Most of us are becoming increasingly aware of the need for calcium in our diets as a necessary factor for our health and longevity. But are we aware that not all forms of calcium are the same? When our health practitioners recommend that we go to the store and get some over the counter calcium tablets and take two daily, are they really aware of how very little of the calcium is bioavailable to the cells of our bodies? Are they educated in biochemical calcium absorption and do they know how much of the calcium we consume in these tablet forms are feeding the sewer systems of America, and not adequately filling our body's critical need for calcium? In this short article I intend to educate you just a little about how our calcium is absorbed and how we can best get the calcium essential for proper metabolism and good health.
About 99% of the body's calcium is deposited in the bones and teeth. The remaining 1% is present in body fluids, approximately equally divided between diffusible calcium and nondiffusible calcium. The diffusible calcium is largely ionic. The nondiffusible calcium is bound to blood proteins, chiefly to albumin, although a small amount is bound by the globulins in the blood.
Ionic calcium (Ca++) is the only physiologically active form of this element. Bone, though considered primarily as the structural support for the soft tissues of the body, also serves as a storage deposit for ionic calcium. Its calcium is available to the body and is drawn upon to maintain normal blood calcium levels during periods of low calcium intake. The protein bound calcium of the blood probably serves as a secondary reservoir, becoming available locally only during excessive loss or use of ionic calcium.
Maintenance of a positive calcium balance by the body depends on dietary intake and the efficiency of absorption of the calcium ion from the intestinal tract. Calcium is one of the more difficult elements for the body to digest and absorb. Because calcium forms insoluble compounds with many of the anions present in food, efficient absorption of calcium is fraught with problems. Normally the phosphate ion is the principal interfering anion. Of the Calcium phosphate complexes, only calcium dihydrogen phosphate is sufficiently soluble to maintain the necessary levels for efficient absorption of the ionic calcium. Unfortunately this salt is stable only in highly acid media, such as stomach acid. And in alkaline media the much less soluble mono-hydrogen phosphate or the highly insoluble tertiary phosphate are the stable forms, and these are not absorbable by the body. Moreover, once calcium has dissolved, its absorption into the body is totally dependent on the presence of vitamin D in the intestine. Vitamin D is, unfortunately, not present in most of our food, so our body is dependent on the action of sunlight on our skin to synthesis vitamin D. Without intestinal vitamin D being present, most of the ionized calcium will pass through the body.
Conditions in the stomach normally provide sufficient acid for the stable existence of the free calcium ion even in the presence of phosphate ions; but absorption cannot take place here. As the contents of the stomach (chyme) is discharged from the stomach and moves through the small intestine, it is neutralized by the alkaline bile. Calcium absorption takes place in the duodenum, but it is apparent that solubility considerations counteract to prevent this uptake, except during the relatively short period of time before the chyme is completely neutralized. Absorption in the remainder of the intestine is pretty much nonexistent, because the calcium by then has been precipitated from solution due to the alkalinity produced by the bile.
It is evident that high phosphate diets (high red meat consumption, carbonated drinks, etc.) are unfavorable to efficient calcium absorption. Because of the common ion effect, excess phosphate will depress solubility of the soluble calcium dihydrogen phosphate stable at the stomach's normally acid pH level. Therapy, involving use of strong bases such as antacids and H-2 blockers (commonly referred to as acid-blockers), decreases the efficiency of calcium absorption because of their alkalizing effects on the pH of the stomach. Tums® or calcium carbonate, an alkaline source of calcium, neutralizes stomach acid needed for calcium absorption. Also, a diet with excess zinc may interfere with calcium absorption. Faulty fat digestion due to high fat intake or an inadequate bile secretion also interferes with calcium absorption through the precipitation of insoluble calcium soaps. Even some plant foods such as rhubarb, Swiss chard, spinach, beet greens, cocoa, soybeans, cashews and kale contain a high oxalate content, which acts as a calcium absorption blocker by binding with calcium producing insoluble salts as calcium oxalate which can not be absorbed.
The average American diet of meats, refined grains, and soft drinks (high in phosphorus) has been documented to contribute to increased bone loss in adults. It is important to remember that proper calcium absorption absolutely requires an adequate level of vitamin D, through diet or by supplement. This vitamin controls the absorption of calcium ion. So one can see that biochemical absorption of calcium is not an easy matter. The excretion of calcium is largely through the mucosa of the small intestines, and a comparatively small quantity (25-35%) is excreted in the urine as phosphate. Since excretion is a normal continuous process, a negative calcium balance can result if dietary intake is too low.