Using biochemical markers to assess the validity of prospective dietary assessment methods and the effect of energy adjustment

The validity of dietary assessment methods in a group of women aged 50-65 y was evaluated with use of biochemical markers. Estimates of nitrogen, potassium, and carotene intakes from weighed-food and estimated records yielded higher correlations with urinary nitrogen, urinary potassium, and serum concentrations of carotenoids than did estimates from food-frequency questionnaires and 24-h recalls. When the residuals method of energy adjustment was used, the correlations between intakes of nitrogen and potassium estimated from food-frequency questionnaires and 24-h recalls and intakes derived from weighed-food records improved, and the high correlations between biochemical markers and estimates from weighed-food records were maintained. In addition, with use of this method, estimates for nitrogen and potassium intakes from food-frequency questionnaires showed the most improvement in comparison with the biochemical markers; however, the correlations of crude nitrogen and potassium with crude energy intake were highest. Carotene intake was not related to energy intake, so that correlations between the intake of carotene assessed by any method and the plasma beta-carotene concentration did not improve with energy adjustment and between-person variability was not reduced. Energy adjustment with either the energy density or residuals method did not alter the ranking of accuracy of various dietary assessment methods in comparison with weighed-food records or biochemical markers in either the total group of subjects or those who were identified as having provided valid weighed-food records.