Sodium loading, treadmill walking, and the acute redistribution of bone mineral content on dual energy X-ray absorptiometry scans

Abstract

The purpose of this study was to assess relationships between plasma sodium concentration ([Na+]) and bone mineral content (BMC) after an acute sodium load plus treadmill walking and then quantify the amount of sodium the dual energy X-ray absorptiometry (DXA) scan could detect. The primary study was a single-blind randomized control crossover trial under two conditions: ingestion of six flour tablets (placebo trial) or six 1-g NaCl tablets (salt intervention trial). The tablets were ingested after baseline blood and urine collection followed immediately by the DXA scan. After 60 min of rest, a 45-min treadmill walk was conducted. Immediately postexercise, blood and urine were collected and the DXA scan was repeated. Main outcomes included changes (∆: post minus pre) in plasma [Na+] and BMC. Additionally, six 1-g NaCl tablets were superimposed over a DXA spine phantom for separate quantification of sodium as BMC. Fourteen subjects completed the primary study. Two-way repeated measures ANOVA tests revealed significant interaction ( F = 13.06; P = 0.0007), condition ( F = 21.88; P < 0.001), and time ( F = 6.51; P = 0.014) effects in plasma [Na+]. A significant condition ( F = 6.46; P = 0.014) effect was also noted in urine [Na+]. Total body BMC∆ was negatively correlated with plasma [Na+]∆ ( r = −0.43; P = 0.02) and urine [Na+]∆ ( r = −0.47; P = 0.01). Total body BMC∆ in the salt intervention trial [−5.5 (27) g] closely approximated the amount of NaCl ingested and subsequently absorbed into the bloodstream. The DXA scan quantified 67% of NaCl tablets as BMC in spine phantom analyses. Total body BMC∆ was negatively related to plasma and urine [Na+]∆ after treadmill walking. Reductions in total body BMC closely approximated the amount of NaCl ingested (~6 g). The DXA scan quantified NaCl as BMC.