Dietary assessment in Whitehall II: comparison of 7 d diet diary and food-frequency questionnaire and validity against biomarkers

The aim of the present cross-sectional study was to examine the agreement a nd disagreement between a 7 d diet diary (7DD) and a self-administered mach ine-readable food-frequency questionnaire (FFQ) asking about diet in the pr evious year, and to validate both methods with biomarkers of nutrient intak e. The subjects were an age- and employment-grade-stratified random subsamp le of London-based civil servants (457 men and 403 women), aged 39-61 years , who completed both a 7DD and a FFQ at phase 3 follow-up (1991-1993) of th e Whitehall II study. Mean daily intakes of dietary energy, total fat, satu rated, monounsaturated and polyunsaturated fatty acids, linoleic acid, tota l carbohydrate excluding fibre, sugars, starch, dietary fibre, protein, vit amin C, vitamin E (as alpha -tocopherol equivalents), folate, carotenes (as total beta -carotene activity), Fe, Ca, Mg, K and alcohol were measured. S erum cholesteryl ester fatty acids (CEFA), plasma alpha -tocopherol and bet a -carotene were also measured as biomarkers. Estimates of mean energy inta ke from the two methods were similar in men, and some 10 % higher according to the FFQ in women. Compared with the 7DD, the FFQ tended to overestimate plant-derived micronutrient intakes (carotenes from FFQ v. 7DD men 2713 (S D 1455) v. 2180 (SD 1188) mug/d, women 3100 (SD 1656) v. 2221 (SD 1180) mug /d, both differences P<0.0001) and to underestimate fat intake. Against pla sma <beta>-carotene/cholesterol, carotene intake was as well estimated by t he FFQ as the 7DD (Spearman rank correlations, men 0.32 v. 0.30, women 0.27 v. 0.22, all P less than or equal to0.0001, energy-adjusted data). Ranking of participants by other nutrient intakes tended to be of the same order a ccording to the two dietary methods, e.g. rank correlations for CEFA linole ic acid against FFQ and 7DD estimates respectively, men 0.38 v. 0.41, women 0.53 v. 0.62, all P less than or equal to0.0001, energy-adjusted % fat). F or alpha -tocopherol there were no correlations between plasma level and es timated intakes by either dietary method. Quartile agreement for energy-adj usted nutrient intakes between the two self-report methods was in the range 37-50 % for men and 32-44 % for women, and for alcohol, 57 % in both sexes . Disagreement (misclassification into extreme quartiles of intake) was in the range 0-6 % for both sexes. The dietary methods yielded similar prevale nces (about 34 %) of low energy reporters. The two methods show satisfactor y agreement, together with an expected level of systematic differences, in their estimates of nutrient intake. Against the available biomarkers, the m achine-readable FFQ performed well in comparison with the manually coded 7D D in this study population. For both methods, regression-based adjustment o f nutrient intake to mean dietary energy intake by gender appears on balanc e to be the optimal approach to data presentation and analysis, in view of the complex problem of low energy reporting.