Passive diffusion does not play a major role in the absorption of dietary calcium in normal adults.

Several authors have indicated recently that passive diffusion is the dominant process by which dietary calcium is absorbed by normal adults. These conclusions have important implications in the maintenance of normal calcium status in humans. They imply that increasing luminal calcium concentration by increasing intake is the most important strategy in promoting calcium absorption. I present several studies that dispute this contention and two studies that suggest that passive diffusion is of little practical significance, even in young children. The contribution of passive diffusion is estimated to be between 8 and 23% of the total calcium absorbed by normal adults. Thus, it clearly does not represent the major path by which they absorb calcium. Children lacking the vitamin D receptor system do not absorb enough calcium by passive diffusion to prevent signs of rickets even when diets contain very large amounts of dietary calcium (>2 g/d). The conclusion drawn from this analysis is that passive diffusion (dictated by calcium intake) is not the major mechanism by which dietary calcium is absorbed by normal adult humans. The vitamin D-dependent processes are more important quantitatively and thus constitute a major determinant of calcium status. Individuals who are not exposed to sunlight may be especially at risk.