Bone Health

Osteoporosis, a disease of bone fragility, can develop in childhood but may not present until the elder years. While as many as 200 million people worldwide may be osteoporotic, about 9 million fractures occur annually.⁸ By the time this bone deficiency is diagnosed, the best treatment opportunities are usually passed. Therefore, optimizing bone development in childhood is critical to building strong bodies and avoiding the dreadful consequences of osteoporosis later in life.

Peak bone mass is dependent on 5 main factors: sex, race, hormones, nutrition, and physical activity. Sex and race are nonmodifiable, while nutrition, physical activity, and hormones are. Bone mass is generally higher in men.⁸ Before puberty, there is not much difference; but after puberty, boys tend to develop greater bone mass. Additionally, African American girls tend to have higher peak bone mass than Caucasian girls. However, after menopause, all women are at risk for osteoporosis, regardless of race.

Testosterone and estrogen are essential for bone mass development. Bone mass in males is clearly augmented by testosterone surges at puberty, while females with amenorrhea trend toward lower bone density.

Keep in mind that bone is a dynamic structure capable of adapting to its activity, nutrition, and hormone environment and is composed of a matrix of collagen, hydroxyapatite crystals, and proteins. Bone strength increases with the deposition of calcium and phosphate into the collagenous matrix. Proper nutrition is critical to bone development, with calcium being the primary component. Peak bone mass is usually achieved by the end of the second decade of life as bone mineral content increases 40-fold from birth to adulthood.³

While all the first 20 years of life are important in bone development, approximately 40% to 60% of adult bone mass is achieved during adolescence.³ Interestingly, 25% of peak bone mass is acquired during the 2-year span around peak height velocity: 12.5 years of age for girls and 14 years of age for boys.³ Nearly all (90%) peak bone mass will have accrued by the age of 18 years,³ often determining our fracture risk for the rest of our lives.

Vitamin D is critical for bone development because it facilitates the active absorption of calcium. Without vitamin D, only 10% to 15% of calcium is absorbed, passively.³ The calcium requirement for growth increases from 200 mg per day for infants up to 6 months of age, while 14- to 18-year-old teenagers require 1300 mg per day for optimal growth.³ Keeping in mind that an 8-oz serving of milk provides 300 mg of calcium, it is clear that the calcium demand for optimal growth is not easily met. I don’t know of many teenagers who drink 4 glasses of milk per day. For those restricted to vegetable diets and who do not consume milk or other calcium-fortified plant milks, the challenge to obtain enough calcium becomes much more difficult.

Clinicians have known for years that physical activity, particularly impact loading of the skeleton, is critical for bone development. Loading the skeleton by walking, running, or jumping, for instance, provides the stimulus for calcium deposition in bone. Questions regarding physical activity have persisted, including how much loading is adequate and how much is too much. Several publications in this issue of Sports Health address these critical musculoskeletal health issues.

From Spain, Gómez-Bruton et al⁴ examined the association between physical fitness and bone strength and structure in 3- to 5-year-old children. These researchers used the PREFIT test battery in 92 children. Hand grip strength, standing long jump, speed/agility, balance, and cardiorespiratory fitness were assessed. As a parent, I would personally like to see how they coaxed those 3-year-olds to participate! Nevertheless, 3 cluster groups emerged: fit, strong, and unfit. The results clearly suggest that global fitness is a determinant for bone structure and strength.

From Portugal, Henriques-Neto et al⁵ examined physical fitness and bone health in young athletes and nonathletes. With 285 boys and 311 girls studied, speed, strength, agility, and cardiorespiratory tests were performed, as well as quantitative ultrasound to evaluate bone health. The hand grip test and vertical jump were the best indicators of bone health.

Both these research groups, from Spain and Portugal, have provided methods to evaluate the growing skeleton in the time course where deficiencies can still be targeted and hopefully corrected. These studies emphasize the benefits of physical activity from preschool to teenage years.

While the majority of American children probably are not maximizing the potential to strengthen their bones through physical activity, there are also some who overdo this parameter at the opposite end of the spectrum. Unfortunately, it is not easy to know how much physical activity is too much until pathology arises. Physique, exercise history, and genetics (to name a few) all contribute to the exercise tolerance threshold. Knowing the guidelines for safe exercise parameters is very helpful for parents, coaches, teachers, and others. The ability to sense the significance of symptoms and anatomic features that point to bone pathology helps tremendously since every ache and pain does not require a physician’s examination, radiography, or magnetic resonance imaging.

The research by Nye et al⁷ presents an algorithm and clinical prediction rule for bone stress injury. Bone tenderness, history of bone stress injury, pes cavus, and significant increases in walking or running distance are valid for detecting bone stress injury and can help decision makers decipher how much activity is too much. This type of clinical prediction rule can be extremely helpful in determining safe amounts of physical activity.

Another helpful parameter for physical activity comes from a study by Field et al² examining 6831 high school girls in a prospective study. Those who participated more than 8 hours per week in running/jumping/pivoting activities were twice as likely to sustain a fracture.

Last, for those concerned about our environment, especially in urban areas, realize that air pollution does affect the risk of osteoporosis.^1,6,9 The amount of particulate matter in the air appears to be a determining factor. Whether those particles alone are capable of stimulating osteoclast activity is uncertain. However, it does appear that cleaner air would benefit our skeletal structure.

In summary, there are a lot of factors to consider in our efforts to decrease our rate of debilitating osteoporosis and subsequent crippling fractures in the elderly; but no doubt, the best place to start is in the very early age of infancy and youth. We may need more milk commercials at the end of the NFL Superbowl!

Edward M. Wojtys, MD
—Editor-in-Chief