The role of obesity in the relation between total water intake and urine osmolality in US adults, 2009-2012

Abstract

Background: Adequate water intake is critical to physiologic and cognitive functioning. Although water requirements increase with body size, it remains unclear whether weight status modifies the relation between water intake and hydration status.

Objective: We examined how the association between water intake and urine osmolality, which is a hydration biomarker, varied by weight status.

Design: NHANES cross-sectional data (2009-2012) were analyzed in 9601 nonpregnant adults aged ≥20 y who did not have kidney failure. Weight status was categorized with the use of body mass index on the basis of measured height and weight (underweight or normal weight, overweight, and obesity). Urine osmolality was determined with the use of freezing-point depression osmometry. Hypohydration was classified according to the following age-dependent formula: ≥831 mOsm/kg - [3.4 × (age - 20 y)]. Total water intake was determined with the use of a 24-h dietary recall and was dichotomized as adequate or low on the basis of the Institute of Medicine's adequate intake recommendations for men and women (men: ≥3.7 or <3.7 L; nonlactating women: ≥2.7 or <2.7 L; lactating women: ≥3.8 or <3.8 L for adequate or low intakes, respectively). We tested interactions and conducted linear and log-binomial regressions.

Results: Total water intake (P = 0.002), urine osmolality (P < 0.001), and hypohydration prevalence (P < 0.001) all increased with higher weight status. Interactions between weight status and water intake status were significant in linear (P = 0.005) and log-binomial (P = 0.015) models, which were then stratified. The prevalence ratio of hypohydration between subjects with adequate water intake and those with low water intake was 0.56 (95% CI: 0.43, 0.73) in adults who were underweight or normal weight, 0.67 (95% CI: 0.57, 0.79) in adults who were overweight, and 0.78 (95% CI: 0.70, 0.88) in adults who were obese.

Conclusion: On a population level, obesity modifies the association between water intake and hydration status.

Keywords: NHANES; effect modification; hydration status; obesity; water intake.

FIGURE 1

NHANES 2009–2012 analytic sample flow diagram.

FIGURE 2

Predicted probabilities (95% CIs) of hypohydration by water intake status and weight status in adults aged ≥20 y in the United States, 2009–2012. n = 9601. Data were generated with the use of marginal standardization from the log-binomial regression model that included an interaction between weight status and water intake status (F = 4.78, P = 0.015). BMI (in kg/m²) categories were as follows: underweight or normal weight, <25; overweight, 25 to <30; and obese, ≥30. Low water intake (dark gray bars) was defined as follows: men, <3.7 L; women, <2.7 L; and adequate water intake (light gray bars) was defined as follows: men, ≥3.7 L; women, ≥2.7 L. Data source: CDC/National Center for Health Statistics, NHANES (26).

FIGURE 3

Predicted probabilities (95% CIs) of hypohydration by weight status across 95% of the range of water intake that was reported in adults aged ≥20 y in the United States, 2009–2012. n = 9601. Data were generated with the use of marginal standardization from the sensitivity analysis (results not shown) that included an interaction between weight status and total water intake (F = 5.37, P = 0.01). BMI (in kg/m²) categories were as follows: underweight or normal weight, <25; overweight, 25 to <30; and obese, ≥30. Data source: CDC/National Center for Health Statistics, NHANES (26).