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Osteoporosis Health: A Review for Fitness Professionals
Namju Lee, Ph.D, Doyeon Kim, MS. and Len Kravitz, PhD.

Osteoporosis (OP) is a skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to increased susceptibility to fractures (World Health Organization, 1994). OP afflicts 10 million people and 18 million people have low bone mineral density (BMD) who are at an increased risk of OP in the US today (Brynin, 2002; Notelovitz, 2003). OP-related fractures added 17 billion dollars to health care costs in the US in 2001 (Notelovitz, 2003) and it is responsible for more than 1.5 million fractures per year (Brynin, 2002). OP has become an important health problem, particularly for women and it is the most prevalent metabolic bone disease in older people (Chan & Duque, 2002). This article will review the categories of OP, how OP is measured, non-modifiable risk factors, modifiable risk factors, bone remodeling, exercise and bone health, and the relationship of diet and race to OP.

The Two Categories of Osteoporosis
There are two categories of OP: primary and secondary. Primary OP (also called type I OP) occurs in women within a few years of menopause. It usually involves bone loss because of cessation of ovarian production of estrogens in women. Secondary OP (also called type II OP) is age-related and usually occurs due to: a) endocrine changes such as hypogonadism (a defect of the reproductive system which results in lack of function of the gonads) and hyperparathyroidism (over activity of the parathyroid glands resulting in excess production of parathyroid hormone); b) chronic use of medications such as corticosteroids (a class of steroid hormones that are produced in the adrenal cortex); c) chronic diseases such as liver disease; and d) nutrient deficiencies such as low levels of calcium and vitamin D (Liggett & Reid, 2000). Type II OP affects both men and women but women are more affected because they have a smaller skeletal mass and live longer than men (Mahan & Escott-Stump, 2004).
How is Osteoporosis Measured?
OP can be measured by various techniques. It is widely accepted that BMD measurements provide the basic diagnosis of OP (Geusens et al., 2002). BMD measurements can be obtained by single energy x-ray absorptiometry (SXA), quantitative computed tomography (QCT), peripheral QCT (pQCT), and dual energy x-ray absorptiometry (DXA). SXA is limited to measuring peripheral bones such as the forearm and heel. QCT and pQCT offer the advantage of measuring the volumetric density of bones. However, QCT, mostly used for lumbar spine BMD, is less reliable and requires higher radiation exposure than DXA. DXA, which is capable of density measurements at multiple skeletal sites, can measure the BMD of the lumbar spine, proximal femur, forearm, and total body. DXA is able to scan faster than other devices and reduce x-ray exposure time. DXA also provides good image quality, allowing better visualization of the scanned region (Geusens et al., 2002; Marcus, Feldman, & Kelsey, 2001).

What are the Non-modifiable Risk Factors Associated with Osteoporosis?
A number of studies (Avioli, 2000; Bono & Einhorn, 2003; Liggett & Reid, 2000; Lin & Lane, 2004; New, 2001; Pachucki-Hyde, 2001; Stewart et al., 2002; Walker-Bone et al., 2001) have classified OP risk factors as non-modifiable and modifiable. Non-modifiable OP risk factors, referred to as intrinsic factors, include family history, ethnicity, gender, age, and menopausal status. BMD has been demonstrated to be lower in daughters of osteoporotic mothers than in women without such a family history. Studies have suggested that 70 to 85% of the variance in BMD might be genetic (Walker-Bone et al., 2001; Pachucki-Hyde, 2001). Yet, the remaining 15 to 30% is a meaningful amount of BMD which may be influenced by methods within individual control such as diet and exercise.

Menopause causes decreased levels of circulating estrogens which often results in rapid bone loss. The size of bone is very important to bone strength because a bone’s resistance to loading is related to its diameter (Kohrt et al., 2004). In addition, age at menarche (first menstrual period), premenopausal ammenorrhea (absence or discontinuation or abnormal stoppage of the menstrual periods), and postmenopausal status may negatively impact OP risk in women.

