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Absorption of calcium |
The absorbability of calcium from spinach was compared with the absorbability of Ca from milk in 13 healthy adults. Absorption was higher from milk in every case, with the mean absorption from milk averaging 27.6% and from spinach, 5.1%. These results conclusively establish that spinach Ca is much less readily available than milk Ca. The presence of lactose (milk sugar), lactase (the enzyme that breaks down lactose), and the acidic amino acids, lysine and arginine, are all factors that improve calcium absorption. On the other hand, decreased absorption is associated with diets high in fiber. Foods such as whole grains and spinach are high in phytates and oxalates, compounds known to bind with calcium and reduce absorption. |
The amount of calcium absorbed in the intestine depends on habitual calcium intake. When intake is low, active transcellular calcium transport in the duodenum is upregulated and a larger proportion of calcium is absorbed by the active process than by the passive paracellular process that prevails in the jejunum and ileum. Bioavailability of the calcium source and digestibility and solubilization plays a role under conditions of low calcium intake but is relatively unimportant when calcium intakes are high (e.g. >800 mg/d in people). Vitamin D intake is a second factor, as active calcium transport is directly and proportionally dependent on the presence in the intestinal cell of calbindin D9k, the biosynthesis of which is totally vitamin D dependent. Passive absorption in jejunum and ileum is the major absorptive process when calcium intake is adequate or high. Passive calcium absorption is a complicated function of solubility in the distal small intestine, the length of sojourn of the chyme in a given intestinal segment, and the rate of paracellular diffusion from lumen to lymph and blood. Calcium that reaches the large intestine undergoes absorption there by both active and passive processes. Probably no more than 10% of total calcium absorption takes place in the large intestine, whether calcium intake is low or high. Calcium absorption by the large bowel can assume nutritional importance under conditions of significant small bowel resection. |