Validation of a food frequency questionnaire measurement of selected nutrients using biological markers in African-American men.

OBJECTIVE: To validate selected nutrients assessed by the food frequency questionnaire (FFQ) used in the Harvard cohort studies in an African-American group.
DESIGN: Blood aliquots were pooled for each decile of intake of two carotenoids and alpha tocopherol as measured by FFQ. These pooled samples were analyzed for nutrient content, and the resultant blood levels were plotted against the median for each decile of intake. In addition, adipose tissue samples taken from each man were analyzed for content of specific fatty acids. We calculated the Spearman correlations comparing intakes of specific fatty acids as percent of total fat intake, adjusted for energy intake, as measured by FFQ, with the percentage of the corresponding fatty acid in adipose tissue. SUBJECTS AND SETTINGS: African-American men (N=104) with prostate cancer were recruited from a Detroit physician's practice and completed a detailed FFQ.
RESULTS: Comparing decile 10 with decile 1 intake of nutrients as measured by FFQ, there was a 32% higher blood level of lycopene, a 288% higher blood level of beta carotene and a 100% higher blood level of alpha tocopherol. The Spearman correlation coefficients between intakes of linoleic acid, alpha linolenic acid, long-chain n-3 fatty acids and trans fatty acid measured by FFQ and the corresponding adipose tissue levels were between 0.10 and 0.47.
CONCLUSION: The FFQ was able to distinguish meaningful differences in biochemical measurements of selected nutrients and presumably corresponding differences in the extremes of intake in African-American men with prostate cancer who were likely to be motivated to report accurately. However, the results found are similar to those found in other populations.