Secretion and excretion of calcium by the human gastrointestinal tract

Fecal excretion of calcium of endogenous origin has been measured in 52 studies in 33 adult human subjects, under full metabolic balance conditions. Endogenous fecal calcium averaged .130 ± .047 Gm. per day, was positively correlated with both fecal calcium and dietary calcium and was inversely correlated with fractional calcium absorption. The regression line relating endogenous fecal calcium with fractional absorption allowed estimates for total intestinal calcium secretion at both zero and 100 per cent absorption; from these estimates it was calculated that approximately 15 per cent of the total intestinal calcium secretion was nonabsorbable even under conditions when dietary calcium was completely absorbed (presumably because it enters the gut caudad of the absorption sites). Total intestinal calcium secretion, calculated so as to allow for this nonabsorbable fraction, averaged .194 ± .073 Gm. per day, and could not be correlated with age, sex, dietary calcium intake, caloric intake, urine calcium, or plasma calcium. A weak correlation with body size (weight, surface area) was observed. The conditions studied were principally disorders of bone, calcium, and parathyroid metabolism, and in most of them total intestinal calcium secretion appeared identical. The sole exception was seen in two studies on a patient with acromegaly, in which total intestinal calcium secretion was over twice the mean value for the remainder of the group