++
Bone atrophy is a problem not only for astronauts and bedridden
patients but also for a growing number of senior citizens and female
athletes. Osteoporosis is found in
most elderly individuals, with earlier onset in women, and is becoming
increasingly prevalent with the increasing mean age of the population.
The condition begins as osteopenia,
reduced bone mass without the presence of a fracture, but often
progresses to osteoporosis, a condition in which bone mineral mass
and strength are so severely compromised that daily activities can
cause bone pain and fracturing (28).
+++
Postmenopausal
and Age-Associated Osteoporosis
++
The majority of those affected by osteoporosis are postmenopausal
and elderly women, although elderly men are also susceptible, with
more than half of all women and about one-third of men developing
fractures related to osteoporosis (83). Although it was once regarded
as primarily a health concern for women, with the increasing age
of the population, osteoporosis is now also emerging as a serious
health-related concern for men (24, 45, 48). Risk factors for osteoporosis include
being female, white or Asian ethnicity, older age, small stature
or frame size, and family history of osteoporosis (71).
++
Type I osteoporosis, or postmenopausal osteoporosis, affects
approximately 40% of women after age 50 (56). The first
osteoporotic fractures usually begin to occur about 15 years postmenopause, with
women suffering approximately three times as many femoral neck fractures,
three times as many vertebral fractures, and six times as many wrist
fractures as men of the same age (56).
++
This discrepancy occurs partially because men reach a higher
peak of bone mass and strength than women in early adulthood, and
partially because of a greater prevalence of disconnections in the trabecular
network among postmenopausal women than among men (67).
++
Type II osteoporosis, or age-associated osteoporosis, affects
most women and also affects men after age 70 (49). After age 60,
about 90% of all fractures in both men and women are osteoporosis-related,
and these fractures are one of the leading causes of death in the
elderly population (79).
++
Although the radius and ulna, femoral neck, and spine are all
common sites of osteoporotic fractures, the most common symptom
of osteoporosis is back pain derived from fractures of the weakened
trabecular bone of the vertebral bodies. Crush fractures of the
lumbar vertebrae resulting from compressive loads created by weight
bearing during activities of daily living frequently cause reduction
of body height. Because most body weight is anterior to the spine,
the resulting fractures often leave the vertebral bodies wedge-shaped,
accentuating thoracic kyphosis (see Chapter 9). This disabling deformity
is known as dowager’s hump. Vertebral
compression fractures are extremely painful and debilitating, and
affect physical, functional, and psychosocial aspects of the person’s
life (83). As spinal height is lost, there is added discomfort from
the rib cage pressing on the pelvis. An estimated 26% of
women age 50 and over have vertebral compression fractures (81).
Alarmingly, the incidence of these vertebral fractures has increased
three- to fourfold in women and more than fourfold in men over the
past 30 years, even when figures are adjusted for age (67). This
underscores the increasing prevalence of lifestyle factors that
negatively affect bone health.
++
As the skeleton ages in men, there is an increase in vertebral
diameter that serves to reduce compressive stress during weight
bearing (67). Thus, although osteoporotic changes may be taking place,
the structural strength of the vertebrae is not reduced. Why the
same compensatory change does not occur in women is unknown.
++
The desire to excel at competitive sports causes some young female
athletes to strive to achieve an undesirably low body weight. This
dangerous practice commonly involves a combination of disordered
eating, amenorrhea, and osteoporosis,
a combination that has come to be known as the “female
athlete triad” (86). This condition often goes unrecognized,
but because the triad can result in negative consequences ranging
from irreversible bone loss to death, friends, parents, coaches,
and physicians need to be alert to the signs and symptoms (39).
++
As many as 62% of female athletes in certain sports
display disordered eating behaviors, with those participating in
endurance or artistic sports such as gymnastics and figure skating
most likely to be involved (86). Prolonged disordered eating can
lead to anorexia nervosa or bulimia nervosa, illnesses that affect
1–10% of all adolescent and college-age women
(34). Symptoms of anorexia nervosa in girls and women include body
weight 15% or more below minimal normal weight for age
and height, an intense fear of gaining weight, a disturbed body
image, and amenorrhea. Symptoms of bulimia nervosa are a minimum
of two eating binges a week for at least three months, a feeling
of lack of control during binges, regular use of self-induced vomiting,
laxatives, diuretics, strict dieting, or exercise to prevent weight
gain, and excessive concern with body image and weight (34). Disordered
eating behavior has been found to be strongly associated with both
menstrual irregularity and low bone mineral density (15).
