PART ONE - EFFECTS OF ELEVATED CARBON-DIOXIDE ENVIRONMENT ON CALCIUM-METABOLISM IN HUMANS

Background: Chronic respiratory acidosis induced by an elevated carbon dioxide (CO2) environment should provoke hypercalciuria with related total body and subsequent bone calcium losses. We examined this hypoth esis in four healthy male volunteers, who were exposed during a 25-d p eriod to an 0.7% CO2 environment within a deep diving isolation chambe r. Three months later the same subjects were reexamined during a secon d campaign being exposed to a 1.2% CO2 atmosphere. Methods: The subjec ts received a constant calcium intake (1.4 g.d(-1)) and vitamin D supp lement (1000 IU.d(-1)) during both campaigns. Calcium balance (oral ca lcium intake minus urinary and fecal calcium output) was evaluated. Se rum calcium concentrations and biomarkers of bone metabolism were meas ured, in order to evaluate bone turnover. Additionally, the response t o an acute oral calcium load was examined as a sensitive measure of ch anges in calcium metabolism. Results: Both, urinary calcium excretion (from 245 +/- 38 to 199 +/- 31 mg.d(-1); mean +/- SE, 0.7% and 1.2%, r espectively) and fecal calcium losses (from 1229 +/- 128 to 996 +/- 62 mg.d(-1)) were significantly reduced in the higher (1.2%) CO2 atmosph ere. Although more calcium was retained in the body during the 1.2% th an during the 0.7% CO2 campaign, serum calcium concentrations and biom arkers of bone formation were significantly lower in the higher CO2 ca mpaign. Furthermore, bone resorption was slightly increased in the 1.2 % experiment. Conclusion: Elevated CO2 atmosphere may dose-dependently preserve body calcium without a parallel improvement of bone substanc e.