56 For red cell folate, ther was .42. 57 The measurement of genomic DNA methylation in blood mononuclear cellsmay also serve as a useful biomarker for dietary folate intake. 53Biomarkers of Isoflavones and LignansIsoflavones and lignans are naturally occurring plant-derived phytoestrogens that mayhave biologically active properties. 58 Lignans are present in grains, beans, green vegetables,fruits, nuts, and grasses, whereas isoflavones are concentrated in soybeans andsoy foods. Common dietary isoflavonoids and metabolites include genistein, daidzein,dihydrodaidzein, O-desmethylangolensin, and equol; common lignans and metabolitesinclude enterolactone, enterodiol, matairesinol, and secoisolariciresinol. In a typicalWestern diet, the daily intake of phytoestrogens is very low (