Amino acid supplementation alters bone metabolism during simulated weightlessness

High-protein and acidogenic diets induce hypercalciuria. Foods or supplements with excess sulfur-containing amino acids increase endogenous sulfuric acid production and therefore have the potential to increase calcium excretion and alter bone metabolism. In this study, effects of an amino acid/carbohydrate supplement on bone resorption were examined during bed rest. Thirteen subjects were divided at random into two groups: a control group ( Con, n = 6) and an amino acid-supplemented group ( AA, n = 7) who consumed an extra 49.5 g essential amino acids and 90 g carbohydrate per day for 28 days. Urine was collected for n-telopeptide (NTX), deoxypyridinoline (DPD), calcium, and pH determinations. Bone mineral content was determined and potential renal acid load was calculated. Bone-specific alkaline phosphatase was measured in serum samples collected on day 1 ( immediately before bed rest) and on day 28. Potential renal acid load was higher in the AA group than in the Con group during bed rest ( P < 0.05). For all subjects, during bed rest urinary NTX and DPD concentrations were greater than pre-bed rest levels ( P < 0.05). Urinary NTX and DPD tended to be higher in the AA group ( P = 0.073 and P = 0.056, respectively). During bed rest, urinary calcium was greater than baseline levels ( P < 0.05) in the AA group but not the Con group. Total bone mineral content was lower after bed rest than before bed rest in the AA group but not the Con group ( P < 0.05). During bed rest, urinary pH decreased ( P < 0.05), and it was lower in the AA group than the Con group. These data suggest that bone resorption increased, without changes in bone formation, in the AA group.