++
The relationship between disordered eating and amenorrhea appears
to be related to a decrease in hypothalamus secretion of gonadotrophin-releasing
hormone, which in turn decreases the secretion of luteinizing hormone
and follicle-stimulating hormone, with subsequent shutting down
of stimulation of the ovary (106). A reported 2–5% of
premenopausal women in the United States have amenorrhea, but the
prevalence in female athletes is higher. Studies of competitive
women athletes in different sports indicate that 3.4–66% have
either primary amenorrhea, with menarche delayed beyond 16 years
of age, or secondary amenorrhea, the absence of three to six consecutive menstrual
cycles (86).
++
The link between cessation of menses and osteoporosis is a low
level of endogenous estrogen (7). Energy deficiency resulting from
disordered eating is also likely to independently contribute to altered
bone metabolism and reduced bone density (22). Although the incidence
of osteoporosis among female athletes is unknown, the consequences
of this disorder in young women are potentially tragic. Among one
group of over 200 premenopausal female runners, those with amenorrhea
had 10% less lumbar bone density than those with normal
menses (38). In other research, young women with anorexia nervosa
were found to have marked trabecular and cortical bone loss on the
order of 4–10% per year (65). This pattern is
of particular concern for adolescent athletes, because roughly 50% of
bone mineralization and 15% of adult height are normally
established during the teenage years (4, 107). Not surprisingly,
amenorrheic premenopausal female athletes have a high rate of stress fractures, with more fractures related to later onset of menarche
(107). Moreover, the loss of bone that occurs may be irreversible,
and osteoporotic wedge fractures can ruin posture for life (86).
++
+++
Preventing and
Treating Osteoporosis
++
Osteoporosis is neither a disease with acute onset nor an inevitable
accompaniment of aging, but is the result of a lifetime of habits
that are erosive to the skeletal system (10). Early detection of low
bone mineral density is advantageous, because once osteoporotic
fractures begin to occur, there has been irreversible loss of trabecular
structure (83). Although proper diet, hormone levels, and exercise
can work to increase bone mass at any stage in life, evidence suggests
that it is easier to prevent osteoporosis than it is to treat it.
The single most important factor for preventing or prolonging the
onset of osteoporosis is the optimization of peak bone mass during
childhood and adolescence (6, 9, 20, 36, 50, 100). Researchers hypothesize
that weight-bearing exercise is particularly crucial during the
prepubertal years, because the presence of high levels of growth
hormone may act with exercise in a synergistic fashion to increase
bone density (6, 36, 50, 100). Activities involving osteogenic impact forces, such as jumping, have been shown to be effective in increasing
bone mass in children (29).
++
Weight-bearing physical activity is necessary for maintaining
skeletal integrity in both humans and animals. Importantly, studies
show that a regular program of weight-bearing exercise, such as walking,
can increase bone health and strength even among individuals with
osteoporosis (5).
++
The American College of Sports Medicine pronouncement on “Physical
Activity and Bone Health” (53) makes recommendations relative
to the role of exercise in preventing and treating osteoporosis.
Children and adolescents should engage in impact activities such
as gymnastics, plyometrics, jumping, and moderate-intensity resistance
training (< 60% of 1 RM), with other
sports that involve running and jumping likely to also be beneficial. The
recommended frequency and duration is at least three times per week
for at least 10–20 minutes, with two or more sessions per
day possibly being more effective. In order to maintain bone mass,
adults should participate in weight-bearing endurance activities
such as tennis, stair climbing, and jogging; activities that involve
jumping, such as volleyball and basketball; and resistance exercise.
Intensity should be moderate to high in terms of bone-loading forces,
and weight-bearing endurance activities should be done 3–5
times per week and resistance exercise 2–3 times per week.
Jumping in place, with 50–100 jumps done in sets of 10
with a frequency of 3–5 times per week, is also recommended
for maintenance of bone mass (110).
