Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Page numbers: 282-290
doi (if applicable): 10.3810/psm.2011.02.1850
Summary of article (review article):
Bone is composed of a calcium-containing mineral called hydroxyapatite deposited on a phosphate-containing matrix. Bone mineral density (BMD) is defined as the amount of inorganic salts (ex. calcium and phosphate) exists per volume of bone. High bone density and high peak bone mass (PBM) are correlated with a lower risk of osteoporosis and fractures. Bone development is especially important during adolescence is order to build strong bones throughout one’s entire lifetime.
Low bone density is one of the conditions of the female triad, which describes three ailments common is overly active young females. The other two conditions are amenorrhea (lack of a monthly period) and low energy availability. Female athletes in negative energy balance experience amenorrhea probably due to a disruption in their hypothalamic-pituitary-gonadal axis. This part of the brain is responsible for generating hormones that signal normal body functions. It is thought that this disruption down-regulates many normal functions, including bone synthesis. The interrelationship between these conditions is strong, therefore making treatment of all three conditions important.
The age at which rate of long bone growth and bone mineral deposition peak is about 1.5 yrs earlier for girls than boys and is roughly between 11.5 and 14 years of age. PBM is also dependent on gender and site of the bone. It is thought that bone is close to the individuals’ maximal density for life soon after sexual and skeletal maturity (when the person is “finished growing”). Males typically experience less fractures than females do; this phenomenon is attributed to their larger and thicker size in general (specifically cross sectional area).
Both estrogen and testosterone are thought to play positive, independent roles in bone development. Studies on patients who lack genes to produce endogenous testosterone or estrogen have shown dramatic increases in BMD and healthy bone composition when supplemented with exogenous hormone. Growth hormone, insulin-like growth factors, and energy-sensing hormones (ex. leptin, insulin) also drive bone growth while glucocorticoids (steroid hormones involved in regulating glucose metabolism and inflammation) can inhibit calcium absorption (therefore reduce bone formation) and thyroid hormone can increase rate of bone turnover. Hormone levels are changing during puberty and in general promote increased bone density.
Inadequate energy intake (one of the conditions of the female athlete triad) can further exacerbate low bone density. Sometimes, low energy availability is also associated with disordered eating. Many hormones will respond to both overtraining and lack of available energy and the athlete’s body will designate less energy to building bone. Several reports concur that there is a high number of adolescent athletes that have disordered eating, possibly up to 18.2%.
Because of the positive effects of exercise, athletes in general commonly have a 5-30% greater BMD than non-athletes. A greater effect is seen with athletes that perform weight-bearing activities, specifically on the bones which bear weight. For example, even amenorrheic gymnasts may have normal to higher than average BMD since their bones must endure up to 12 times their own body weight. If healthy athletes with high BMDs maintain their nutritional and physical practices, their fracture risk could be reduced 50-80% at the time and later in life. One study showed that men who had stopped playing a weight-bearing sport up to 6.5 yrs prior had maintained larger bones (by volume) than sedentary subjects.
Bone is constantly being remodeled, and the entire bone-remodeling cycle takes 3-4 mos. Therefore, exercise intervention (at least 4 hrs/wk) is required to change an individual’s bone composition. It has been shown that intervention in children at various stages of development is positive in regard to BMD, and the largest effects were seen in prepubescent children.
One must be cautious, however, since exercise itself can increase risk of injury. Stress fractures (small cracks in the bone due to overuse), for instance, are possible with repeated force, such as distance running. These types of injuries are more common when available energy is low.
Calcium, phosphorus, and protein are all essential nutrients for maintenance of healthy bones. The Institute of Medicine recommends 1300 mg of calcium per day for children and adolescents 9-18 yrs old. About 500 mg of calcium is the maximum amount that can be absorbed at one meal, so sources should be spread out throughout the day. Milk (cow’s and soy), cheese, other dairy products, molasses, beans, quinoa, almonds, and sesame seeds are all good sources of calcium. Vitamin D is necessary for bone formation and deficiencies are common even in the Western world. Cholesterol can be converted into vitamin D with adequate sun exposure, but getting sun exposure is difficult in the winter and in cloudy regions. Fatty fish is a good source of vitamin D, as are fortified foods such as breakfast cereals. Vitamin D supplements are also an option. Athletes in particular, because of their higher macro- and micronutrient need, should eat a healthy, balanced diet complete with fruits, vegetables, lean meats or other protein sources, and whole grains.
Interpretation of findings/Key practice applications:
It is imperative to build strong, dense bones in childhood and adolescence in order to reduce the risk of osteoporosis and fractures later in life. This can be accomplished with regular exercise (especially weight-bearing exercise such as jumping rope or weight lifting) and a healthy diet rich in calcium, vitamin D, and protein. The female athlete triad (inadequate nutrition, low bone density, and a disrupted menstrual cycle) is very dangerous for bone health because all three conditions contribute to further reduction of bone density. Careful monitoring of young athletes and education about its danger can likely encourage healthy practices among female athletes.
This article discusses many different types of bone types and bone tissue but did not adequate distinguish between them or explain the differences in their response to exercise, nutrition, or the female athlete triad.