rando, R. R. Wolfe, and S. M. Smith. Amino acid supplementation alters bone metabolism during simulated weightlessness. J Appl Physiol 99: 134 -140, 2005. First published February 3, 2005 doi:10.1152/japplphysiol.01406.2004.-High-protein and acidogenic diets induce hypercalciuria. Foods or supplements with excess sulfurcontaining 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 ϭ 6) and an amino acid-supplemented group (AA, n ϭ 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 Ͻ 0.05). For all subjects, during bed rest urinary NTX and DPD concentrations were greater than pre-bed rest levels (P Ͻ 0.05). Urinary NTX and DPD tended to be higher in the AA group (P ϭ 0.073 and P ϭ 0.056, respectively). During bed rest, urinary calcium was greater than baseline levels (P Ͻ 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 Ͻ 0.05). During bed rest, urinary pH decreased (P Ͻ 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. bone resorption; methionine; n-telopeptide; acidosis; bed rest BONE LOSS AND FRACTURE RISK are significant concerns for astronauts and the general public. Diet can have a profound effect on acid-base balance of the body, which in turn could affect bone metabolism. Acid-base balance is determined mainly by the partial pressure of carbon dioxide and endogenous acid production from acid components in the diet (7, 21). Net endogenous acid production results from incomplete oxidation of organic acids and production of sulfuric acid by oxidation of sulfur-containing amino acids. In contrast, acid production can be neutralized by sources of bicarbonate in the diet or endogenous sources (7). Vegetables mainly provide base rather than acid precursors; they contain high concentrations of bicarbonate and other organic anions that can be converted to bicarbonate. Bicarbonate is one of the body's main buffering systems, and bone serves as a major body reservoir for bicarbonate (33).In vitro studies show that low pH induces bone resorption through a direct physiochemical effect as well as a cellular effect on bone (1). It is currently debated whether excess aci...