Young female athletes are at risk of low bone density. Building strong bones can help prevent sports injuries now and osteoporosis later.

Most teenage girls don't spend much time thinking about their bones. The threat of osteoporosis — a disease that makes bones fragile and prone to break — just is not on their radar. But adolescence is a critical time for bone health, because maximum bone development takes place in the years between puberty and age 18.

For girls and young women who train hard at sports, the problem of weak bones isn't just in the future. Female athletes are more likely than are nonathletes to have low bone mineral density. Bone mineral density (BMD) refers to how much calcium and other minerals are in bones. A bone's strength depends on its size and density. The higher the bone mineral content, the denser and generally stronger the bone is.

About 16 to 22 percent of high school female athletes have low BMD, which increases the risk of having stress fractures or other bone-related injuries.

Bone is living, growing tissue that is constantly changing. Throughout life, bits of old bone are broken down and removed, and new bone is formed, a process known as bone remodeling.

In young girls, the body makes new bone faster than it breaks down old bone. The skeleton grows in both size and density. Bone mass — the amount of bone tissue in the skeleton — increases rapidly. The most critical time for girls to build bones is between ages 10 and 16. By age 18, girls have reached 90 percent of their peak bone mass, while boys do so by age 20. Peak bone mass is the maximum amount of bone mass a person will achieve in life.

Most individuals reach peak bone mass by age 30. After that, bone remodeling continues, but slightly more bone mass is lost than is gained. Think of it as a bank account. The higher peak bone mass is, the more bone is "in the bank" to draw from later in life. This reduces the likelihood of developing osteoporosis.

For young female athletes, doing everything possible to bank bone also pays off in the present. Strong bones help prevent injuries to bones and to muscles.

Low BMD is one of the three components of the "female athlete triad," along with consuming fewer calories than the body needs for its activity level (low energy availability) and menstrual cycle disturbances. And either of these other two conditions can also cause low BMD.

Teenage girls who don't menstruate or who have longer than normal times between periods produce less estrogen, the hormone that regulates the rate at which bone is broken down (bone resorption). If estrogen becomes chronically low, then bone resorption can occur at a faster rate than it should. The result? Less bone mass is accumulated, and BMD is lower.

But even female athletes who have regular menstrual periods can still have low BMD. The physical and emotional stress of intense training can cause changes in growth hormone and cortisol production that also negatively affect BMD. A diet that is low in calcium, vitamin D and other nutrients needed for bone formation can all result in lower BMD.

Some aspects of bone mass aren't in an individual's control. Genetic factors account for more than half the influence on how strong and large one's bones will be. In general, girls will have a lower bone mass than boys will. But there are many habits any individual can follow to build and maintain strong bones.

Good nutrition is the starting point. The body needs protein, minerals and vitamins to make and regenerate bone.

Calcium: Calcium is the most important nutrient for attaining peak bone mass and preventing osteoporosis. A diet that is low in calcium contributes to diminished BMD, early bone loss and an increased risk of fractures.

Children between the ages of 9 and 18 should consume at least 1,300 milligrams (mg) of calcium a day. For adults ages 19 to 50, the recommended amount is 1,000 mg of calcium a day. Many children and teens don't get enough calcium, however. On average, adolescent girls consume 900 mg a day, compared with 1,200 mg a day for adolescent boys.

Good sources of calcium include dairy products, almonds, broccoli, kale, canned salmon with bones, sardines and soy products, such as tofu. An 8-ounce (237 milliliter) glass of milk provides 300 mg of calcium. Many packaged foods are fortified with calcium, including cereals, juices, breakfast bars, pastas and waffles.

Girls and young women who find it difficult to get enough calcium from their diets should talk to a doctor, registered dietitian or other health care provider about taking calcium supplements. Calcium supplementation has been shown to improve bone mineral density by 1 to 2 percent.

Vitamin D: The body needs vitamin D to absorb calcium. A lack of vitamin D can weaken bones and increase the risk of fracture. As with calcium, however, many children and adults don't consume the recommended daily value of 600 international units (IU).

Good sources of vitamin D include oily fish, such as tuna and sardines, egg yolks, and fortified milk. Sunlight also contributes to the body's production of vitamin D. Many experts recommend that female adolescent athletes take a daily vitamin D supplement of 500 to 1,000 IU, or even more if a deficiency has been identified.

Magnesium: This mineral can contribute to BMD. Food sources of magnesium include green vegetables, almonds, black beans, kidney beans, peanut butter and whole-wheat bread. A magnesium nutritional supplement also can be helpful for bone health.

Vitamin K: Vitamin K is needed for normal bone metabolism and helps prevent excess bone loss. Vitamin K is found in leafy green vegetables such as spinach, kale and broccoli, as well as peas and green beans.

Young female athletes (and women of any age) can benefit from practicing habits that promote good bone health.

  • Incorporate weight-bearing exercise. Weight-bearing exercises — activities that involve bones supporting the body's weight — will stimulate bones and make them stronger throughout life. For both younger and older women alike, this form of exercise is one of the most important factors for reaching and maintaining peak bone mass. Weight-bearing exercises include running, dancing, tennis, gymnastics, basketball, volleyball, soccer and weight training. If a person's main sport isn't weight bearing — such as swimming or bicycling — then a weight-bearing activity several times a week should be added.
  • Maintain a healthy weight. Being underweight or overweight — at any age — increases the risk of osteoporosis.
  • Don't smoke. Smoking has been linked to low bone density in adolescents, and teens who smoke develop less bone mass than nonsmokers do.
  • Limit alcoholic beverages (and don't drink at all if underage!). Regularly having more than two alcoholic drinks a day is harmful to bone health in many ways. And for women, experts recommend drinking no more than one drink a day for overall health. Alcohol interferes with the body's ability to absorb calcium and form new bone.
  • Limit soda and sugary drinks. Soda, energy drinks, sweetened teas and other high-sugar drinks add calories but not nutrition. Teens who drink these beverages (including diet soda), instead of milk, are forgoing an excellent source of calcium.

How do you know if someone's bones aren't as strong as they should be? Bone density can be measured, but not every female athlete needs to be tested. A doctor will recommend bone density testing when a girl or young woman:

  • Has not had a period for more than six months
  • Has had a stress fracture and a history of missing periods or a family history of osteoporosis

Bone density testing utilizes an X-ray machine to measure how many grams of calcium and other bone minerals are packed into a segment of bone. The most accurate testing method is dual-energy X-ray absorptiometry (DXA), which uses two different X-ray beams to estimate bone density in the spine and the hip. Strong, dense bone allows less of the X-ray beam to pass through it. The test is easy, fast, safe and painless.

Bone density test results report two different numbers: T-score and Z-score. For younger people, the Z-score is more appropriate. The Z-score compares the individual's bone density with what's normally expected for someone of the same age, sex, weight, and ethnic or racial origin. The Z-score is the number of units — called standard deviations — that bone density is above or below that reference value.

A Z-score below -2 is considered to be low BMD. A young female athlete who has a Z-score between -1 and -2, as well as a history of stress fractures, is considered to be at risk of low BMD and likely needs further testing.

Oct. 16, 2019