Reply to Safer, Tasci, Cintosun, and Binay Safer.

TO THE EDITOR: We appreciate the letter by Dr. Safer and colleagues (2) and are pleased to respond. First, we would like to inform the readers that the volunteers in our study are the exact same subjects as described by Hastings et al. (1), participating in exactly the same study; therefore we did indeed stratify our groups by baseline fitness. Although we thought that we made this point clear in the manuscript, on careful re-read it appears that this fact was implicit rather than explicit. We regret this omission and appreciate the opportunity for clarification. Second, we are preparing another paper describing the effects of rowing ergometry on mineral metabolism in collaboration with our colleague Dr. Joe Zerwekh, which will include all the measurements and markers used in his previous studies of bed rest and bone metabolism and follow his previously published protocols (3) in the context of our study. Although to avoid biasing publication, we cannot provide data in this response, we would like to reassure our readers that many serum and urinary biomarkers of bone metabolism, including vitamin D, were measured in this experiment. In addition, we also examined the effects of potassium magnesium citrate supplementation on bone turnover and renal stone formation risk in a subgroup of our study. Vitamin D was— apart from what our standardized diet provided—not specifically supplemented to avoid introducing further intervention bias through its effects on calcium and phosphorus homeostasis. We look forward to future work by Dr. Safer’s group, which we presume will test the interesting hypotheses presented in their letter about the role of vitamin D on muscle structure and function during bedrest in a direct and controlled fashion.