Better Bones

Skeletal mass can be viewed as a retirement account: The more bone mass a woman has in her account, the longer it will last into her later years. Building bone mass starts in childhood and is particularly critical during adolescence, when nearly half of a woman’s adult bone mass is gained.1 The skeleton is a dynamic organ with ongoing processes of bone formation and resorption. During the prepubertal and adolscent years, bone formation dominates over resorption with resulting net bone gain. This time defines the window of opportunity for a woman to maximize her skeletal reserves that will last her a lifetime; peak bone mass is reached in young adulthood and the cash in this retirement account then sustains the individual’s skeleton forlife.1

Genetics, dietary choices, lifestyle, and reproductive physiology all affect net bone gain during young adulthood. The importance of a well-balanced diet and regular physical activity in helping a young girl achieve and optimize her peak bone mass cannot be too highly stressed. While eating well and exercising regularly are important to bone health throughout life, these lifestyle choices are especially critical during this intense growth phase, when accumulation of bone mass is greatest.

Nutrition: Calcium, Vitamin D, and Protein
These three nutrients maximize bone mass accrual during childhood and adolescence.

Recommended Calcium Intakes in a 9-18 y is 1300 mg/d
In adults 19-50 y it is 1000 mg/d and in 50 y and older it is 1200 mg/d

The role of calcium in building and maintaining strong bones is well established. However, most people, including children, do not get enough of this essential nutrient in their diet.

Dairy products are an excellent source of calcium and are the most widely consumed source of calcium among Americans. Milk is a particularly good way to get enough calcium because of its high bioavailability. For example, although spinach is a good source of calcium, the body absorbs only about one-tenth the amount of calcium from spinach as it does from milk. Milk also provides other nutrients important for bone health, including vitamins A, B12, and D, potassium, magnesium, phosphorous, riboflavin, and protein. Because whole milk dairy products are high in fat, low-fat alternatives are best for girls choose dairy products as the mainstay of their calcium intake. There are also several nondairy sources of calcium for those who are lactose intolerant or who prefer nonmilk products; supplements can be used also

Nondairy Sources of Calcium
Some vegetables, including broccoli, kale, cabbage, and bok choy

Dark leafy greens, such as collard greens, turnip greens, and beet greens

Canned fish with the bones, such as salmon and sardines

Beans and legumes, including soybeans, tofu, baked beans, and black beans

Fortified foods, such as ready-to-eat cereal, waffles, orange juice, and instant oatmeal

Vitamin D Vitamin D is essential because the body needs it to absorb dietary calcium. Vitamin D is derived from exposure to the sun, from the diet, and from supplements. As little as 15 minutes in the sun without sunscreen a few times a week is all that is needed for most fair-skinned people to meet the recommended daily intake of 400 to 600 IU/d of vitamin D. Dietary sources of vitamin D are few and include eggs, fatty fish, and liver. Foods fortified with vitamin D include milk, some fruit juices, and some prepackaged cereals and are the primary means of vitamin D intake in the American diet.

According to a recent study, 7 out of 10 children in the United States are not getting enough vitamin D.7 Girls; children who are African-American, Mexican-American, or obese; those who drank milk less than once a week; and those who spend more than 4 h/d in front of “screens” (TV, video games, computers) are at highest risk for low vitamin D levels.

Clinicians should also be aware that, in 2008, the American Academy of Pediatrics revised the recommended minimum daily intake of vitamin D for infants, children, and adolescents, increasing it from 200 IU/d to 400 IU/d.

Protein Protein is a major component of the bone matrix and a scaffold that determines subsequent bone mineralization and hence bone density. Interestingly, a recent study found the benefits of protein on bone growth were dependent on getting enough calcium in the diet.Another recent study found that, during a 12-year period (from preschool through their teen years), children whose diets included two or more servings of dairy products per day had much stronger bones than children who did not. However, the strongest bones belonged to those children who had two or more servings of dairy as well as four or more servings of meat or nondairy protein each day.

The Institutes of Medicine has set the recommended daily intake of protein for girls according to age as follows:
Ages 9 to13: 34 g/d
Ages 14 to18: 46 g/d
Beans, nuts, and whole grains are good sources of vegetable protein. To avoid overconsumption of fats, the best sources of animal protein are fish and poultry. Although red meat, such as beef, pork, or lamb, is a good source of protein too, choosing the leanest cuts minimizes fat intake.

Physical Exercise: A Healthy Habit From the Start
While weight bearing activity acts as a stimulus for bone formation, additional benefits of an active lifestyle include improved muscle coordination, which reduces the likelihood for falls. Outdoor physical activity holds the additional benefit of helping optimize bodily vitamin D stores through sun exposure. Studies have found that most adolescents who participate in regular physical exercise continue to be more physically active as young adults than their inactive peers. They also maintain their bone strength, showing the benefits of exercise to bone health in childhood carry over into young adulthood.13

From a bone health perspective, the most beneficial physical activity is weight-bearing exercise. Activity that uses the feet and legs to support the body’s weight, including walking, running, jumping rope, climbing stairs, and just about any team sport, is considered weight bearing.The benefits on bone health are obvious after only a short time, and they last into young adulthood.

Ideally, children and adolescents should get at least 60 minutes of physical activity, preferably every day.

References:
Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food, and Nutrition Board, Institute of Medicine. Calcium. The National Academies Press;
The Food and Nutrition Board of the National Academy of Sciences
U.S. Dept of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 22. 2009; and Brown SE. Nutrition & bone health. Better Bones Web site.

Modern Medicine

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