Bones are the structural component of the body that shield our brain and organs from injury and make it possible to move. Minerals make up approximately two-thirds of the mass of the skeletal system and are involved in growth and maintenance of cellular membranes and connective tissues. Three major minerals of particular importance for bone formation and maintenance are calcium, phosphorus, and magnesium; with crystals of primarily calcium and phosphorus laid down on a matrix of proteins (predominantly collagen, a structural protein found in connective tissue) during bone formation. (INFOGRAPHIC 13.2)

Calcium (Ca) is the most abundant mineral in the body, with 99% found in bones and teeth, where it provides an essential structural component for their formation. Bones provides a reservoir of calcium that can be tapped to supply calcium to body fluids when its concentration in blood decreases. The other 1% is located in the body cells and fluids, where it is necessary for many essential functions such as blood clotting, hormone secretion, muscle contraction, and nerve transmission.

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Because so many critical body functions depend on calcium, its concentration in blood is tightly regulated so that it remains nearly constant regardless of dietary calcium intake. When calcium in blood falls even slightly, it will be released from bone to maintain steady blood calcium levels. The mechanism by which the body maintains calcium levels in the blood is known as calcium homeostasis. When blood calcium levels fall, the parathyroid gland releases parathyroid hormone (PTH), which stimulates the production of the active form of vitamin D (calcitriol) and thereby increases calcium absorption from the intestine. (Refer back to Infographic 10.4 to review this process.) PTH and activated vitamin D work together to mobilize calcium from the bone and decrease calcium excretion from the kidneys.

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Calcium plays an indispensable role in bone and tooth formation. In fact, bone is constantly being broken down and rebuilt in a process known as remodeling. Bone remodeling is necessary not only to maintain blood calcium levels, but it is also required during bone growth in the young, to allow bone to adapt to strain, and to repair the microscopic damage that occurs daily.

The Institute of Medicine has set the AI for calcium at 1,000 milligrams per day for men and women aged 19 to 50 years; the Tolerable Upper Intake Level (UL) is set at 2,500 milligrams. Calcium-rich foods include milk, yogurt, and cheese, as well as some legumes, and certain dark-green leafy vegetables, such as Chinese cabbage, kale, and broccoli. Because these vegetables are low in oxalates the bioavailability of calcium is high: about 50% is absorbed. In contrast, though spinach is high in calcium, it is also high in oxalates that bind calcium and inhibit its absorption. Consequently, the bioavailability of calcium in spinach is low: only about 5% is absorbed. Calcium is also added to some grains, juices, tofu, and cereals. Milk, an excellent natural source of calcium, is usually also fortified with vitamin D, which works with calcium to promote bone health. Most studies indicate that calcium from food is better absorbed than calcium from supplements. This is likely due to improved absorption with meals and the tendency of people to consume smaller amounts of calcium more frequently, which likely improves absorption efficiency. (INFOGRAPHIC 13.3)

Question 13.1

With age, the balance of calcium release and deposition in bone changes. During years of growth, such as childhood, more calcium is added to bone in relation to the amount lost, but as we get older, bone breakdown often exceeds formation. Peak bone mass is established at around age 30, so it is important, during the formative years of bone development, to consume adequate amounts of calcium and vitamin D. If intake is low or absorption is impaired, bone loss occurs because the body uses the calcium in bone to maintain blood levels and support calcium-dependent bodily functions.

Most people realize that adequate calcium status is important for optimal bone health. However, a significant number of Americans have low bone mass. Although some bone loss is a normal consequence of aging, bone loss accelerates in postmenopausal women because of low levels of the hormone estrogen. This reduced bone mass, or bone density, along with reduced mineral content, can lead to a condition called osteopenia. When osteopenia becomes severe, and bone loss worsens to cause bones to be fragile and porous, a person develops osteoporosis, or “porous bones,” and the risk of bone fractures is dramatically increased.

Osteoporosis afflicts more than 10 million Americans; approximately one-half of all women and one-quarter of all men older than 50 years of age experience osteoporosis-related bone fracture. This is particularly worrisome as the risk of mortality increases by as much as four times in the first four months following an osteoporosis-related fracture. Risk factors for osteoporosis include advanced age as well as a history of inactivity, smoking, excess alcohol consumption, and a family history of osteoporosis. To reduce their risk, individuals should maintain a healthy diet with adequate intake of calcium and vitamins D and K throughout life. Vitamin K is needed for the functioning of several proteins that are involved in regulating bone formation. Some studies have found that the risk of bone fractures decrease as intake of vitamin K increases. It is also important to participate in regular weight-bearing exercises, such as walking, running, or tennis, as well as perform resistance exercises to maintain bone health and reduce risk of osteoporosis. (INFOGRAPHIC 13.4)

Although it is tempting to think that supplements are a “sure thing” to confirm that you are getting enough calcium, supplementation can potentially push intake to higher-than-recommended levels, especially when added to daily food sources that are fortified with calcium. Studies show that more than 60% of women older than 60 years take calcium supplements, which could push their intake close to the UL. High dietary intakes may cause constipation and can interfere with iron absorption. Although rare in healthy people, excess intake may contribute to hypercalcemia, or excess calcium in the blood. Contrary to what is commonly believed, there is little evidence that high calcium intake promotes the formation of kidney stones.

Magnesium (Mg) is a cofactor in more than 300 chemical reactions in the body. An adult body contains about 25 grams of magnesium, most of it—about 50% to 60%—is found in the bones; the rest is in the cells and fluids of the body. Magnesium plays a role in the transport of ions across the cell membrane, a process that is important to muscle contraction, nerve impulse conduction, and maintaining the rhythm of the heart. Magnesium is involved in extracting energy from carbohydrates, fats, and protein, as well as using that energy to perform work; and it plays a role in protein production. Magnesium is also necessary to convert vitamin D into its active form (calcitriol) to increase calcium absorption and thus plays an important role in bone health.

Approximately 60% of American adults do not consume the recommended intake level of magnesium, but outright deficiency symptoms are rare, because the kidneys limit excretion when intake is low and the body may absorb more. However, low intakes of magnesium are a risk factor for osteoporosis. Marginal or moderate magnesium deficiencies may also increase the risk of atherosclerosis, cancer, diabetes, and hypertension. There is ongoing research about the role of magnesium in preventing and managing these disorders. Excess consumption from the diet is also rare, but toxicity can occur from supplement misuse. (INFOGRAPHIC 13.5)

Phosphorus (P) is the second most abundant mineral in the body and is present in every cell of the body. It too plays a critical role in bone health and is an essential component of bone and cartilage, phospholipids, DNA, and RNA. It is also involved in energy metabolism, and a multitude of enzymes and other proteins depend upon phosphorus to regulate their activity. Phosphorus is important in the maintenance of proper acid-base balance in the body. Phosphorus deficiency is rare, in part because the mineral is found in most protein-rich foods, such as meats and dairy, and because many food additives also contain phosphorus. In fact, there is some concern that Americans may be chronically overconsuming phosphorus, a potential problem considering that increased blood levels of the mineral have been associated with cardiovascular and other types of chronic diseases, particularly in people with kidney disease.