Word count: 3357 (before edit)



Word count: 3357 (before edit)

BONE MINERAL DENSITY AND AGING

Patricia C. Fehling and Denise L. Smith

University of Connecticut Health Center, Osteoporosis Center, Farmington CT., Department of Physical Education and Dance, Skidmore College, Saratoga Springs, NY

Bone is a metabolically active tissue. There are distinct phases through the life span where the predominate metabolic activity will change in the skeletal system. The predominant activity of children's bone is that of bone growth and modeling (shaping the bone). After puberty and through approximately age thirty, bone is thought to be in an equilibrium of bone remodeling (a coupled reaction of bone loss followed by bone formation). In the later years there is an age-related loss of bone mass. All of the phases are important in developing and maintaining a healthy skeletal integrity. Measures of bone mass and bone mineral density are used to describe bone health. Bone mass refers to the absolute amount of hydroxyapatite (calcium phosphate crystals in the bone matrix) measured in grams. Bone mineral density (BMD) is defined as the relative value of bone mineral content per measured bone area, expressed as grams per centimeter squared (g/cm2). This paper will discuss the basics of bone physiology and cover the changes that occur in BMD during important phases of the life span. Finally, it will briefly discuss the influences of mechanical (physical activity) and non-mechanical (hormone replacement therapy and calcium) stimuli on the age-related changes in BMD.

Bone Physiology

Bone tissue, also called osseous tissue, is a dynamic, living tissue that is constantly undergoing change. In fact, 10-30% of an adult skeleton and 100% of an infant skeleton is typically replaced each year by the process of bone remodeling. Bone remodeling refers to the continual process of bone breakdown (resorption) and the formation (deposition) of new bone. Bone remodeling plays an important role in regulating blood calcium levels and in replacing old bone with new bone to ensure the integrity of the skeletal system. The shape and mass of the bones depend largely upon the stress placed upon them. The more the bones are stressed (by mechanical loading in the form of physical activity), the more they increase in volume and mass, specifically at the site of mechanical loading.

Bone Tissue

There are two types of bone tissue: cortical and trabecular bone. Cortical bone is densely packed and makes up a majority of the skeleton (approximately 80%). Trabecular bone is more porous and is surrounded by cortical bone. Individual bones are composed of both types of bone tissue, what varies is the relative proportion of trabecular and cortical bone. In general, bones of the axial skeleton (vertebra, ribs) have a much greater percentage of trabecular bone; whereas, bones of the appendicular skeleton (legs, arms) have a greater percentage of cortical bone.

Cortical bone is composed of concentric layers (called lamellae) of matrix that surround widely dispersed cells. The matrix is the intercellular space and is made up of organic and inorganic substances. The functional unit of bone is called an osteon (Haversian System).

Trabecular bone is composed of a network of branching projections or struts (called trabeculae) which form a lattice-like network of interconnecting spaces. Trabecular bone has the same microscopic anatomy as the cortical bone, but has a greater degree of porosity. Trabecular bone has more rapid remodeling due to the large surface area and increased blood supply. It is also in the trabecular bone that the greatest age-related loss in bone mineral density occurs. Therefore, it is logical that most osteoporotic fractures occur in areas which are composed predominately of trabecular bone (i.e., wrist, hip, and spine).

Bone Cells and Remodeling

The cells are the "living" part of bone. The cells represent a small fraction of the total composition of bone, yet they are responsible for the constant remodeling of bone. There are three types of bone cells: osteoclasts, osteoblasts, and osteocytes. Osteoclasts are responsible for bone resorption (breakdown). These cells secrete digestive enzymes and phagocytize the matrix to perform their function of bone resorption. As the bone is degraded, the mineral salts (primarily calcium and phosphate) are dissolved and move into the blood stream. Osteoblasts are responsible for producing the bone matrix, which will become calcified and harden when minerals are deposited. Osteocytes are located within bone and appear to initiate the process of calcification.

The action of the osteoclasts and the osteoblasts are coupled, meaning that they work together to remodel bone. In fact, osteoclasts must first cause bone resorption before the osteoblasts can form new bone. First, the osteoclasts are stimulated and cause the resorption of bone, resulting in cavity. Osteoblasts then appear and deposit bone matrix where the cavity had existed (the matrix is called osteoid until it is calcified). Calcification of the new bone occurs as calcium and phosphate minerals are deposited in the osteoid. The bone then returns to the resting or quiescent phase.

The end result of bone remodeling may result in greater bone mass, the same bone mass, or a reduction in bone mass. Through young adulthood, the amount of bone resorbed is less than the amount of bone formed and thus there is an increase in bone mass. This strengthens the bone and accounts for the increase in bone mineral density that occurs during this period of life. When bone remodeling is in equilibrium, the amount of bone resorbed is equal to the amount of bone formed; thus, bone mineral density remains constant. In the elderly, the amount of bone resorbed is greater than the amount of bone formed, and a decrease in bone mineral density occurs.

Bone Mineral Density Changes Through The Life Span

The major changes associated with puberty, the attainment of peak bone mass, and age-related bone loss are discussed in the following section.

Pubertal Changes

Remodeling of bone provides the means for skeletal growth particularly during developmental years. During childhood through adolescence, the skeleton will go through extensive changes. It is thought that approximately 45% of the skeletal volume is formed during the adolescent growth spurt. The skeleton will proceed through growth spurts that will result in an elongation of the bone, as well as a rapid formation of bone mineral content (BMC). The remodeling sequence has been estimated to be ten times higher in a child than an adult. These appear to be important events in the maturation process of the skeletal system and are closely linked to the gonadal hormones.

