16.3: Calcium in Foods

The main source of calcium in the American diet is milk and foods made with milk. Milk is a particularly good source— the vitamin D, protein, and lactose in the milk promote the absorption of calcium. In the U.S., calcium intake generally is related to the amount of milk or milk products in the diet. (Dark, leafy, green vegetables are a source of calcium in this country for those who don’t drink much milk.)

Calcium-fortified foods (foods with calcium added) are an increasingly popular source of calcium, and include calcium-fortified orange juice and calcium-fortified soy milk. Some foods eaten more commonly in other cultures contain a fair amount of calcium. Traditional Mexican corn tortillas are rich in calcium. Corn itself isn’t rich in calcium, but the corn is soaked in lime water (a solution of calcium oxide) before being ground to make the tortillas. (This calcium-fortified cornmeal is called masa harina.)

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The calcium-rich antlers of deer shed onto the forest floor provide dietary calcium for small animals.

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Soybean curd (tofu) is also made rich in calcium by processing in a calcium-rich solution. Sesame seeds are rich in calcium, and a paste of ground sesame seeds is the main ingredient in halvah, a Turkish confection. Fish bones (as in canned sardines and salmon) and fish sauce (bagoong) also provide calcium.

When animals (including humans) need a lot of calcium but must subsist on foods that are low in calcium, they must eat tremendous amounts. Cows spend virtually all their waking hours munching on grass or hay.

The amount of dietary calcium absorbed from the intestine varies, depending on a number of factors. Spinach is rich in calcium, magnesium, and iron, but also contains lots of oxalic acid, a substance that can bind to these minerals and hinder their absorption. In contrast, as discussed in the previous chapter, vitamin D promotes calcium absorption.

Table 16-1: Minerals

Mineral	Function	Source	Deficiency	Possible Toxicity
Calcium	Bone and teeth structure, muscle and nerve function	Milk, milk products, sardines, collard greens, calcium-set tofu, masa harina	Stunted growth, osteoporosis	Kidney stones
Phosphorus	Bone and teeth formation, energy production	Meat, fish, milk, eggs, legumes, cereals, nuts, soft drinks	Irritability, weakness, muscle ache	Poor bone mineralization if low calcium intake
Magnesium	Cofactor in metabolism	Green leafy veggies, whole grains, nuts, legumes, shrimp, broccoli, seeds, milk	Weakness, muscle pain, cramps, spasms	Diarrhea, nausea, abnormally slow heartbeat, weakened reflexes
Sodium	Water balance	Salt, soy sauce, fast foods, processed foods	Weakness, cramps	High blood pressure
Chloride	Fluid balance, stomach acid	Salt, soy sauce, processed foods	Rare; upset acid-base balance	High blood pressure
Potassium	Nerve function	Dried fruits, winter squash, legumes, potatoes, spinach, tomatoes, citrus fruits	Weakness, irregular heartbeat	Irregular heart beat

The rate of calcium absorption improves as the body adapts to a lower calcium diet over a long period, and also in times of higher need (e.g., during early childhood, pregnancy, and lactation). On the other hand, absorption is diminished in older adults. This variability in calcium absorption is taken into consideration in setting the recommended intake.

It’s important to get enough calcium, particularly during childhood, adolescence, and early adulthood when bone is growing and increasing in calcium content. The recommended intake for calcium is the same for both sexes, but males generally meet the recommendation, whereas females 11 years and older generally don’t.

Girls and women drink less milk, and eat less food than males the same age. The more food we eat, the more nutrients we take in.

In population studies, adequate calcium intake is associated with a lower risk of colon cancer and high blood pressure.

Calcium Supplements

Different calcium supplements contain varying amounts of calcium. Calcium carbonate is the most concentrated form (40% calcium). Calcium citrate is 21% calcium; calcium lactate is 13%; calcium gluconate is 9%. Also, to be effective, the supplement must dissolves easily in the acid fluid of the stomach.

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Some brands of supplements don’t dissolve as they should in the stomach. Put a tablet into a half-glass of vinegar at room temperature, and stir vigorously every 5 minutes. If the tablet isn’t completely dissolved in 30 minutes or less, try another brand.

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Taking supplements of ground-up dolomite (a natural rock rich in calcium) or bone (“bone meal”) isn’t advised—they might be contaminated with lead or other toxic elements. Look for calcium supplements having a USP (United States Pharmacopeia) seal of approval.

Calcium supplements are better absorbed when taken with a meal, and in doses of 500 mg or less. Calcium supplements can increase total calcium intake far beyond recommended amounts. Excessive calcium can interfere with the absorption of other essential minerals.

Osteoporosis

Osteoporosis literally means porous bones. The interior of bone is normally porous (styrofoam-like), but the size of the pores increase (become lace-like) as bone minerals—mainly calcium—are lost (see Fig. 16-3). Osteoporosis is the condition in which bones become so porous and brittle that they (especially the bones of the wrist, spine, and hip) are easily fractured. These fractures can lead to severe pain and disability. Osteoporosis is especially common in white, thin, older women.

Figure 16-2: Calcium moves between blood and bone. The amount of calcium in blood is kept steady by hormones (calcitonin and parathormone), which move calcium in and out of bone.

After age 50, wrist fractures become more common, usually from extending the arm to break a fall. While wrist fractures aren’t usually serious, they can indicate underlying osteoporosis. Fractures of the spinal vertebrae, which tend to occur after about 55, are often painless, though some people suffer severe pain and disability. The outward signs of these fractures are a loss of height and the stoop of old age—often called “dowager’s hump” (see Fig. 16-4).

Fractures of the hip are the most serious and are most common among women over age 70. Many of these women either die from complications, or are so disabled that they can no longer live independently.

A key factor in the development of osteoporosis is the density (calcium content) of bone in early adulthood. The higher the bone density at its peak, the less likely that osteoporosis will occur later (see Fig. 16-5).

At comparable ages, osteoporosis is more common among women than men, and more common among whites than blacks. The increased susceptibility of women and whites can be explained by their lower peak bone density and increased longevity, compared to men and blacks. Women also have higher risk because of about 5 years of accelerated bone loss that begins at menopause. Estrogen has a protective effect on bone, and estrogen production by the ovaries falls after menopause.

Figure 16-3: Dense vs. Porous

Although osteoporosis can be detected to some extent by taking note of losses in a person’s height, osteoporosis usually is brought to medical attention by a bone fracture.

A puzzling aspect of osteoporosis is that the amount of bone loss doesn’t necessarily correlate with the amount of pain or disability that a person suffers. One person may have compression fractures of the vertebrae and severe loss of bone density yet not suffer pain or disability, while another person may have much less bone loss and experience debilitating pain.

Osteoporosis has been a hard disease to study because most of the studies of changes in bone mass compare people of different ages within a population rather than following the same people as they age (which gives better information).

Another complicating factor is that bone is not lost evenly throughout the skeleton or even throughout a single bone. Certain bones are often studied because of convenience, cost, etc. The heel bone may be easier to study than the spine, but what one sees in the heel may not be indicative of what’s happening in the bones of the spine or hip.

In other words, much of what is known about the development of osteoporosis is still rather tentative. Osteoporosis is a very active area of research, and more sophisticated methods of measuring bone loss, e.g., CAT (computerized axial tomography) scans and DEXA (dual energy x-ray absorptiometry), now provide better information. DEXA is also used as a screening test to identify people with or at high risk of osteoporosis.

Figure 16-4: Osteoporosis often results in a loss of height and a misshapen body.