24h urinary sodium excretion and subsequent change in weight, waist circumference and body composition

Abstract

Background: In the same period as the increasing obesity epidemic, there has been an increased consumption of highly processed foods with a high salt content, and a few studies have suggested that a diet with a high salt content may be associated with obesity.

Objective: To investigate the association between 24 h urinary sodium excretion and subsequent change in body weight (BW), waist circumference (WC), body fat (BF) and fat free mass (FFM) among adults.

Design: A longitudinal population study based on the Danish part of the MONICA project, with examinations in 1987-1988 and 1993-1994. Complete information on 24 h urinary sodium excretion along with repeated measures of obesity, as well as on potential confounders, was obtained from 215 subjects. Linear regression was used to examine the association between sodium excretion, as a measure of salt consumption, and subsequent changes in BW, WC, BF and FFM, and further evaluated by restricted cubic splines. Stepwise adjustments were made for selected covariates.

Results: Neither the crude nor the adjusted models showed any statistically significant associations between sodium excretion and change in BW or WC. Likewise, we found no significant association between sodium excretion and change in BF and FFM in the unadjusted models. However, after adjusting for potential baseline confounders and the concurrent BW change, we found a significant increase in BF of 0.24 kg (P = 0.015, CI: 0.05 to 0.43) per 100 mmol increase in 24 h urinary sodium excretion (equivalent to 6 g of salt), during the 6-year study period. Moreover, during the same period, we found a significant association with FFM of -0.21 kg (P = 0.041, CI: -0.40 to -0.01).

Conclusions: These results suggest that a diet with a high salt content may have a negative influence on development in body composition by expanding BF and reducing FFM.

Figure 1. Restricted cubic splines of the association between 24 h urinary sodium excretion and change in BF and FFM.

3 knots positioned at the 10th, 50th and 90th empirical percentiles. Prediction lines presented over the empirical 5th-95th percentiles sodium excretion range. Abbreviations: BW, body weight; BF, body fat; FFM, fat free mass. The association between sodium excretion and change in BF and FFM are shown for a six-year period (1987–88 to 1993–94). Crude (Red line): No adjustments. Basic model (Blue line): Adjusted for baseline age, sex, education level, smoking, physical activity, alcohol consumption, menopausal status for women, height and baseline measure of outcome. Full model (Green line): Adjusted for variables in the basic model + baseline total energy intake and change in BW between baseline and follow-up.