Calcium plays a critical role in building strong and healthy bones. Low calcium consumption and inadequate weight-bearing exercise contribute to osteoporosis, the weakening of the bone that can occur late in adulthood and causes 1.5 million bone fractures a year. About 10 million Americans have osteoporosis. In addition, an estimated 41 million Americans may develop osteoporosis or low bone mass by 2015 unless steps are taken to prevent, detect, and treat the disease.

Two important factors that influence the incidence of osteoporosis are peak bone mass attained during the first two to three decades of life and the rate at which bone is lost in the later years.³ Childhood and adolescence are critical periods for bone development because most bone mass accumulates during this time. By the time adolescents finish their "growth spurt" around the age of 17, approximately 90% of their adult bone mass will have been established. Bones then continue to grow more dense until around age 30, when peak bone mass is reached. At this point, bone mass and density may remain steady, or bone loss may begin to occur at a rate of up to about 1% per year.

NICHD believes that osteoporosis is a pediatric preventable disease. A recent NICHD-supported study found that supplementing the diets of girls, age 12-16, with 500 mg of calcium citrate-malate produced a 14% increase in bone density in comparison to unsupplemented girls. The implications of this increase are striking: for every 5% increase, the risk of fracture later in life declines by 40%. However, subsequent study has shown that without continued supplementation at this level, the difference in the bone densities of the two groups becomes indistinguishable.⁴ Therefore, adequate growth, development, and maintenance of the skeletal system requires continuing calcium intake over a lifetime.

Unfortunately, most children and teens do not meet the dietary calcium recommendations that can help build maximum bone mass and protect against osteoporosis. Half of all children under five and about 85% of females age 12-19 do not meet the 1989 Recommended Dietary Allowance (RDA) for calcium. And even fewer of these children and adolescents could attain the government's new guidelines, the Dietary Reference Intakes (DRIs), set in 1997 (see Table 1).

These new guidelines set Adequate Intake (AI) values for calcium designed to lead to the fewest diet-related osteoporotic fractures later in life. Prior to the DRIs, optimal levels of calcium were recommended by the National Institutes of Health (NIH) Consensus Conference in 1994 and were slightly higher for most age groups. Parents should ensure that their children and teens get enough calcium and weight-bearing exercise to help them reach their maximum bone density.

Table 1

QUESTIONS AND ANSWERS ABOUT CALCIUM INTAKE

Q: How does bioavailability affect calcium absorption?
A: Bioavailability is the degree to which the intestinal system can absorb calcium. Absorption depends on the overall level of calcium in the food and the type of food being consumed. Foods vary in their ability to enhance or inhibit calcium absorption. The efficiency of calcium absorption is fairly similar for foods such as dairy products and grains. However, calcium may be poorly absorbed in foods high in oxalic acid (spinach, sweet potatoes, and beans) or phytic acid (unleavened bread, raw beans, seeds, and nuts). Oxalates in particular are strong inhibitors of calcium absorption. As a  result, additional servings of many calcium-rich foods are needed to compensate for their low calcium absorption. According to the National Academy of Sciences report on the new DRIs, the body absorbs about a tenth as much calcium from spinach as it does from milk.

Q: What other factors can affect bone development?
A: Vitamin D - Vitamin D is an essential factor in bone mineralization, assisting in calcium absorption. Vitamin D acts to increase absorption from the gastrointestinal tract by encouraging active transport of calcium through the epithelium of the ileum.⁵ Deficiencies of this vitamin are responsible for serious abnormalities in the mineralization of newly formed bone and result in reduced absorption.

Physical Activity - Weight bearing activity determines the strength, shape, and mass of bone. Activities such as running, jumping, and dancing, as well as those that increase strength, can help bone development. Studies show that absence of exercise will result in a loss of bone mass, especially during long periods of immobilization.

Table 2

Q: Why choose milk as a source of calcium?
A: While calcium can be found in a variety of foods, including vegetables, grains, and dairy products, the 1994 NIH Consensus Conference on Optimal Calcium Intake designated dairy products as the preferred source of calcium.⁶ The NICHD has selected low-fat or fat-free milk as the best source of calcium because it has a high calcium content and the calcium can be easily absorbed by the body. Milk is also a widely available dairy product already a part of many American diets.

In fact, milk products provide about 3/4 of the calcium in the U.S. food supply. While other foods, such as green leafy vegetables, are healthy sources of calcium, it takes 11-14 servings of kale a day to get the same amount of calcium in 3-4 glasses of milk.

In addition to calcium, milk provides other essential nutrients which are important for optimal bone health and human development including: vitamins D, A, and B12; potassium, magnesium, protein, phosphorus, and riboflavin.

Q: How can drinking milk fit in as part of a heart-healthy diet?
A: The NICHD encourages consumption of low-fat or fat-free milk rather than whole milk after two years of age. No matter which type, milk contains about 300 mg of calcium per 8 oz glass. Incorporating low-fat and fat-free milk can be a great complement to a heart-healthy diet while still providing an excellent source of calcium.

