From The February 2000 Issue of Nutrition Science News
By Marilyn Sterling, R.D.
We all learned in grade school that calcium is a key nutrient. But our very familiarity with calcium may prevent us from fully appreciating how life depends upon this ion necessary for processes ranging from setting the body's biological cycle to movement. Calcium is the conduit for most cellular communication. Nerves and muscles depend on electrical impulses controlled by calcium switches. Perhaps because a steady supply of the element is so critical, the body has evolved a skeletal structure that stores extra calcium and doles it out when needed.
Getting Down to the Bone
Bone's solid appearance is misleading. Its living tissue is constantly in flux, bending with movements and being broken down and rebuilt. The amount of bone in the body is continually adjusted to achieve the best balance between lightweight structure and strong support.
Nearly 2 to 5 percent of bone is made up of bone cells, including bone-dissolving osteoclasts and bone-forming osteoblasts. Bone activity in any stage of life continually undergoes an orderly set of procedures. The first step, activation, is a retraction of the protective lining cells that cover normal resting bone. After the bone is directly exposed to the blood stream, osteoclasts swarm to its raw surface by way of a rich system of blood vessels. Once there, the osteoclasts dissolve the bone, after which they die or become inactive. Osteoblasts then arrive to rebuild the damage. They make bone by producing rows of collagen fibers, changing the orientation of the fiber in each row. This collagen matrix is filled in by bone mineral, or hydroxyapatite, made up of 50 percent phosphate, 40 percent calcium and 10 percent carbonate. The osteoblasts remain in the bone tissue where they continue to live. Any remaining osteoclasts transform into harmless lining cells.
During any 10-year period, virtually all bones are completely rebuilt. In children, the process of resorption and regrowth allows bone shape to match the child's rapid growth.
Bone remodeling is controlled by several factors including hormones, physical stress and the availability of building blocks such as calcium. During growth, children incorporate substantial amounts of calcium into their bonesup to 500 mg daily. In fact, the amount of calcium children ingest may determine their bone density. One study of 84 twins ages 9 to 14 found that within the first six months of an 18-month study, the twin given 1,000 mg calcium/day grew heavier bones than the one taking a placebo. 
Although height and bone growth are obviously connected, maximum bone building in puberty occurs a year or two after the fastest growth spurt13.5 years for boys and 11 years for girls.  Drinking more calcium-rich milk from childhood through the teenage years also increases bone density.  About 90 percent of maximum bone density is reached by age 17, but bone density continues to increase slowly throughout young adulthood. Supplementing young adults in their 20s with calcium stimulates bone growth, possibly maximizing their peak bone mass.  If peak bone mass is high, the inevitable bone loss of old age may have less serious repercussions.
During pregnancy and breast-feeding, calcium from a mother's body builds her baby's bones. Bone resorption is stepped up during lactation to provide needed calcium, but as soon as breast-feeding stops, a mother's bone growth is restimulated. Although it is commonly believed that enough calcium is resorbed from bone to maintain the calcium content of breast milk regardless of a mother's intake, a recent study found higher third-trimester calcium intake led to higher levels of calcium in breast milk. 
Can Calcium Stop Osteoporosis?
Until menopause, bone growth and resorption proceed at an equal rate, but at menopause the bone loss increases by 2 to 6 percent a year for roughly five years. The rate of bone loss slows to 1 percent annually about 10 years after menopause, continuing at that rate until age 75, when it may again increase. Men also lose bone at an annual rate of 1 percent after age 50. When bone density slips far enough below that of a healthy young person, osteoporosis has set in. Osteoporosis is bone loss to such an extent that bone can no longer support the body and fractures easily. Although it is associated with aging, young people can also develop osteoporosis if they have a condition such as anorexia or bulimia, or if they exercise to excess, diet severely or use corticosteroids. All can interfere with calcium function or bone growth.
Extensive research has focused on whether calcium may slow or prevent age-related bone thinning. Most studies find that calcium supplementation does slow the loss of bone density, and numerous intervention studies document a decrease in fractures in postmenopausal women and older men given calcium supplements. [6,7] Several of the studies, however, also provided another bone-buildervitamin Dalong with the calcium. One study of 248 perimenopausal women found supplementation with up to 2,000 mg/day calcium actually increased bone density. 
Although these studies support the beneficial effects of calcium, not all research indicates that more calcium helps bones. Two large studies surveyed more than 32,000 postmenopausal women to determine how much calcium they usually consumed. Neither found any correlation between dietary calcium and the incidence of broken hips among the women. [9,10] Likewise, cross-cultural studies reveal that societies with the highest intake of calcium and dairy products experience the highest rates of osteoporosis and hip fractures. Statistical studies show that many traditional societies, such as those in Greece and China, have low calcium intakes and low osteoporosis rates. Another study failed to find any correlation between bone density and risk of hip fractures.  Some researchers argue that osteoporosis results from loss of the collagen bone matrix, which is more responsible for bone strength than its mineral filling. 
