Change in methodology for collection of drinking water intake in What We Eat in America/National Health and Nutrition Examination Survey: implications for analysis.

OBJECTIVE: To provide updated estimates of drinking water intake (total, tap, plain bottled) for groups aged ≥1 year in the USA and to determine whether intakes collected in 2005-2006 using the Automated Multiple-Pass Method for the 24 h recall differ from intakes collected in 2003-2004 via post-recall food-frequency type questions.
DESIGN: Cross-sectional, observational study.
SETTING: What We Eat in America (WWEIA), the dietary intake component of the US National Health and Nutrition Examination Survey (NHANES).
SUBJECTS: Individuals aged ≥1 year in 2003-2004 (n 8249) and 2005-2006 (n 8437) with one complete 24 h recall.
RESULTS: The estimate for the percentage of individuals who reported total drinking water in 2005-2006 was significantly (P < 0·0000) smaller (76·9 %) than that for 2003-2004 (87·1 %), attributable to a lower percentage reporting tap water (54·1 % in 2005-2006 v. 67·0 % in 2003-2004; P = 0·0001). Estimates of mean tap water intake differed between the survey cycles for men aged ≥71 years.
CONCLUSIONS: Survey variables must be examined before combining or comparing data from multiple WWEIA/NHANES release cycles. For at least some age/gender groups, drinking water intake data from NHANES cycles prior to 2005-2006 should not be considered comparable to more recent data.