Quantifying The Contributions Of Sodium And Calcium In Bone Using Serial Dxa Scans

Abstract

Bone has been hypothesized to serve as a sodium reservoir to buffer acute and chronic derangements in plasma sodium concentration ([Na+]). However, the contributions of sodium in bone have yet to be quantified in imaging studies. PURPOSE: To: 1) quantify the contributions of 6g sodium chloride (NaCl) on bone mineral content (BMC) and density (BMD) using dual energy x-ray absorptiometry (DXA) scans; 2) compare the contributions of 6g NaCl vs. 6g calcium citrate (Ca3Cit2) on both BMC and BMD; and 3) assess whole body BMC changes in humans 2hrs after oral ingestion of 6g NaCl. METHODS: 1) Ten DXA scans serially performed on a spine phantom (alone) and then with 6g NaCl placed over the spine as: solid 1g tablets; crushed tablets in 1.5mL Eppendorfs (tubes); and as a solution in tubes. 2) Ten DXA scans performed on a spine phantom plus 6g Ca3Cit2 as solid 1g tablets and as crushed tablets in tubes. 3) 14 healthy humans ingested 6g NaCl, (6x1g tablets) with DXA scans taken immediately after ingestion (+tablets; just swallowed) and then repeated 2hrs following oral tablet ingestion (-tablets; due to GI absorption). RESULTS: Mean spine phantom BMC (51.38±0.26g;CV=0.51%) was significantly lower compared with spine phantom plus: NaCl tablets (55.39±0.23;p<0.0001); NaCl tubes (54.87±0.26g;p<0.0001); NaCl solution (52.49±0.28g;p<0.0001); Ca3Cit2 tablets (55.05±0.31g;p<0.0001); and Ca3Cit2 tubes (53.68±0.39g;p<0.0001). Mean spine phantom BMD (0.989±0.004g/cm2;CV=0.50%) was statistically lower when compared with phantom plus: NaCl tablets (1.063g/cm2±0.005;p<0.0001); NaCl tubes (1.056±0.005g/cm2;p<0.0001); NaCl solution (1.006±0.005g/cm2;p<0.0001); Ca3Cit2 tablets (1.058±0.006g/cm2;p<0.0001); and Ca3Cit2 tubes (1.021±0.005g/cm2;p<0.0001). Absorption of 6gm NaCl tablets into the bloodstream 2hrs after oral ingestion reduced whole body BMC by -5.5±27g. CONCLUSIONS: 6g NaCl caused significant increases in BMC and BMD in DXA scans when placed over a spine phantom in: tablet form, when crushed in tubes; and dissolved in solution. Increases in BMC and BMD were similar between 6g NaCl and 6g Ca3Cit2. Alternatively, GI absorption of 6g NaCl tablets after oral ingestion resulted in a ~6g decrease in total body BMC. Thus, changes in BMC and BMD may reflect Na+ and Ca++ mobilization in DXA scan measurements.