A recently published review article by seven leading osteoporosis experts offered a critical overview of known factors that impact the achievement of skeletal mass. In conducting this thorough review, the authors’ intent was to provide a strong basis for the development of both public policy and public education programs concerning the development of peak bone mass. This issue of NIH ORBD~NRC NEWS provides a synopsis of key points of this important article.*
Bone mass (density) is an essential and measurable component of bone strength. The significance of attaining optimal bone mass cannot be underestimated: higher peak bone mass early in life is associated with greater fracture protection later in life. While genetic factors account for about three-fourths of the variance in peak bone mass, select environmental factors have been found to either positively or negatively impact the acquisition of bone.
Calcium and Peak Bone Mass
Several nutrients are believed to play a role in the acquisition of bone, such as protein, vitamins C, D and K, phosphorous and calcium. While most North American diets tend to supply an adequate amount of most of these nutrients, calcium intakes are generally far below current recommendations. Experts estimate that variations in calcium nutrition in youth may account for a 5-10% difference in peak adult bone mass, and a 25-50% difference in hip fracture risk in later life.
Surveys have found that adolescent girls are less likely than adolescent boys to achieve the recommended dietary intakes for calcium. Contrary to what is observed in boys, calcium intake in girls begins to decline around the time of puberty. Data from the National Health and Nutrition Examination Survey (NHANES III, 1988-94) demonstrate that approximately 90% of girls between the ages of 9 and 18 fail to consume the recommended calcium intake. Declining calcium intakes in teenage girls may be due to an avoidance of milk, a beverage many girls mistakenly believe causes weight gain. However, studies have found that girls who drink milk experience less weight gain and have lower percent body fat than those teens who take calcium supplements.
Vitamin D, which promotes the transport of calcium across the intestinal mucosa, is essential for calcium absorption. Vitamin D is important during adolescence, given the high calcium demands of the teenage years. Due to the routine supplementation of vitamin D in infancy and childhood, and the fortification of cow’s milk with this vitamin, vitamin D deficiency is rarely observed in children in North America.
The Impact of Other Nutrients
While phosphorous is an important nutrient for bone health, it may have a negative effect on calcium status. High phosphate intakes, particularly in the presence of low calcium levels, have been linked to bone loss in laboratory animals. Preliminary research suggests that excessive consumption of phosphate-containing soft drinks may contribute to low peak bone mass, though the mechanism for this effect is not fully understood. It is possible that these beverages are detrimental to bone health because they are ingested in place of milk.
Dietary sodium exerts an important influence on calcium balance and bone acquisition. As is the case in adults, sodium is the primary determinant of urinary calcium excretion in adolescent girls. Excessive protein intake triggers urinary calcium losses in adults, but may not be as detrimental to the skeletal health of girls. What seems to be more important in younger women than dietary calcium or protein alone is the calcium:protein ratio of the diet.
Exercise and the Skeleton
The skeletal benefits of physical activity in youth are well-established. High impact/high loading exercises (such as gymnastics) tend to produce the greatest benefit to bone. On the other hand, excessive exercise in youth can cause injury to growth plates and can actually trigger bone loss in girls if the menstrual cycle is interrupted.
The precise exercise prescription for optimizing peak bone mass has not been established. Exercises like walking, running and dancing are those activities most commonly recommended. Evidence suggests that 20-30 minutes of moderate to intense physical activity several times per week produces a favorable skeletal benefit.
The Effects of Tobacco and Alcohol
Tobacco use has been linked to low bone density in adolescents and is associated with other unhealthy behaviors such as alcohol use and sedentary lifestyle. The Centers for Disease Control and Prevention (CDC) reports that in 1997 about 36% of high school students admitted to current cigarette smoking, compared with about 25% of students in 1991. CDC also found that more than two-thirds of ninth grade students in 1997 reported trying cigarette smoking. The negative impact that smoking has on peak bone mass is further exacerbated by the fact that those who begin smoking at a younger age are more likely to be heavier smokers later in life. These older smokers are at further risk for bone loss and even fracture.
The impact of alcohol on the achievement of peak bone mass is not clear. The effects of alcohol on bone have been more extensively studied in adults, where high consumption of these beverages is associated with declining bone density. Experts assume that high intakes of alcohol in youth have a similar adverse effect on skeletal health.
Other Lifestyle Issues
Oral contraceptives are widely used by younger women. Studies exploring the impact of these agents on bone density suggest that combination estrogen-progestin pills do not hinder gains in peak bone mass. (These agents may actually enhance the accrual of bone at cortical-rich sites in the skeleton.) Conversely, the progestational contraceptive medroxyprogesterone acetate has been shown to decrease estrogen levels and negatively impact bone density in young women.
Approximately one percent of adolescent girls develop anorexia nervosa, an eating disorder characterized by an irrational fear of weight gain. Several studies confirm that girls and women with anorexia experience extreme deficits in bone mineral density. The younger the age at which the eating disorder begins, the greater the toll on the skeleton. Amenorrhea from excessive exercise and restricted caloric intake can also precipitate substantial reductions in bone mineral density. Clearly, body weight, estrogen status, exercise, and nutrition are the significant determinants of peak bone mass.
Developing a National Agenda for Optimizing Peak Bone Mass
While many significant research questions remain unanswered, researchers are calling for a concerted national effort to meet the following objectives:
- To improve calcium intakes and enhance nutritional status of pre-teens, teens and young adults;
- To increase physical activity levels of youth and
- To ensure adequate intakes of vitamin D throughout the period of skeletal growth.
The authors suggest the following strategies to achieve these goals:
- Develop curricula in middle and high schools stressing the importance of nutrition and exercise for bone health.
- Develop additional bone health and nutrition badges for Boy and Girl Scouts and develop 4-H lesson plans on the same.
- Restrict access to carbonated beverages in schools.
- Maintain school physical education programs.
- Incorporate nutrition and exercise objectives into pediatric and family practitioner training programs and include questions on these topics on board examinations.
- Establish incentives for appropriate calcium and vitamin D fortification of healthy foods.
- Encourage the dairy industry to develop attractive packaging of dairy beverages and more flavored and shelf-stable low fat milk beverages.
- Encourage the dairy industry to fortify yogurt products.
- Encourage milk drinking and yogurt consumption in television sit-coms and videos.
- Develop educational programs for parents concerning their ability to serve as role models for their children by consuming low-fat dairy products and other calcium-rich foods.
- Intensify efforts to reduce smoking and alcohol consumption by adolescents.
*R. P. Heaney, S. Abrams, B. Dawson-Hughes, A. Looker, R. Marcus, V. Matkovic and C. Weaver. Osteoporosis International. Volume 11, pp. 985-1009.