The negative effect of unloading exceeds the bone-sparing effect of alkaline supplementation: a bed rest study.
Potassium bicarbonate was administrated to an already alkaline diet in seven male subjects during a 21-day bed rest study and was able to decrease bed rest induced increased calcium excretion but failed to prevent bed rest-induced bone resorption.


INTRODUCTION
Supplementation with alkali salts appears to positively influence calcium and bone metabolism and, thus, could be a countermeasure for population groups with an increased risk for bone loss. However, the extent to which alkalization counteracts acid-induced bone resorption or whether it merely has a calcium and bone maintenance effect is still not completely understood. In the present study, we hypothesized that additional alkalization to an already alkaline diet can further counteract bed rest-induced bone loss.


METHODS
Seven healthy male subjects completed two parts of a crossover designed 21-day bed rest study: bed rest only (control) and bed rest supplemented with 90 mmol potassium bicarbonate (KHCO3) daily.


RESULTS
KHCO3supplementation during bed rest resulted in a more alkaline status compared to the control intervention, demonstrated by the increase in pH and buffer capacity level (pH p = 0.023, HCO3p = 0.02, ABE p = 0.03). Urinary calcium excretion was decreased during KHCO3 supplementation (control 6.05 ± 2.74 mmol/24 h; KHCO3 4.87 ± 2.21 mmol/24 h, p = 0.03); whereas, bone formation was not affected by additional alkalization (bAP p = 0.58; PINP p = 0.60). Bone resorption marker UCTX tended to be lower during alkaline supplementation (UCTX p = 0.16).


CONCLUSIONS
The more alkaline acid-base status, achieved by KHCO3 supplementation, reduced renal calcium excretion during bed rest, but was not able to prevent immobilization-induced bone resorption. However, advantages of alkaline salts on bone metabolism may occur under acidic metabolic conditions or with respect to the positive effect of reduced calcium excretion within a longer time frame.


TRIAL REGISTRATION
Trial number: NCT01509456.
['Frings-Meuthen|P|P|http://orcid.org/0000-0001-5291-4419', 'Bernhardt|G|G|', 'Buehlmeier|J|J|', 'Baecker|N|N|', 'May|F|F|', 'Heer|M|M|']
D000328:Adult D001510:Bed Rest D001639:Bicarbonates D015415:Biomarkers D001862:Bone Resorption D002118:Calcium D018592:Cross-Over Studies D019587:Dietary Supplements D006801:Humans D006863:Hydrogen-Ion Concentration D007103:Immobilization D008297:Male D010012:Osteogenesis D017680:Potassium Compounds D016474:Weight-Bearing D055815:Young Adult
Bed rest Potassium bicarbonate Calcium excretion Bone metabolism