Ability of self-reported estimates of dietary sodium, potassium and protein to detect an association with general and abdominal obesity: comparison with the estimates derived from 24 h urinary excretion

As under-reporting of dietary intake, particularly by overweight and obese subjects, is common in dietary surveys, biases inherent in the use of self-reported dietary information may distort true diet-obesity relationships or even create spurious ones. However, empirical evidence of this possibility is limited. The present cross-sectional study compared the relationships of 24h urine-derived and self-reported intakes of Na, K and protein with obesity. A total of 1043 Japanese women aged 18-22 years completed a 24h urine collection and a selfadministered diet history questionnaire. After adjustment for potential confounders, 24h urine-derived Na intake was associated with a higher risk of general obesity (BMI >= 25 kg/m(2)) and abdominal obesity (waist circumference >= 80 cm; both P for trend=0.04). For 24h urine-derived protein intake, positive associations with general and abdominal obesity were observed (P for trend=0.02 and 0.053, respectively). For 24h urine-derived K intake, there was an inverse association with abdominal obesity (P for trend=0.01). Conversely, when self-reported dietary information was used, only inverse associations between K intake and general and abdominal obesity were observed (P for trend=0.04 and 0.02, respectively), with no associations of Na or protein intake. In conclusion, we found positive associations of Na and protein intakes and inverse associations of K intake with obesity when using 24h urinary excretion for estimating dietary intakes. However, no association was observed based on using self-reported dietary intakes, except for inverse association of K intake, suggesting that the ability of self-reported dietary information using the diet history questionnaire for investigating diet-obesity relationships is limited.