In general, calcium absorption becomes less efficient as we age. During infancy and childhood, 50-70 percent of the calcium ingested may be absorbed, whereas an adult might use only 30-50 percent of dietary calcium in his or her body. It is likely this is based on natural body needs. Various factors can improve our calcium absorption. Besides vitamin D, vitamins A and C can help support normal membrane transport of calcium. Protein intake helps absorption of calcium, but too much protein may reduce it. Some dietary fat may also help absorption, but high fat may reduce it. Lactose helps calcium absorption, and because of this as well as the protein-fat combination, the calcium content of milk is a reliable source of easily assimilated calcium. For other reasons, though, milk is not an ideal food for many people, especially the homogenized variety fortified with synthetic vitamin D, making milk a less-than-perfect (and definitely not the only) source of calcium. Nonfat milk does not improve calcium absorption and, in fact, may decrease it.

Gastric hydrochloric acid helps calcium absorption. The duodenum is the main location for absorption of calcium because farther down the small intestine the local environment becomes too alkaline. A fast-moving intestinal tract can also reduce calcium absorption. Exercise has been shown to improve absorption, and lack of exercise can lessen it. Stress also can diminish calcium absorption, possibly through its effect on stomach acid levels, digestion, and intestinal motility. Though calcium in the diet improves the absorption of the important vitamin B12, too much of it may interfere with the absorption of the competing minerals magnesium, zinc, iron, and manganese.

Many dietary factors also reduce calcium absorption. Foods that are high in oxalic acid, such as spinach, rhubarb, chard, and chocolate, can interfere with calcium absorption by forming insoluble salts in the gut. Phytic acid, or phytates, found in whole grain foods or foods rich in fiber, may reduce the absorption of calcium and other minerals as well. Protein, fat, and acid foods may help calcium absorption, but high-protein diets may increase calcium elimination through the intestines. Calcium absorption is sensitive and requires energy to transport it into the body. Calcium is often chelated with proteins or amino acids (specifically, glutamic or aspartic acid) to make it more absorbable.

Increased by:

• Body needs—growth, pregnancy, lactation
• Vitamin D
• Milk lactose
• Acid environment—hydrochloric acid, citric acid, ascorbic acid (vitamin C)
• Protein intake and amino acids such as lysine and glycine
• Fat intake
• Exercise
• Phosphorus balance

Decreased by:

• Vitamin D deficiency
• Gastrointestinal problems
• Hypochlorhydria (low stomach acid)
• Stress
• Lack of exercise
• High fat intake
• High protein intake
• Oxalic acid foods (beet greens, chard, spinach, rhubarb, cocoa)
• Phytic acid foods (whole grains)
• High phosphorus intake

Because of the many complex factors affecting calcium absorption, anywhere from 30-80 percent may end up being excreted. Some may be eliminated in the feces. The kidneys also control calcium blood levels through their filtering and reabsorption functions. Excess salt intake can lead to increased calcium losses in the urine. Sugar intake may reduce the reabsorption of calcium and magnesium and cause more to be eliminated. The table on the previous page summarizes the factors affecting calcium absorption.