ESPN RISE

Exercise, Nutrition, and Healthy Bones

by Chris Jensen, Ph.D, MPH, RD

[FOOTBALL]
Playing football means high impact - to compete on the gridiron is more than a matter of muscle; it also demands strong, healthy, dense bones. But as a football player, you may have a higher risk of broken bones or stress fractures. Weight-bearing exercise and resistance training are excellent ways to help strengthen your bones. Proper nutrition, including vitamin and mineral intake, as well as consumption of adequate calories, also helps reduce the risk of bone loss, stress fractures, and breaks. This article will give you the information you need to promote optimal bone health, to keep you on the field, and off the injury list.

[RUNNING]
You feel every step you run in your bones, the strength, density, and health of which is pivotal to your performance. But as a runner, you may have a higher risk of broken bones or stress fractures. Weight-bearing exercise and resistance training are excellent ways to help strengthen your bones. Proper nutrition, including vitamin and mineral intake, as well as consumption of adequate calories, also helps reduce the risk of bone loss, stress fractures, and breaks. This article will give you the information you need to promote optimal bone health, to keep you on the trails, and off the crutches.

[VOLLEYBALL]
Volleyball players need the physical power to move with speed and agility, and to drive the ball where it needs to go. Muscle is part of the equation, but your bones also need to be strong and healthy. But as a volleyball player, you may have a higher risk of broken bones or stress fractures. Weight-bearing exercise and resistance training are excellent ways to help strengthen your bones. Proper nutrition, including vitamin and mineral intake, as well as consumption of adequate calories, also helps reduce the risk of bone loss, stress fractures, and breaks. This article will give you the information you need to promote optimal bone health, to keep you on the court, and off the crutches.

PHYSIOLOGY OF BONE

Seemingly 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 OSTEOPENIA

A 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 BONE

The 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 SWORD

So 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 CRUCIAL

It 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 24 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 ATHLETES

The 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 HEALTHY

Awareness 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.