Age is an important determinant of BMD. BMD changes that occur with increasing age are thought to be related to the decline in osteoblastic (bone formation) function (Mahan & Escott-Stomp, 2004). BMD may be lost at a rate of 5% for every 5 years after the age of 65 (Pachucki-Hyde, 2001).

What are the Modifiable Risk Factors Associated with Osteoporosis?
Modifiable OP risk factors, also called extrinsic factors, include physical inactivity, diet, smoking habits, alcohol consumption, body weight, and medication use. In females, cigarette smoking negatively affects bone by decreased production and increased degradation of circulating estrogens and by decreasing dietary calcium absorption (Mahan & Escott-Stomp, 2004; Marcus et al., 2001). Alcohol abuse is believed to be associated with an increased risk of fracture (Lau & Cooper, 1996). Alcohol directly impedes osteoblastic function (this function is to form the bone tissue and minerals that give bone its strength) and may also increase osteoclastic (functions in the breakdown and resorption and/or removal of bone tissue) activities, because excessive alcohol increases urinary calcium loss and reduces absorption of calcium from the intestine (Lau & Cooper, 1996; Marcus et al., 2001).
Heavier people usually have stronger bones while light or underweight people have higher risk for OP. The reason for the greater BMD in heavier persons relates largely to the load (weight) that is constantly borne by the skeleton (Mahan & Escott-Stomp, 2004). Maintaining appropriate body weight is an essential factor of protecting for occurrence of OP. People with thin or small body frames often have higher risk for osteoporotic fracture (Ensrud et al., 1997).

What is Bone Remodeling?
Bone is a dynamic tissue that serves mechanical and metabolic functions. Bone modeling is the process by which bones grow in size and change their longitudinal and cross-sectional dimensions. Modeling is a process that occurs primarily during growth and in response to mechanical loading. Bone modeling allows optimizing shape or size in response to forces. Remodeling is a process that includes bone resorption and formation. Bone resorption is mediated by the action of osteoclasts (a type of bone cell that removes bone tissue by removing the bone's mineralized matrix). Bone formation is a building process that is mediated by the action of osteoblasts (a specialized cell that is responsible for bone formation). Under normal conditions, bone formation and resorption are balanced, which is necessary for overall health of bone. In abnormal states of bone metabolism, remodeling processes become unbalanced, and when resorption exceeds formation there is a net loss of BMD (Bono & Einhorn, 2003). Bone remodeling is the lifelong process of the skeleton by which bone continually repairs itself and adapts to external strains (Mahan & Escott-Stomp, 2004). The purpose of remodeling is to maintain the mechanical integrity of the tissue by replacing fatigue-damaged older bone with new bone. Factors such as mechanical loading, calcium intake, and reproductive hormones regulate remodeling activity (Marcus et al., 2001).
Bone adapts, grows and remodels itself to the mechanical and functional demands that are placed on it, which is referred to as Wolff’s Law (Pearson & Lieberman, 2004). The goal of bone development is to provide a structure or framework for the body that allows efficient locomotion with an appropriate level of bone mass to resist fracture (Dalsky, 1990). Frost (1990) proposed that when the loads applied to the skeleton exceed or fall below upper and lower threshold levels respectively, cellular activity alters to adjust bone mass and strength until strains fall within a defined range. Once adaptation to regularly applied loads has occurred, the stimulus no longer falls outside the threshold and further modeling is not stimulated, unless the load changes.

What is the Exercise and Bone Health Relationship?
Weight-bearing endurance exercises such as running and jumping are likely to benefit bone health (Kohrt et al., 2004; Turner & Robling, 2003). It is more effective to provide a higher intensity stimulus than simply to extend the duration of lower intensity loading activities (Bennell et al., 1997; Marcus, 2001). Exercise such as weight training, in which load is progressively increased, is very effective in improving BMD. Bone-loading exercise at least three times per week for 10-20 minutes is recommended to maintain bone health (Kohrt et al., 2004). Turner and Robling (2003) found that the effectiveness of an exercise protocol increased by as much as 50% if the daily exercise was divided into two short sessions on 5 days per week.

