It is well-known that both a high-salt diet and obesity are risk factors for the development of hypertension and cardiovascular disease. Several studies have suggested a link between dietary sodium and obesity. However, recent literature suggests this relationship may be independent of energy intake. In addition, a series of novel studies suggest that sodium may be stored in the skin, however, the effect this may have on the relationship between dietary sodium and obesity remains unknown. The African Prospective study on the Early Detection and Identification of Cardiovascular Disease and Hypertension (African-PREDICT) study provides us with the ideal population to investigate the sodium intake-adiposity link, as participants are young and apparently healthy with detailed nutritional, anthropometric and cardiovascular measures.
The aim of this study was to determine whether there was a relationship between sodium intake and obesity-related measures in a young healthy, black and white South African population, when adjusting for potential confounders.
This study used cross-sectional data from the first 761 participants with complete data sets at baseline. Data with regard to age, sex, ethnicity and dietary intake were collected using various questionnaires. Socio-economic status was calculated using a point system adapted from the Kuppuswamy's Socioeconomic Status Scale 2010. Participants provided a 24hr urine sample for analysis. Anthropometric measurements and bioelectrical impedance were collected as well as physical activity (accelerometery) and 24hr ambulatory blood pressure. Additionally, venous blood samples were taken from the brachial vein branches and analysed for a wide range of biochemical markers. Results
Based on 24hr urine sodium analysis, the total group consumed on average 7.65 g of salt per day with 79.9% consuming above the daily recommended salt intake of 5 g per day. In the total population, 46% were classified as overweight or obese (26% overweight, body mass index (BMI) 25-29.9 kg/m2; 20% obese, BMI >30 kg/m2). In Pearson correlations, all anthropometric measures correlated with 24hr sodium, with body surface area (BSA) showing the strongest correlations. Multivariate-adjusted regression analysis of associations between either BSA or BMI and 24hr urinary sodium showed both BSA and BMI associated positively with 24hr urinary sodium in the total group, black and white men (unadjusted model; all p≤0.032) but not in women. In Model 1, adjusted for total energy expenditure (TEE) only, BSA associated positively with 24hr urinary sodium in both the total group and white women (all p≤0.037). Following adjustment for energy intake only (Model 2), BSA remained associated with 24hr urinary sodium in the total group and additionally in black and white men (all p≤0.012), BMI also associated positively with 24hr urinary sodium in the total group and white men (both p≤0.026). Model 3, adjusted for both TEE and energy intake, showed an association between BSA and 24hr urinary sodium in the total group only (p=0.005). In Model 4, we replicated the analysis of a previous study that found a positive relationship between BMI and sodium intake when adjusting for age, socio-economic score, TEE, dietary energy intake, self-reported tobacco and alcohol use. We also found a significant positive association but only between BSA and 24hr urinary sodium in the total group and white women (both p≤0.043). Lastly, we ran a fully adjusted model including systolic blood pressure and a range of additional covariates (age, socio-economic status, cotinine, gamma glutamyl transferase, aldosterone, C-reactive protein, low and high-density lipoprotein cholesterol and glucose; Model 5). In this model, BSA, but not BMI, remained significantly associated with 24hr urinary sodium in the total group (p=0.039). Within the total group, when using the fully adjusted model, we additionally tested for associations between all other measures of obesity and 24hr urinary sodium using the same models and no consistent independent associations were found. Conclusion
In conclusion, we found a continuous and robust positive relationship between BSA and 24hr urinary sodium in the total population independent of energy intake, expenditure and numerous potential confounds. As BSA is also used as an estimate of skin surface area, the relevance of this finding in terms of obesity remains unclear, especially as salt intake was not independently associated with any of the more traditional obesity markers||en_US