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Meeting Calcium Needs for Optimal Bone Health

By By Connie Weaver, Ph.D.
Department of Foods and Nutrition
Purdue University

Calcium and Bone Health

Osteoporosis, a condition of reduced bone density, is the major underlying cause of bone fractures affecting more than 25 million individuals in the United States with an associated annual health care cost exceeding $13.8 billion¹. Even small differences in bone density can have a great impact on risk of fracture; a 5% increase in bone density translates into a reduction of fracture risk of approximately 40%. Increasingly, osteoporosis is being thought of as a completely preventable disease if an individual consumes a lifelong adequate intake of calcium, participates in weight bearing exercise, and, for females, takes estrogen replacement therapy after the menopause.

Bone mass accrues until the peak is reached by early adulthood and frequently decreases in later life by about 1% per year. Maximizing peak bone mass within an individuals' genetic potential requires optimal calcium intakes throughout growth, but especially during adolescence when the rate of calcium accretion is highest. The age of attaining peak bone mass differs with specific sites. Peak is achieved in the femoral neck by age 14-18 years^2,3. The age at which selected percentages of total body peak bone mineral content is attained is given in Table 1⁴.

Men attain higher peak bone mass than women, but lose bone at a similar rate than women except for immediately following the menopause. Blacks attain greater peak bone mass than whites or Asians and have higher bone mineral densities as adults. Lower levels of bone resorption in blacks are thought to reduce bone loss later in life.

Calcium Requirements

The new Dietary Reference Intakes (DRI) for calcium, magnesium, phosphorus, fluoride, and vitamin D were released August 13, 1997 by the Food and Nutrition Board of the National Academy of Science (NAS). The bone related nutrients were the first of a series of reports to be released. Six additional reports--on folate and B vitamins, antioxidants, macronutrients, trace elements, electrolytes and water, and other food components--will follow.

Unlike the RDAs, which established the minimal amounts of nutrients needed to be protective against possible deficiency, the new values are designed to reflect the latest understanding about nutrient requirements based on optimizing health in individuals and groups. The new recommendations, which included four categories of reference (see Table 1), were made by a group of more than 30 U.S. and Canadian scientists who examined the results of hundreds of nutritional studies on both the beneficial aspects of nutrients and the hazards of taking too much of a nutrient. Where the scientific evidence allowed, the committee made recommendations aimed at helping individuals at different stages of life obtain enough of a nutrient to promote bone strength and to maintain normal nutritional status.

Suggested uses of the DRIs are summarized in Table 2.

Calcuim recommendations were set at levels associated with maximum retention of body calcium, since bones that are calcium rich (high bone density) are known to be less susceptible to fractures. Maximal retention is determined as the intake which provides no additional benefit to calcuim retention/bone accretion. In addition to calcium consumption, other factors that are thought to affect bone retention of calcium and the risk of osteoporosis include high rates of growth in children during specific periods, hormonal status, exercise, genetics, and other diet components.

Our laboratory determined maximal retention calcium intakes for adolescents to be 1300 mg/d. The model developed by our research group was used to determine maximal calcium retention for other age groups. The new DRIs for calcium are given in Table 3.

Achieving Adequate Calcium Intakes

The gap between current calcium intakes (See Table 4 from CSF II 1994 survey data) and requirements is even wider for some age groups. Our challenge and responsibility as educators, health professionals, and caretakers of individual's diets, is to close this gap.

Although the NAS report does not prescribe a means for increasing individual consumption of calcium, it suggests that possible methods for doing so include educating consumers to eat more calcium-rich foods, fortifying foods, and recommending dietary supplements. Individuals who wish to increase their calcium can consume more low- or non-fat dairy products or fortified food products. According to the report, taking supplements such as calcium tablets may be appropriate for those at high risk of health problems due to low calcium intake.

Dairy foods provide 75% of the calcium in the U.S. diet. This is problematic if 80% of the world is lactose insufficient. Vegetables and grains can provide calcium, but the quantities required to replace the amount of calcium in even a single glass of milk practically limit the role of other unfortified foods⁶. (See Table 5). Fortified foods could provide additional choices for meeting calcium requirements with the caution that individuals who consume less milk also have lower intakes of vitamin A, folate, riboflavin, vitamin B6, magnesium, and potassium⁷.

Calcium bioavailability in soybeans is comparable to milk⁸. However, the calcium content of soy products varies widely. Some soy products are calcium rich and can be substituted for milk on an equivalent basis. For example, a 100-gram serving of calcium-set tofu provides as much absorbable calcium as a glass of milk. Calcium fortified soy milk is a good source of calcium, which should be offered as an alternative to milk in school lunch programs.

Some dietary constituents increase urinary calcium loss, and thus, impact calcium retention even though they do not affect calcium absorption. For every gram of dietary salt consumed, approximately 26 mg calcium is lost in the urine. For every gram of metabolizable protein, approximately 1 mg additional calcium in the urine is lost. Thus, choices present themselves to meet individual needs. Diets can be constructed which are higher in calcium or lower in salt, protein, caffeine, and other constituents which lead to calcium loss.

I recommend that individuals try to get at least one calcium-rich source at every meal.

REFERENCES

1. Consensus Development Conference. Diagnosis prophylaxis, and treatment of osteoporosis. Am. J. Med. 94:646-649, 1993.

2. Gitsany, V., Gibbens, D.T., Carlsen, M., Boechat, M.I., Cann, C.E., and Schulz, E.E. Peak trabecular vertebral density: A comparison of adolescent and adult females. Calcif. Tissue Int. 43:260-262, 1988.

3. Theintz, G., Buchs, B., Pizzoli, R., Slosman, D., Clavien, H., Sizonendo, P.C., and Benjour, J.P. Longitudinal monitoring of bone mass accumulation in healthy adolescents: Evidence for a marked reduction after 16 years of age at the levels of lumbar spine and femoral nect in females subjects. J. Clin. Endocrinol. Metab. 75:1060-1065, 1992.

4. Teegarden, D., Proulx, W.R., Martin, B.R., Zhao, J., McCabe, G.P., Lyle, R.M., Peacock, M., Slemenda, C., Johnston, Jr., C.C., and Weaver, C.M. Peak bone mass in young women. J. Bone Min. Res. J. Bone Min. Res. 10:711-715, 1995.

5. Jackman, L.A., Millane, S.S., Martin, B.R., Wood, O.B., McCabe, G.P., Peacock, M., and Weaver, C.M. Calcium retention in relation to calcium intake and postmenarcheal age in adolescent females. Am. J. Clin. Nutr. 66:327-333, 1997.

6. Weaver, C.M., Plawecki, K.L. Dietary calcium: Adequacy of a vegetarian diet. Am. J. Clin. Nutr. 59:1238S-41S, 1994.

7. Fleming, K.H. and Heimback, J.T. Consumption of calcium in the U.S.: Food sources and intake levels. J. Nutr. 124:1426S-1430S, 1994.

8. Heaney, R.P., Weaver, C.M., and Fitzsimmons, M.L. Soybean phytate content: effect on calcium absorption. Am. J. Clin. Nutr. 53:745-747, 1991.