We reviewed data on calcium absorption in humans, mainly focusing on concentrations of soluble calcium in the gut, kinetics of absorption, correlation of net absorption with serum 1, 25-(OH) 2-vitamin D, and adaptation to different intakes. We conclude:(a) most of the unabsorbed food calcium is insoluble when it reaches the terminal ileum;(b) calcium absorption in jejunum is both active and passive with active component being dependent on vitamin D; ileal absorption is also highly dependent on vitamin D, but it is uncertain whether this action is on the active or passive component;(c) the relative role of vitamin D-dependent and vitamin D-independent mechanisms to dietary calcium absorption depends upon serum 1, 25-(OH) 2-D level and calcium intake;(d) over short periods, the gut reduces calcium absorption much more in response to a high-calcium diet than it increases absorption in response to a low-calcium diet. We also evaluated the measurement of'true'calcium absorption by isotopic methods. By ignoring isotopic equilibration, these methods overestimate both the amount of calcium secreted into and absorbed from the gut. Since altered gut physiology and pathology can variably affect isotopic equilibration,'true'absorption may not reflect absorption of dietary calcium in an individual patient. Moreover, only net absorption has metabolic and nutritional significance.