Exercise has two primary ways to improved bone health (Kohrt et al., 2004). The first is to directly influence the skeleton. Exercise increases bone mass and strength. Several cross-sectional studies have demonstrated higher BMD among athletes who engage in weight bearing and impact sports when compared to their non-athlete counterparts (Davee et al., 1990; Stewart & Hannan, 2000), as well as when compared to their non-weight-bearing athlete counterparts (Pettersson et al., 2000; Taaffe et al., 1997).

The other important link between exercise and bone health is to decrease the osteoporotic fracture risk by decreasing fall risk (Pheifer et al., 2004; Schwartz, Nevitt, Brown, & Kelsey, 2005). Greater than 90% of hip fractures and as many as 40-60% of vertebral fractures occur due to a fall (Nevitt et al., 2005; Stevens & Olson, 2000). Improved muscle strength, muscle power, and dynamic balance can be achieved with exercise participation in older adults, and these improvements are likely to reduce fall risk (Pheifer et al., 2004; Stevens & Olson, 2000).
Regular exercise may be an optimal preventive strategy for decreasing OP risk by its positive benefits in the following: a) to increase BMD during growth and improving peak bone mass; b) to increase or maintain BMD in early to middle adulthood; c) to decrease rates of bone loss in older adults; and d) to reduce falls that may lead to fracture. Physical activity and exercise alone may decrease fracture risk by 20 - 45% in older adults (Feskanich, Willett, & Colditz, 2002). Exercise has been shown to improve muscle strength, power, and dynamic balance, thereby reducing the risk of a falls, which directly lead to the fracture (Marcus et al., 2001). It is important to note that exercise may have a different influence on BMD at different body sites and for people of different ages, possibly related to the progression of bone formation and resorption (Stewart et al., 2002). Because of the multi-factorial nature of osteoporosis, a specific exercise recommendation (as is seen with the improvement cardiovascular fitness) is not to be found. However, regular weight-bearing activities that involve doing aerobic exercise with the bones supporting body weight such as in walking, dancing, jogging, and mixed-impact aerobics are highly recommended. For bone loading, the use of free weights, weight machines, resistance bands or water exercises that strengthen the muscles and bones of the body is very important, as is the incorporation of balance exercises and programs such as Tai Chi (to help prevent falls).

What is the Diet and Bone Health Relationship?
Nutrition plays an important role in maintaining bone health. It is beneficial for bone health to have adequate calcium, vitamin D, vitamin K, magnesium, phosphorus, and potassium intake (New, 2001). The most common dietary factors studied with respect to bone health are calcium and vitamin D intake. Optimal calcium intake is the amount a person needs to reach maximum peak bone mass, maintain BMD, and minimize bone loss later in life. Insufficient dietary calcium intake forces hormones such as parathyroid hormone to increase bone resorption to maintain adequate blood levels. In a normal and healthy American diet, dairy products supply about 80% of the daily calcium requirement (Mahan & Escott-Stomp, 2004). Dietary calcium is important for bone health because it is the primary mineral of the skeleton. Vitamin D, instrumental in the absorption of calcium, and calcium supplementation have been shown to significantly reduce fracture rate (McCabe et al., 2004; New, 2001).
Other dietary factors, such as soy protein (Brynin, 2002) and fresh fruit and vegetable intake (New et al., 2000) have been studied more recently as they relate to bone health. Soy protein intake has been found to be important for bone health notably in populations with low calcium intake, such as in Asian cultures (Greendale et al., 2002) and especially among postmenopausal women (Mei et al., 2001). Epidemiological studies (Greendale et al., 2002; Horiuchi, Onouchi, Takahashi, Ito, & Orimo, 2000; Mei et al., 2001) have also found soy intake is beneficial in maintaining or improving BMD in postmenopausal women. Further, at least one study has found that soy protein supplementation in postmenopausal women may be more beneficial to the skeleton of those not taking hormone replacement (Arjmandi, Khalil, Smith, Lucas, Juma, Payton & Wild, 2003). These researchers suggest a link between an increase in insulin-like growth factor (IGF-I) with soy protein intake. IGF-I is known to exert anabolic effects on bone.

