Minerals are inorganic substances (elements) required in small amounts by the body. They function in a number of processes including formation of bones and teeth, fluid balance, nerve conduction, muscle contraction, signaling, and catalysis. [Note: Several minerals are essential enzyme cofactors.] Like the organic vitamins (see Chapter 28), minerals are micronutrients required in mg or µg amounts. Those required by adults in the largest amounts (>100 mg/day) are referred to as the macrominerals. Minerals required in amounts between 1 and 100 mg/day are the microminerals (trace minerals). Ultratrace minerals are required in amounts <1 mg/day (Fig. 29.1). [Note: The classification of specific minerals into these categories can vary among sources.] Mineral concentrations in the body are influenced by their rates of absorption and excretion.
II. MACROMINERALS
The macrominerals include calcium (Ca2+), phosphorus ([P] as inorganic phosphate [Pi, or PO43−]), magnesium (Mg2+), sodium (Na+), chloride (Cl−), and potassium (K+). [Note: The free ionic forms are electrolytes.]
A. Calcium and phosphorus
These macrominerals are considered together because they are components of hydroxylapatite (Ca5[PO4]3OH), which makes up bones and teeth.
1. Calcium: Ca2+ is the most abundant mineral in the body, with ~98% being found in bones. The remainder is involved in a number of processes such as signaling, muscle contraction, and blood clotting. Ca2+ binds to a variety of proteins including calmodulin (see p. 133), phospholipase A2 (see p. 213), and protein kinase C (see p. 205) and alters their activity. [Note: Calbindin is a vitamin D–induced intracellular Ca2+-binding protein involved in Ca2+ absorption in the intestine (see p. 392).] Dairy products, many green vegetables (for example, broccoli, but not spinach), and fortified orange juice are good dietary sources. Although dietary deficiency syndromes are unknown, average Ca2+ intake in the United States is insufficient for optimal bone health. Toxicity is seen only with supplements (tolerable upper limit [UL] = 2,500 mg/day for adults). Hypercalcemia (elevated serum Ca2+) can result from overproduction of parathyroid hormone (PTH). This may cause constipation and kidney stones. Hypocalcemia (low serum Ca2+) can result from a deficiency of PTH or vitamin D. It can lead to bone demineralization (resorption). [Note: The hormonal regulation of serum Ca2+ levels was presented in the vitamin D section of Chapter 28 and is reviewed in 3. below.]
Bone mass increases from infancy through the early reproductive years and then shows an age-related loss in both men and women that increases the risk for fracture. This loss is greatest in postmenopausal Caucasian women. Some studies have shown that supplementation with Ca2+ and vitamin D decreases this risk.