Adequate calcium is critical intake to the achievement of peak bone mass in the first several decades of life, the retention of bone during middle adulthood and the minimization of bone loss during the last several decades.
The amount of calcium absorbed via the non-saturable, paracellular mechanism is dependent upon adequate supply of calcium in the intestinal lumen.
It is a passive process with no carriers or energy needed that occurs throughout the small intestine , but mostly in the jejunum and ileum.
Increased absorption via this mechanism becomes possible only when there is an increased intake of the mineral. With restricted calcium intake, the large intestine may play a role in calcium homeostasis.
Although absorption of calcium is variable among individuals, the average absorption is approximately 30% of intake, with absorption being more efficient in males than females.
Some factors influencing absorption of the mineral other than the physiologic state of the individual and availability of adequate vitamin D include ingestion of food along with the calcium source and the type and amount of fiber in the diet.
The active form of vitamin D which control absorption of calcium is calcitrol.
Some calcium supplements, in particular calcium carbonate, appear to be absorbed more readily when they are ingested with food.
Even persons with achlorhydria can absorb calcium when supplement is taken with food, a circumstance indicating that acid in itself is not necessary for calcium absorption.
Calcium absorption increased is observed during growth, pregnancy, primary hyperparathyroidism, sarcoidosis and estrogen and growth hormone administration.
Fiber, specifically cellulose and hemi cellulose, decrease calcium absorption not only by increasing the bulk of intestinal contents and decreasing transit time but also by stimulating microbial proliferation, a mineral requiring process.
Diets in high fiber reduce the bioavailability of dietary calcium. In the gut, calcium binds to fiber in proportion to its phytate content and bound calcium has reduced bioavailability.
The source of fiber also is important with respect to its effect on calcium absorption. For example, wheat fiber has a more deleterious effect on calcium availability that vegetable fiber.
Controversy exists about the importance of phytate in decreasing the absorption of calcium.
Researchers believe that depression of mineral absorption by fiber is due more to its phytate content that to the absolute amount of fiber, while others contend that phytate has little effect on the absorption of calcium.
Wheat bran and oxalate reduce calcium absorption. High supplement doses of phosphorus and magnesium, may also interfere with calcium absorption.
Those that increase absorption include protein – specific amino acids, lysine and arginine and lactose in infants.
Calcium absorption diminishes in old age, partly because of lower calcitrol levels. Other physiological or pathological factors that decrease intestinal calcium absorption include chronic insufficiency, hypoparathyroidism, and vitamin D deficiency.
Calcium absorption
