Strong bones are absolutely integral to your success as an athlete.
Your skeletal structure provides the strength and rigidity that allows
you to train and compete in your sport. Ask any athlete that has
suffered a stress fracture: When the health and integrity of your bones
is compromised, so too is your ability to perform athletically.
Exercise
puts a positive stress on bone, and just like with muscle, your bones
generally respond by becoming stronger. But some athletes are actually
at high risk for weak bones and stress fractures. This article delves
into the physiology of bones, what makes them strong, what increases
your risk of low bone mass and stress fractures, and what to do to help
ensure that your bones are as strong as possible.
Physiology of BoneSeemingly
hard as rock, bone is actually a living and dynamic tissue that is
constantly undergoing a regenerative process known as remodeling. Your
bones are made up of a protein matrix, which is imbedded with minerals
like calcium and phosphorus that make bones hard. The remodeling
process follows a cycle. In the first phase of the cycle, cells
dissolve away and demineralize bone in a process called resorption.
Following this, other cells remineralize and rebuild bone in a process
called formation. None of this happens overnight. A full remodeling
cycle typically takes months, and at any given time, there are minute
areas within the same bone that are being resorbed, while other areas
are in the formation stage.
Bone Mineral Density, Osteoporosis, and OsteopeniaA
key measure of the health of your bones is how much mineral — primarily
calcium and phosphorus — can be found in them. Called bone mineral
density or bone mass, the greater the density, the stronger your bones,
and the less chance you have of suffering a stress fracture during
exercise or a fracture later in your senior years.
Bone
mineral density is measured with a high-tech scanner. Testing can be
done throughout the body, but the usual sites of emphasis are the hip,
lower spine, and the forearm. These sites are focal points, because
they’re areas where bone fractures frequently occur, especially in
older persons.
Individuals with low levels of bone mineral
density are said to have osteoporosis. This is a condition where there
is too little calcium and phosphorus in bone. As a result, the bones
are weak and at higher risk of fracture. Osteopenia is a milder form of
the same low bone mass condition. Sports osteopenia refers to low bone
mass seen in athletes.
Exercise To Build or Maintain BoneThe
strain of bearing your body weight and the load from the mechanical
pull that occurs with muscle contraction during exercise stimulates the
process of bone remodeling. About one-fourth of the total mineral
deposited in bone occurs during the roughly 2-year period that
encompasses puberty, and about 95% of your peak bone mass is achieved
by the end of your teenage years. Therefore, this is an important
window of opportunity for beefing up your bones. Studies suggest that
higher-impact, weight-bearing sports and activities prior to and during
puberty seem to be most effective at building stronger bones.
Activities that involve jumping are particularly useful. Thus, a key
strategy for helping to ensure a lifetime of strong bones is to
maximize bone mineral density during the adolescent growth spurt and in
early adulthood by encouraging regular participation in a variety of
physical activities and sports.
The peak in your bone mass
typically occurs by the third decade of life. For awhile thereafter,
the rate of bone resorption and formation is pretty stable. But in your
40s, resorption begins to outpace the rate of formation and you start
to experience a net loss in bone. The average age for menopause is
about 50 years, and this marks a time when bone loss increases very
rapidly in women. The speedier rate of bone loss is tied to a decrease
in the circulating concentration of the hormone estrogen. Estrogen
inhibits the activity of cells that breakdown bone. However, with less
estrogen available, these bone-degrading cells aren’t inhibited.
Consequently, the rate of bone resorption accelerates, and bone
formation can’t keep up. The end result is often a sharp drop in bone
mass during the menopausal years.
Fortunately, exercise seems
to have a positive effect on bone mass throughout the adult years. Most
studies of the relationship between exercise and bone mineral density
have been conducted in women because they have a greater risk of
suffering a bone fracture with age in comparison to men. The studies
suggest that whether you walk, jog, or run, these weight-bearing
exercises tend to have a positive impact on bone mass. And this seems
to be true for women before and after menopause. Progressive resistance
training using lifts that load the hip and back may be even more
effective for building bone mineral density in premenopausal and
postmenopausal women. In young women, progressive resistance training
and running both produced an increased bone mineral density in the
lower back. And although men haven’t been adequately tested to date,
similar beneficial effects of exercise are anticipated.
The
key message from these studies is that weight-bearing exercise and
progressive resistance training can help to protect the health and
strength of your bones, regardless of your age and gender. For adults,
the American College of Sports Medicine recommends a combination of
weight-bearing exercise virtually every day, along with progressive
resistance exercise 2–3 times per week.
Exercise: A Double-Edged SwordSo
the good news is that all the exercise you’re doing can stimulate the
building of stronger bones. The bad news is that certain athletes have
low bone mass or sports osteopenia. And unfortunately, this is all too
commonly diagnosed in female athletes. As a consequence, they are at
high risk for stress fractures during exercise. A stress fracture is a
partial or complete break in the bone. It’s caused by the bone’s
inability to withstand repeated stress of a non-violent nature, such as
running. Stress fractures occur because microscopic damage in the bone
accumulates and is not adequately repaired by the remodeling process.
An increase in the load on a bone can cause a stress fracture. In
addition, factors that interfere with bone remodeling and repair, or
those that decrease bone strength, are culprits.
The question
is, if exercise is such a positive stimulus for making bones stronger,
why do some athletes have low bone mineral density and weak bones?
