The human body contains more calcium than any other mineral. The body of a 154-pound person contains approximately 4 pounds of calcium. Ninety-ninepercent of that calcium is found in the skeleton and teeth. The remaining 1% is found in the blood.
Functions.Calcium, in combination with phosphorus, is a component ofbones and teeth, giving them strength and hardness. Bones, in turn, provide storage for calcium. Calcium is needed for normal nerve and muscle action, blood clotting, heart function, and cell metabolism.
Regulation of Blood Calcium.Each cell requires calcium. It is car-ried throughout the body by the blood, and its delivery to the cells is regulated by the hormonal system. Normal blood calcium levels are maintained even if intake is poor.
When blood calcium levels are low, the parathyroid glands release a hor-mone that tells the kidneys to retrieve calcium before it is excreted. In addition, this hormone, working with calcitriol (the active hormone form of vitamin D), causes increased release of calcium from the bones by stimulating the activity of the osteoclasts (cells that break down bones). Both of these actions increase blood calcium levels. If calcium intake is low for a period of years, the amount withdrawn from the bones will cause them to become increasingly fragile. Osteoporosis may result.
If the blood calcium level is high, osteoblasts (cells that make bones) will increase bone mass. During growth, osteoblasts will make more bone mass than will be broken down. Bone mass is acquired until one is approximately 30 years old. With adequate consumption of calcium, phosphorus, and vitamin D, bone mass will remain stable in women until menopause. After menopause, bones will begin to weaken owing to the lack of the hormone estrogen. A special X-ray, a DEXA scan, can be taken to determine bone density. If a person is at risk for injury due to decreased bone density, the phy-sician will decide the best course of action. Drugs that help prevent further loss of bone mass are available.
Sources.The best sources of calcium are milk and milk products. Theyprovide large quantities of calcium in small servings. For example, 1 cup of milk provides 300 mg of calcium (Figure 8-1). One ounce of cheddar cheese provides 250 mg of calcium.
Calcium is also found in some dark green, leafy vegetables. However, when the vegetable contains oxalic acid, as spinach and Swiss chard do, the calcium remains unavailable because the oxalic acid binds it and prevents it from being absorbed. When the intake of fiber exceeds 35 grams a day, calcium will also bind with phytates (phosphorus compounds found in some high-fiber cereal), which also limits its absorption.
Factors that are believed to enhance the absorption of calcium include adequate vitamin D, a calcium-to-phosphorus ratio that includes no more phos-phorus than calcium, and the presence of lactose. A lack of weight-bearing exercise reduces the amount of calcium absorbed.
Requirements.The estimated requirement for calcium is now given asan Adequate Intake (AI) level. Calcium is measured in milligrams (mg).
The AIs for calcium at different ages and conditions are shown in Table 8-4. The recommendations were made to achieve optimal bone health and to reduce the probability of fractures in later life.
Calcium supplements are recommended for persons who are lactose intoler-ant, those who dislike milk, and those who are unable to consume enough dairy products to meet their needs. Calcium carbonate, the form found in calcium-based antacid tablets, has the highest concentration of bioavailable calcium. Calcium supplements appear to be absorbed most efficiently when consumed in doses of 500 mg.
When purchasing calcium supplements, check for the USP (United States Pharmacopeia) seal of approval on the product you select (Figure 8-2). USP-approved products are unlikely to contain lead or other toxins. Avoid bone meal products because they may contain lead.
Deficiency.Calcium deficiency may result in rickets. This is a disease thatoccurs in early childhood and results in poorly formed bone structure. It causes bowed legs, “pigeon breast,” and enlarged wrists or ankles. Severe cases can result in stunted growth. Insufficient calcium can also cause “adult rickets” (osteomalacia), a condition in which bones become soft. And although the pre-cise etiology of osteoporosis is not known, it is thought that long-term calcium deficiency is a contributing factor. Other factors contributing to osteoporosis include deficiency of vitamin D and certain hormones.
Insufficient calcium in the blood can cause a condition characterized by involuntary muscle movement, known as tetany. Excessive intake may cause constipation, or it may inhibit the absorption of iron and zinc.