A high level of fresh fruit and vegetables in the diet shows a positive relationship to bone health (Brynin, 2002; New et al., 2000). These relationships were independent of confounding factors of body weight, height, smoking, and physical activity (New et al., 2000). Lower consumption of fresh fruits and vegetables may increase OP risk. New and colleagues (2000) found that a high intake of fruit was significantly associated with higher femoral BMD in women aged 45 to 54 years. Theoretically, a higher consumption of fruits and vegetables decreases urinary calcium excretion, by buffering pH changes due to dietary protein.

How is a Person’s Race Related to Bone Health?
Caucasian and Asian races have higher risk for OP and suffer more osteoporotic fractures than African-Americans and Hispanics (Lau & Cooper, 1996; Walker-Bone et al., 2001). Asian races may have higher risk for OP because they have smaller bones (Bhudhikanok et al., 1996), arrive at menarche (first menstruation) at a later age, and reach menopause at an earlier age (Ku et al., 2004) as compared to Caucasians. With the rapid aging of the population in Asia, it is expected that 50% of the world’s hip fractures will occur in Asian women by the year 2050 (Mei et al., 2001). Asian and Asian Americans are at great risk for OP because they have lower BMD, smaller bones, and low levels of PA (Lin & Lane, 2004).

A variety of intrinsic and extrinsic factors contribute to higher OP risk among Asian women. Activity levels of Asians are lower than Caucasians. In fact, 38.6% of Asians meet the recommended levels of lifestyle PA compared with 45.8% of the total US population and approximately 24% are inactive during their leisure time (Centers for Disease Control and Prevention, 2005). McCabe and colleagues (2004) found that African-Americans consumed higher calcium intake than Caucasian Americans and had higher BMD. It is interesting to note that older people (over 65 years) of all races and those with less formal education are less likely to engage in regular PA (CDC, 2005).

Final Thoughts
Osteoporosis is a skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to increased susceptibility to fractures. It has become an important health problem, particularly for women and is the most prevalent metabolic bone disease in older people. Susceptibility to OP appears to involve the interaction of multiple environmental and genetic factors. Factors associated to increased OP risk include physical inactivity, increased age, Asian and Caucasian races, female gender, low body weight or size, family history, menopause or premature loss of menses, some dietary patterns including limited calcium and vitamin D intake, use of cigarettes, excessive alcohol consumption, and prolonged use of certain medications that affect bone metabolism.
The health and fitness professional is encouraged to be an advocate of osteoporosis prevention by promoting regular bone loading and weight bearing aerobic exercise activities for clients. Educating and encouraging students for life long bone health will indeed lead to their improved quality and enjoyment of life.

Author Biographies:
Namju Lee, Ph.D, is now a part time lecturer at Ewha Womans University, Seoul, South Korea. She earned her doctorate degree in exercise and sport science in 2006 at the University of Utah and master ’s degree in sports health in 2001 at Texas Tech University. Her research interests are Women's Triad and exercise nutrition.

Doyeon Kim, MS, is a doctoral student in the exercise science program at the University of New Mexico, Albuquerque (UNMA). He earned his master ’s degree in exercise and sport science in 2005 at the University of Utah and has research interests in the childhood obesity, body composition, sports training adaptation and exercise prescription._
Len Kravitz, Ph.D., is the Program Coordinator of Exercise Science and Researcher at the University of University of New Mexico where he recently won the "Outstanding Teacher of the Year" award. Len was honored with the 1999 Canadian Fitness Professional “International Presenter of the Year” and the 2006 Canadian Fitness Professional “Specialty Presenter of the Year” awards and chosen as the American Council on Exercise 2006 "Fitness Educator of the Year”.


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