Under- and overreporting of energy intake using urinary cations as biomarkers: relation to body mass index.

Under- and overreporting of energy intake in relation to body mass index (BMI, kg/m2) were examined by using 24-hour urinary sodium and potassium as biomarkers. The data were obtained from 2,124 men and 1,998 women aged 25-74 years who participated in the 1981-1984 Belgian Interuniversity Research on Nutrition and Health study conducted in Belgium. The ratios of dietary intake to urinary excretion of sodium and potassium, as a measure for relative underreporting, were inversely associated with BMI (for men, beta = -0.019 for sodium ratio and beta = -0.026 for potassium ratio; for women, beta = -0.017 for sodium ratio and beta = -0.019 for potassium ratio; all p < 0.0001) independent of age, smoking, alcohol intake, and educational level. Since 77% of dietary potassium was reported to be excreted in the urine, subjects for whom the (dietary potassium x 0.77)/urinary potassium ratio was <1 were considered underreporters and >1 as overreporters. The percentage of underreporters increased with increasing pooled sex-specific deciles of BMI (beta = 1.88, p < 0.0001) and was higher than the percentage of overreporters in 13 of 20 deciles. At a BMI of 25.4, the percentage of under- and overreporters equalized. In conclusion, the relative underreporting of energy intake and the percentage of underreporters increased with increasing BMI.