++
Research on animals, however, has shown that the positive effect
of jumping on bone formation saturates after about 40 jumps, with
100 jumps resulting in no greater stimulus for bone formation than
40 (101). A related recent research finding is that the insertion
of a rest interval between impacts greatly stimulates bone formation,
possibly by enhancing the flow of fluid within the bone matrix and
the related stimulation of osteocytes (35). In fact, it has been
shown that the insertion of these “rest” periods
between impacts reduces the effects of impact saturation and can double
the effects of mechanical loading on bone building (76). In practical
terms, incorporating a 10- to 15-second pause between hops into
the childhood game of hopscotch could be much more effective in
building skeletal mass than hopping without the pause.
++
Although regular physical exercise promotes bone mineralization
and bone health, it is clear that hormonal factors also exert a
powerful influence, given the prevalence of osteoporosis in amenorrheic
female athletes. High bone mineral density has been associated with
early menarche, late menopause, and a long reproductive period,
all of which support the effectiveness of estrogen in maintaining
bone (30). Conversely, low levels of estrogen in females and low
levels of testosterone in males promote bone loss in both children
and adults. Estrogen deficiency potentially damages bone by diminishing
the efficiency with which calcium is absorbed in the body and facilitating
the resorption of bone by osteoclasts (88). Bone is also more responsive
to mechanical loading when the number of estrogen receptors in bone
cells is high, as occurs during adolescence, and less responsive
when the number is low, as occurs during amenorrhea or postmenopausally
(57). Estrogen replacement therapy acts to maintain bone density
by reducing bone turnover, with high doses actually stimulating
osteoblast activity and formation of new bone (105). Because of
its effectiveness and relatively low cost, estrogen replacement
therapy is considered the first-line pharmacologic therapy for preventing
osteoporosis in postmenopausal and amenorrheic women (1). Although
the mechanism by which testosterone influences bone mass is not
understood, testosterone replacement therapy also stimulates bone
building (88).
++
Increased dietary calcium intake exerts a positive influence
on bone mass for women with a dietary deficiency, with the amount
of calcium absorbed influenced positively by calcitriol (the active
form of vitamin D) and negatively by dietary fiber (76, 90). Although
adequate dietary calcium is particularly important during the teenage
years, unfortunately the median American girl falls below the recommended
daily intake of 1200 mg per day by age 11 (20). A modified diet
or calcium supplementation can be critical for the development of
peak bone mass among adolescent females at a dietary deficiency.
The role of vitamin D in enabling absorption of calcium by bone
is also important, with over half the women receiving treatment
for low bone density in North America having a vitamin D deficiency
(41). Clinicians are now recognizing that a predisposition for osteoporosis
can begin in childhood and adolescence when a poor diet interferes
with bone mass development (12).
++
Other lifestyle factors also affect bone mineralization. Known
risk factors for developing osteoporosis include physical inactivity;
weight loss or excessive thinness; tobacco smoking; deficiencies
in estrogen, calcium, and vitamin D; and excessive consumption of
protein and caffeine (21, 78, 89, 102, 111). A study of female twins,
one of whom smoked more heavily than the other, showed that women
who smoke one pack of cigarettes a day through adulthood will have
a reduction in bone density of 5–10% by the time
of menopause, which is sufficient to increase the risk of fracture
(40). Although caffeine consumption may negatively affect bone mineral
density among postmenopausal women who consume low amounts of dietary
calcium, it has been shown not to affect
bone mineral density among young women (17). Genetic factors also
influence bone mass, but do not appear to be as important as diet
and exercise (26).
++
Research indicates that in the future the use of pharmacologic
agents that stimulate bone formation may enable the prevention and
eventually the elimination of osteoporosis (3, 73). Recent clinical
trials suggest that low doses of growth factors can stimulate osteoblast
recruitment and promote bone formation, and researchers have begun
to identify genetic markers for assessment of fracture risk (64,
77). However, until much more is understood about osteoporosis,
young women in particular are encouraged to maximize peak bone mass
and to minimize its loss by engaging in regular physical activity
and avoiding the lifestyle factors that negatively affect bone health.