Q: At what age can children begin to drink low-fat or fat-free milk?
A: The American Academy of Pediatrics recommends that children drink only breast milk or iron-fortified formula during their first year. Because children under age two should not have fat-restricted diets, children between the ages of one and two may begin to drink whole milk. After age two, they should drink fat-free or low-fat milk.

Q: Aren't some people lactose intolerant and unable to consume milk?
A: Recent controlled studies indicate that many of those who are lactose intolerant may still consume smaller portions of milk products without suffering ill effects. In one study of 30 people who classified themselves as severely lactose-intolerant, 9 were found able to absorb lactose, and the other 21 could tolerate 240 ml per day over a week period.⁷

There are however, specific populations that are genetically lactose intolerant, including about 85 percent of Asian- Americans and 50 percent of African-Americans. Lactose intolerance is least common among people of northern European origin, affecting only about 10% of Caucasians.⁸ It also occurs infrequently in infants and young children. Studies have shown that lactase is high at birth in all infants regardless of race, and wanes in non-Caucasians and other populations without dairy traditionally in their diet by age 5-6.

There are some strategies for people who can not easily digest lactose. Drinking milk in servings of one cup or less and drinking milk with other food can diminish symptoms. Other dairy products, such as cheese or yogurt, can also be easier to digest. In addition, lactose containing foods may also be treated with commercial preparations of lactase.⁹ There are a variety of digestive aids, including drops that can be added to milk, and tablets taken with lactose-containing foods.

Q: Are there risks associated with high calcium intake?
A: The 1997 DRIs set a Tolerable Upper Intake Level (UL) for calcium and other nutrients. The UL is set at the highest level that is unlikely to pose adverse health risks to almost all individuals. For calcium, the UL is 2,500 mg per day for all age groups, including pregnant and lactating women. This level of calcium is far above the AI values recommended for any age group and corresponds to an average of more than 8 glasses of milk a day. This level is likely to be exceeded only by individuals who overuse calcium carbonate antacids. The efficiency of calcium absorption decreases as intake increases, protecting the body from absorbing too much calcium. In addition, there is an association between calcium and oxalate and the formation of kidney stones. However, while certain foods can affect stone formation in susceptible people, researchers do not believe eating any specific food causes stones to form in people not susceptible.¹⁰

Q: Can children be given calcium supplements to augment their calcium intake?
A: Experts believe calcium should be obtained from natural dietary sources whenever possible. The 1994 NIH Consensus Conference Statement on Optimal Calcium Intake indicates that dairy products are the best source of calcium and contain far more utilizable calcium than other foods. However, if calcium cannot be ingested dietarily, calcium supplements can be given to children. For optimal absorption, no more than 500 mg of supplemental calcium should be taken at any one time. Children and young adults, who absorb calcium relatively well, are perhaps best off taking calcium supplements between meals.

Q: Are there any special recommendations for pregnant or lactating adolescents?
A: During pregnancy, approximately 25 to 30 grams of calcium are transferred to the fetus, mostly during the third trimester. Although studies do not show conclusively that higher calcium intake is needed during this time, pregnant adolescents may benefit from a high calcium intake due to their need to support their own bone growth as well as that of the fetus.

Increasing dietary calcium does not prevent the loss of calcium that occurs during lactation, and the calcium lost seems to be regained following weaning. Therefore, the DRIs do not recommend increasing calcium intake in lactating adolescents above normal levels for that age group. However, the 1994 NIH Consensus Conference statement recommends that lactating adolescents and young adults increase their ingestion of calcium to up to 1,500 mg per day.

REFERENCES:

1  USDA Continuing Survey of Food Intakes by Individuals, 1994, Table 3.

2  National Teen Nutrition Research - Final Report. Teenage Research
    Unlimited, June 1996.

3  NIH Consensus Conference Statement: Optimal Calcium Intake, June 6-8,
   1994; 12(4): 3.

4  Lloyd, Tom et al. Calcium supplementation and bone mineral density in
    adolescent girls. JAMA 1993; 270(7): 841-844.

5  Guyton, A.C. Textbook of Medical Physiology, 8th Edition. Philadelphia:
    W.B. Saunders Company: 1996; 786.

6  NIH Consensus Conference Statement: Optimal Calcium Intake, June 6-8,
    1994; 12(4): 24.

7  Suarez, F.L., Savaiano, D., Arbisi, P., & Levitt, M.D. Tolerance to the
    daily ingestion of two cups of milk by individuals claiming lactose
    intolerance. Am J Clin Nutr 1997; 65(5): 1502-6.

8  Dietary Reference Intakes for Calcium, Phosphorus, Magnesium,
    Vitamin D, and Fluoride. Food and Nutrition Board, Institute
    of Medicine. Washington, DC: National Academy Press, 1997: 4-6.

9  Miller, G.D., Jarvis, J.K., McBean, L.D. Handbook of Dairy Foods and
    Nutrition. Boca Raton: CRC Press, 1995: 203-209.

10  Kidney Stones in Adults. National Institute of Diabetes and Digestive
      and Kidney Diseases, National Institutes of Health: April 1994.