Attainment of Peak Bone Mass

The balance between bone resorption and bone formation is in near equilibrium during the early adult years and bone mass remains relatively stable after peak bone mass is attained. The attainment of peak bone mass is thought to occur sometime in the fourth decade of life of cortical bone and possibly earlier in trabecular bone. There remains controversy in the literature as to the exact timing of peak bone mass, which appears to vary depending upon the site measured and the technology used. It has been reported that, at the mid-radial site, peak bone mass occurs late during the second decade of life. At the lumbar site, peak bone mass has been reported anywhere from the second to the fourth decade of life.

Age-Related Bone Loss

Age-related bone loss occurs as a result of increased resorption, followed by the inability of formation to replace these losses. These age-related bone losses are characterized as either the slow or fast bone loss. Slow bone loss is the result of the uncoupling of the processes of bone resorption and formation. Accelerated bone loss is seen in women shortly after the onset of menopause and is characterized by high bone turnover. This increased turnover is thought to be the result of increased of osteoclasts activity after the withdrawal of the protective effect of estrogen.

Cortical bone loss begins at approximately age 40 for both sexes and proceeds at a rate of 0.3-0.5% per year. Accelerated loss occurs at a rate of 2-3% per year, beginning at menopause and lasting for 8 to 10 years, before resuming the slow loss rate. Trabecular bone loss may begin at age 30 to 35 years. It appears that there is a curvilinear decrease in trabecular bone in women that amounts to a 2-4% loss per year. Males experience a 1-2% linear decrease. Riggs and Melton (7) go over this process in a fine and in-depth review.

A summary of the life span changes in BMD is presented in Figure 1 (5). Three general phases can be identified which affect BMD. The first phase is associated with bone growth. This occurs during the first two decades of life and is influenced by puberty in both sexes. Both males and females gain considerable BMD between the ages of 10-20 years. Peak bone mass (represented as 100% BMD) occurs between the ages of 20-40 years, with men achieving a greater absolute BMD. Somewhere after age 40 years, there is a characteristic bone loss in women. The rapid phase of bone loss is associated with menopause. Also represented is the influence of physical activity and non-mechanical influences (estrogen and calcium) on BMD throughout life.

Factors Effecting Bone Mineral Density

The following section discusses several factors that influence BMD.

Physical Activity

Physical activity is necessary to develop and maintain a healthy skeletal system. However, the exact amount and the type of activity that is required is not clear. In the past 10 to 20 years, considerable research attention has focused on the affect of physical activity on bone. In general, these studies suggest that physical activity does have a positive effect on bone density. The skeletal adaptation to exercise, however, is dependent on the type of bone being measured (trabecular versus cortical bone) and the type of activity.

Research clearly indicates that a lack of weight-bearing exercise is detrimental to the skeleton (i.e., results in a loss of bone mineral density). This has been shown in astronauts, patients confined to bed rest or immobilized in a cast. These studies have consistently found that weight-bearing bones are affected to a greater degree and that trabecular bone is lost at a greater rate than cortical bone. In one study, it was reported that astronauts lost an average of 4% of trabecular bone and 1% of cortical bone on a monthly basis.

One approach to studying the effect of increased physical activity on bone density is to compare the dominant limb to the non-dominant limb in sports such as tennis and baseball. These studies report that the dominant arm has greater bone mineral density than the non-dominant arm. This appears to be true for both females and males and across a wide age span.

Another approach to studying the effect of activity on bone is to compare different athletic groups with one another and with control groups. These studies collectively suggest that individuals who are involved in athletics or participate in vigorous fitness training have greater bone mineral density than sedentary controls. Furthermore, those individuals who are involved in weight-bearing or impact-loading sports tend to have higher bone mineral density than those involved in non-weight-bearing or weight-supported activities.

In a longitudinal training study that investigated changes in bone mineral content with training and subsequent detraining, Dalsky and colleagues (3) reported that 18 months of weight-bearing exercise caused a significant increase (6.2%) in lumbar bone mineral content. When subjects discontinued exercise training (or trained fewer than 3 days per week), however, bone mineral content returned toward baseline values. After one year of detraining, bone mineral content was increased only 1.1% above baseline values. These data suggest that the increase in bone mineral that resulted from exercise are lost if exercise is not continued. Bone responds to activity and to inactivity.

Type of Physical Activity

Research data suggests that the type of activity will greatly influence skeletal adaptations. Weight-bearing or impact-loading activities, such as strength training, stairclimbing or volleyball, are more likely to stimulate increases in bone mass than non-weight-bearing or weight-supported activities, such as swimming or cycling. The type of activity that should be chosen depends upon the health and preference of the individual. One major concern when considering exercise for individuals with low bone mineral content is the risk of falling. Activities that carry a high risk of falls or collisions would not be recommended for certain segments of the population, such as the elderly.

In summary, physical activity is necessary for a healthy skeletal system. Research indicates a higher bone mineral density in athletic or highly active groups when compared to their sedentary counterparts. Further research is needed to clarify the exercise prescription that is most appropriate for developing and maintaining bone health. In the meantime, there is sufficient data to act upon - particularly in recommending physical activity as an important component in achieving and maintaining bone health.

Hormonal Control of Bone Remodeling

Bone remodeling reflects the interrelationship between the structural and metabolic functions of bone. Although calcium is necessary to provide the structural integrity of bone, it is also essential to the proper functioning of the heart, skeletal muscles and neural tissue. Only about one gram of calcium ( ................
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