Calcium is the most abundant mineral in the human body. The average adult skeleton contains about 2-3 Ib of calcium, with 99% found in the bones and teeth. The remaining 1% of body calcium is found in the blood and within cells, where calcium helps with dozens of metabolic processes. This 1% of blood and cellular calcium is so tightly maintained within normal ranges that the body will draw on calcium stores in the bones, even at the expense of causing osteoporosis. Good dietary sources of calcium include all dairy products and several vegetables, like broccoli, bok choy, and kale. A cup of milk contains about 300 mg of calcium. Because of the predominant role of calcium in bone health, it is covered in this chapter, but calcium is also an essential mineral for maintenance of blood pressure and nerve conduction. Therefore, dietary supplements containing calcium are frequently marketed for controlling blood pressure, reducing the risk of colon cancer, and alleviating certain symptoms of PMS.
Calcium is cheap, widely available, and well tolerated as a supplement. Practically nobody consumes enough calcium in a "normal" daily diet, so calcium is one of the nutrients for which supplementation is highly recommended. When it comes to choosing a calcium supplement, a prudent approach is to select a product that combines calcium with other bone-specific nutrients such as vitamin D, vitamin K, and magnesium. The form of calcium, however, seems to be less important. Although a few studies suggest that one form of calcium may have superior intestinal absorption compared with another form, most comparison studies show no appreciable differences among the absorption rates of the various forms of calcium available as dietary supplements (milk, milk powder, carbonate, citrate, hydroxyapatite, and others).
The most obvious need for calcium is to help build and maintain strong bones, but calcium is also important for blood clotting, muscle contraction, nerve transmission, and maintenance of normal blood pressure. There is also some evidence that calcium supplements may be helpful in reducing the risk of colon cancer, regulating heart rhythms, and treating PMS.
For decades, we have known about the important role that calcium plays in achieving and maintaining strong bones and helping to prevent osteoporosis. Numerous studies have shown the skeletal benefit of using calcium supplements (generally, 500-1,000 mg/day) to increase bone mass in women from adolescence to old age (Martini and Wood, 2002; Stear et al, 2003). Not all studies, however, have found that an increase in calcium intake translates into a reduction in fracture risk (Feskanich et al., 2003), suggesting that vitamin D and vitamin K status may be as important as calcium intake for optimal absorption, transport, and skeletal assimilation of bone minerals.
The dietary source of calcium does not appear to be as important as the overall level or its combination with vitamins D and K. Martini and Wood (2002) have shown that the bioavailability of calcium and suppression of parathyroid hormone (PTH, a reliable index of calcium status) does not differ between calcium supplements provided as calcium citrate malate (in fortified orange juice), skim milk, or calcium carbonate.
Data from various nutrition questionnaires and surveys indicate that 50-70% of American men and women fail to consume the recommended amounts of calcium (Dawson-Hughes et al., 2002); thus, it is prudent for most Americans to consider a daily calcium supplement as one approach to increasing total calcium intake. Although calcium, when supplemented alone, has been shown to significantly improve bone density at various skeletal sites (Cleghorn et al., 2001), other studies suggest that the optimal bone-sparing effects of calcium are achieved in combination with vitamin D (Jensen et al., 2001). Such combined dosing is also likely to enhance the efficacy of estrogens and bisphosphonates, approximately doubling the increase in bone mass at various skeletal sites in supplemented versus nonsupplemented subjects (Nieves et al., 1998).
More recent research, much of it conducted over the past 5 years, has suggested a number of other beneficial health effects of getting adequate calcium in the diet. Among the more exciting research, scientists have recently shown that eating more calcium-rich foods reduces the risk of colon cancer in men and that taking daily calcium supplements can cut in half women's premenstrual symptoms (pain, bloating, mood swings, and food cravings), In other studies, researchers found that adequate calcium intake (along with vitamin D) can reduce blood pressure in women with mild hypertension and in black teenagers (two groups who rarely consume enough calcium). The hypertensive effects of a high-salt diet tend to be most pronounced among people whose diets are low in calcium. In addition, women who take calcium supplements during pregnancy tend to give birth to children with healthier blood pressure levels (lower than average for the first seven years of life), which may reduce the child's risk of developing high blood pressure later in life.
Side effects from calcium supplements are rare, but nausea, diarrhea, constipation, or other gastrointestinal effects may be possible at extremely high intakes. The UL for calcium is 2,500 mg/day. Intakes above 1,500 mg/day have not been associated with any greater benefits than more moderate intakes in the RDA ranges of 1,000-1,300 mg/day for men and women.
The recommended dietary reference intake (DRI) for calcium are 1,300 mg for ages 9-18; 1,000 mg for adults aged 19-50; and 1,200 mg for adults over age 50. For postmenopausal women not taking hormone replacement therapy, 1,500 mg/day of calcium is recommended.