Calories Are CrucialIt
turns out that calorie intake is a critical factor in how strong your
bones are. Some athletes, particularly women involved in endurance
sports such as running, or sports where being lean is considered ideal,
such as dancing and gymnastics, restrict their intake of calories while
still training and competing at a high level.
The body adapts
to the calorie shortfall, and these athletes may have very stable body
weights, despite consuming a low level of calories. But the physiologic
cost is high. The precious calories consumed end up preferentially used
to meet the daily energy demands of training and competing.
Unfortunately, this means there aren’t enough calories left over to
fully support other normal physiologic functions. One casualty of
consuming too few calories is reproductive function. Often, these
athletes stop ovulating and stop having their normal monthly menstrual
periods. While at first glance that may seem like a welcome
convenience, the effect on bone is devastating.
Menstrual
periods stop because hormones that are involved in reproduction, such
as estrogen, are in short supply when the body is lacking in calories.
However, just as in menopause, when you take away the inhibiting
effects of estrogen on cells that break down bone, suddenly bone
resorption far outpaces bone formation. As the number of missed
menstrual cycles accumulates, bone mineral density continues to
decline, leading to weaker bones. In fact, physically active women with
menstrual irregularities have 2–4 times the risk of stress fractures
than women with regular periods. Also, women who suffer from stress
fractures are more likely to suffer debilitating and painful
osteoporotic fractures later in life. Moreover, the current thinking is
that the loss of bone mass due to low calorie consumption leading to
missed menstrual cycles can’t be reversed later. So it’s a problem that
needs to be identified quickly, and resolved sooner, rather than later.
Amenorrhea: All Too Common in Female AthletesThe
medical term for the cessation of menstrual periods in women of
reproductive age is amenorrhea, and unfortunately it is a condition
that seems to be disturbingly common in certain female athletes. While
the condition occurs in about 2–5% of women of reproductive age in the
general population, in small studies among athletes, amenorrhea has
been found in 69% of dancers and 65% of long-distance runners! Thus,
the data suggest that certain female athletes are at very high risk for
a condition that is silently but inexorably undermining the health of
their bones.
Ensure That Your Bones Remain HealthyAwareness
and prevention are critical to protecting the health of your bones. If
you’re a woman engaged in a sport where it’s a competitive advantage to
be lean, it’s important to be aware that you are at risk for consuming
too few calories to support normal physiologic function. Remember, you
may be able to compete successfully and your weight might even be
stable, but that doesn’t mean you have enough calories available to
support other important physiological functions needed for long-term
health.
Your calorie shortage may be inadvertent. After all, as
an athlete, your primary focus is probably on consuming enough carbs
each day to replenish depleted glycogen stores so that you can keep up
with the demands of training and competing. And of course you pay
attention to calories, because no one wants to lug around more body
weight than is necessary. But if your periods have become irregular or
have stopped altogether, it is important to understand that you’ve
crossed an unhealthy threshold, and that your bones are suffering the
consequences with each period missed.
Fortunately, normal
menstruation and reproduction can be restored by increasing your
available calories, and this can reestablish the normal bone remodeling
process. So the first order of business is to increase the availability
of calories. Do this by either upping your intake of calories,
decreasing your exercise, or a combination of both. Whichever approach
you take to making more calories available to support normal
physiologic function, stick with it until your regular periods resume
and they continue while you are training and competing.
In
addition to freeing up calories, make sure that you’re supplying the
other key nutrients needed to support optimal bone remodeling,
including calcium, vitamin D, and protein. According to the American
College of Sports Medicine, adequate daily amounts of these
bone-building nutrients include 1,000–1,300 mg calcium and 400–800 IU
of vitamin D. Good sources of both calcium and vitamin D are dairy
products. For example, an 8-oz glass of milk provides about 300 mg
calcium and 100 IU vitamin D. A 6-oz tub of yogurt provides about 300
mg of calcium and 80 IU vitamin D. Other calcium-rich dairy products
include cheeses, cottage cheese, frozen yogurt, and ice cream. If
you’re looking for a non-dairy source of calcium, tofu provides about
150 mg calcium in a 3-oz serving. Also, if your intake of dairy foods
is restricted, supplements of calcium and vitamin D may be necessary to
consistently achieve optimal intakes for bone health.
The
daily recommendation for protein to support strong bones is 0.5–0.7
grams per lb (1.2–1.6 grams per kg) body weight. This equates to about
63–88 grams of protein daily for a 125-pound (57-kg) athlete. Most
athletes consume this amount of protein, although vegetarians may need
to be extra diligent in ensuring that their protein intake is adequate.
Finally, other nutrients important in the bone remodeling process
include vitamin C, vitamin K, zinc, copper, and manganese. Thus,
consuming a variety of foods will help to ensure an adequate supply of
the full range of nutrients needed for healthy bones.
REFERENCES
Nattiv A, Loucks AB, Manore MM, Sanborn CF, Sundgot-Borgen J, Warren MP. American College of Sports Medicine Position Stand. The Female Athlete Triad. Med Sci Sports Exerc 2007; 39:1867–1882.
Nichols DL, Sanborn CF, Essery EV. Bone density and young athletic women. Sports Med 2007; 37:1001–1014.
Kerr D, Khan K, Bennell K. Bone, exercise and nutrition. In: Burke L, Deakin V. Clinical Sports Nutrition. 3rd ed. McGraw-Hill, 2006; 237–261.
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