Calcium's Other Applications
Although calcium's role in bone development gets the most attention, research into its effect on other parts of the body has been going on for years. Here's an update.
Atherosclerosis The connection between calcium intake and atherosclerosis is unclear. Atherosclerosis, the most common arterial disease, is marked by cholesterol deposits in the artery walls. Some research shows higher calcium intake protects against atherosclerosis in humans and in animals. But not all calcium sources provide the same benefit. Although calcium from most dietary sources and supplements is associated with decreased heart disease risk, one prevalent calcium sourcedairymay actually increase the chance of heart disease. This may be because other components in milk increase body levels of atherosclerotic homocysteine, an amino acid implicated in heart disease. Other studies show calcium intake has no effect on heart disease, and some researchers have even suggested a connection between calcium deposition in arterial walls and calcium intake. [13-17]
Research on calcium and high blood pressure has also yielded mixed results. It appears that high calcium intake or calcium supplements may reduce blood pressure in only certain segments of the populationAfrican-Americans, the elderly and diabetics as well as those whose blood pressure increases with salt intake. 
Colon Cancer The large-scale Iowa Women's Health Study of 34,702 women found that women with the highest intakes of calcium and vitamin D from foods cut their risk of rectal cancer by almost half.  A case-control study of colon and rectal cancer among 1,190 Wisconsin women also found that those with cancer consumed less calcium than healthy women.  High calcium intake also reduces the recurrence rate of colonic or rectal tumors, especially in women with high-fat diets.  Although no one is sure why calcium appears protective, it may be that it binds with potentially carcinogenic fatty acids and bile, rendering them harmless.
Pregnancy Because calcium supplementation significantly lowers blood pressure in pregnant women, researchers speculated that it might prevent preeclampsia, a pregnancy disorder marked by high blood pressure and responsible for most maternal deaths in the United States. The hypothesis was supported by lower rates of preeclampsia found in women taking calcium supplements for Japanese studies. [22,23] Hopes for an easy solution were dashed when a recent National Institute of Health clinical trial of 4,000 U.S. women found calcium supplementation ineffective for preventing preeclampsia.  It is possible that the high salt intake in Japan and genetic differences between the two populations account for the differing results.
PMS On a more definite note, calcium may help women's health by reducing PMS symptoms. A recent, highly publicized study of 441 women found a 1,200 mg calcium supplement decreased most premenstrual symptoms by 48 percent. It should be noted that the placebo group reported a 30 percent decrease in PMS symptoms. 
How much calcium is enough? Official U.S. calcium requirements are determined from metabolic studies. The calcium Dietary Reference Intake for adults older than 50 was raised in 1997 from 1,000 mg to 1,200 mg to reflect new information about calcium's role in bone health. Because excess calcium can interfere with absorption of other minerals, people should take no more than 2,000 mg daily.
Most children older than five, adolescents and adults do not get enough calcium according to U.S. government food surveys. Calcium intakes of men and women older than 50 tend to fallin many cases dropping to half of the 1,200 mg RDA for seniorsalthough their calcium requirements rise. 
Dairy products are not the only, or even necessarily the best, calcium sources. Enriched tofu has as much calcium as milk. Tofu made with nigari is also high in calcium. Cruciferous vegetables, especially the leaves of kale, collard, and mustard greens are high in calcium. Molasses, almonds, peanuts, corn tortillas and dried beans are other good sources. Fortified orange juice contains as much calcium as milk but no vitamin D. Fortified soy and rice milks are also good sources.
Calcium supplements come in several forms: calcium citrate, calcium carbonate and calcium gluconate. In one recent study, calcium citrate proved 2.5 times more bioavailable than calcium carbonate.  Another study, however, suggests that calcium citrate may increase aluminum absorption. Oyster shell and bone meal calcium sources may also contain heavy metals. [28,29]
Calcium-balance studies, which measure calcium excretion to determine replacement needs, are often difficult to interpret because calcium metabolism fluctuates according to intake. Calcium metabolism is part of a complex web of interactions among parathyroid hormone and other hormones, and is interwoven with intakes of other nutrients. For example, high levels of phosphates, found in soft drinks and protein, increase the amount of calcium secreted during digestion and therefore lost.
Sodium intake is one of the most important, yet one of the most overlooked determinants of calcium need. Sodium carries calcium with it as it travels from the blood to the urine, so a high salt intake essentially drains the body's calcium supply. It has been calculated that increasing sodium consumption 1 g/day causes bone loss of 1 percent annually unless calcium loss is compensated for.  Americans eat astronomical amounts of sodium compared with most other cultures other than Japan. 
Researchers are still fitting together pieces of the calcium puzzle. Until there is a clearer understanding of calcium metabolism and requirements, encourage your customers to get enough calcium for long-term bone health by eating calcium-rich foods and taking supplements when dietary calcium is inadequate.
U.S. Calcium Dietary Reference Intakes, 1997
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