Nutritional Calcium Supply Dependent Calcium Balance, Bone Calcification and Calcium Isotope Ratios in Rats.

Serum calcium isotopes (δCa) have been suggested as a non-invasive and sensitive Ca balance marker. Quantitative δCa changes associated with Ca flux across body compartment barriers relative to the dietary Ca and the correlation of δCa with bone histology are unknown. We analyzed Ca and δCa by mass-spectrometry in rats after two weeks of standard-Ca-diet (0.5%) and after four subsequent weeks of standard- and of low-Ca-diet (0.25%). In animals on a low-Ca-diet net Ca gain was 61 ± 3% and femur Ca content 68 ± 41% of standard-Ca-diet, bone mineralized area per section area was 68 ± 15% compared to standard-Ca-diet. δCa was similar in the diets, and decreased in feces and urine and increased in serum in animals on low-Ca-diet. δCa was higher in animals on low-Ca-diet, lower in the diaphysis than the metaphysis and epiphysis, and unaffected by gender. Independent of diet, δCa was similar in the femora and ribs. At the time of sacrifice, δCa inversely correlated with intestinal Ca uptake and histological bone mineralization markers, but not with Ca content and bone mineral density by µCT. In conclusion, δCa was bone site specific, but mechanical stress and gender independent. Low-Ca-diet induced marked changes in feces, serum and urine δCa in growing rats. δCa inversely correlated with markers of bone mineralization.