Lifestyle risk factors for osteoporosis

MedSurg Nursing, Oct, 1997 by Terry N. Thomas

Osteoporosis is a threat to women's health. It leads to hip, wrist, and vertebral fractures in 1.5 million women per year in the United States (Paier, 1996). There are many factors that put women at risk for osteoporosis. Nonmodifiable factors include postmenopausal status, age, and genetics. Modifiable factors include low calcium intake, lack of physical activity, and ingestion of substances that affect bone mineral density, including cigarettes, alcohol, and caffeine. Teaching women how they can modify lifestyle factors to decrease their risk for osteoporosis is an important nursing intervention. Nurses are in a position to facilitate health protection and promotion with their clients. After reviewing the basic physiology behind bone health, four lifestyle risk factors--lack of physical activity, smoking, caffeine ingestion, and alcohol intake--will be discussed.

Bone Health

Bone is continually being restructured. Bones are made or kept strong by keeping a positive balance between bone resorption and bone formation. There are two basic types of bone in the body--cortical and trabecular. Cortical bone constitutes the outer structure of the bone and is less metabolically active. The shafts of the long bones in the body are made mostly of cortical bone (Wardlaw, 1993). Trabecular bone is found in the flat bones such as the vertebrae and pelvis, and at the end of the long bones. It is more active metabolically than cortical bone, but can not be replaced once lost. The trabecular bone forms the internal support structure of the bone (Wardlaw, 1993). Both men and women lose trabecular bone as they age, but women lose much more trabecular bone than men. During the phase of menopause, women have rapid bone loss related to decreasing estrogen levels (Erickson & Jones, 1992).

Bone is constantly being remodeled through a process of resorption and formation. Osteoclasts bind to the bone and cause resorption. Certain substances released by the osteoclastic activity attract osteoblastic cells to the site. These osteoblastic cells replace the osteoclasts and synthesize new bone matrix. This process takes anywhere from 3 to 4 months with 10% of the bone being involved at any one time (Wardlaw, 1993).

Lack of Physical Exercise

Prolonged immobility, such as bed rest, decreases bone mineral density through the loss of bone minerals (Sardana, 1992). Lack of physical activity leads to decreased mechanical (muscle) stress on the bone. This appears to increase bone resorption without a corresponding increase in osteoblastic activity (Wardlaw, 1993). Conversely, athletes who have larger muscle mass than nonathletes tend to have greater bone density, supporting an association between mechanical stress and bone density (Erickson & Jones, 1992). Repetitive mechanical stress seems to be the most beneficial type of activity so weightlifting and weight-bearing exercise, such as walking and running, are examples of exercise which may increase bone mineral density.

It is difficult to ascertain, however, how much and what kind of exercise is most beneficial in preventing osteoporosis. Walker and Holm (1993) point out the difficulty of comparing studies that use different definitions and tools for measuring exercise. They found the largest differences in results when questionnaires and interviews were used to measure exercise. Nevertheless, a recent comprehensive study (Kushi et al., 1997) on physical activity and mortality in postmenopausal women found that both vigorous and moderate physical exercise significantly reduced mortality in this age group. Therefore, some form of weight-bearing exercise is recommended for the overall health of postmenopausal women.

Smoking

Smoking puts women at risk for osteoporosis because smoking decreases serum estrogen. Estrogen most likely has an effect on osteoblasts, which causes an increase in new bone formation (Moore & Noonan, 1996). Loss of estrogen leads to decreased osteoblastic action progressing to an imbalance between resorption and formation. Estrogen also plays a role in the absorption of calcium, an essential nutrient in forming strong bones (Wardlaw & Weese, 1995).

Increased weight is also associated with increased bone density presumably because of increased stress on weight-bearing bones. Smokers tend to have lean body masses (Mazess & Barden, 1991) perhaps because of the interference of smoking with eating. The combination of decreased estrogen and low body weight leads to increased risk for osteoporosis.

It is important to note that estrogen replacement therapy will not protect against osteoporosis as well in women who smoke as opposed to women who do not smoke (Lappe, 1994). Therefore, women who continue to smoke and wish to take estrogen replacement therapy for preventing osteoporosis will need their doses adjusted accordingly.

Alcohol Intake

Alcohol intake increases women's susceptibility to osteoporosis by three different mechanisms. First, excessive alcohol consumption depresses bone formation by decreasing osteoblastic activity (Lappe, 1994). This leads to an imbalance between the resorptive osteoclastic activity and the formative osteoblastic activity progressing to decreased bone mineral density. Excess alcohol intake also leads to interference with proper nutrition, especially calcium and vitamin D intake.