Top-up the calcium
Studies of male athletes have shown that exercise induced calcium losses from sweating can be substantial. It is widely accepted that women are generally at greater risk than men from calcium insufficiency, mainly because of inadequate intake. But what about women who take part in regular exercise? Does exercise add to their calcium insufficiency because of the extra calcium they are losing when sweating? US researchers recently completed a study to quantify the increase in total calcium losses over 24 hours through the skin and net changes in calcium retention, in active sportswomen after a strenuous 60-min exercise session (‘Exercise and calcium supplementation: Effects on calcium homeostasis in sportswomen’, Medicine and Science in Sports and Exercise2007: 39 (9) 1481-1486).
The researchers also wanted to examine the role of calcium supplementation to see whether it could correct exercise-induced calcium losses. Twenty-six premenopausal sportswomen took part in the study, which demonstrated that women who exercise strenuously for an hour a day have a small but significant increase in calcium loss through sweat (13mg). The women would need to consume an additional 40mg of calcium a day to make up that lost during exercise.
Break and break again
Stress fractures are common among young female competitive athletes, especially in track and field. A range of factors is thought likely to increase the risk level of fracture, including:
* current or past menstrual irregularity
* low bone mineral density
* low lean body mass
* late onset of menarche
* low body weight
* disordered eating
* low calcium and dairy product intake.
All these factors require more research to establish definite links to an increased likelihood of fracture. A team of US researchers recently investigated further (‘Risk factors for stress fracture among young female cross-country runners’, Medicine and Science in Sports and Exercise 2007: 39 (9) 1457-1463).
One hundred and twenty-seven competitive female cross-country runners aged 18 to 26 took part in the study. To be eligible, the runners had to run at least 40 miles a week at peak training times, compete in races and must not have used oral contraceptives or other hormonal contraceptives within six months of entering the study. Participants were monitored for any signs of stress fracture over a period of just under two years.
Almost one in three (31%) reported having previously had one or more definite stress fractures, 57% had a history of menstrual irregularity, and 40% had previously used oral contraceptives. During the trial period, 18 runners had at least one stress fracture. Ten of the first stress fractures occurred in the tibia, six in the foot and two in the femur. Four runners had a second stress fracture during the trial period: two in the tibia, one in the foot, and one in the femur. The researchers found that women who had reported a previous stress fracture were more than five times more vulnerable to fracture during the trial than women without such a history.
The researchers also found that the following factors were associated with increased rates of fracture:
* lower bone mass * low average daily dietary calcium intake and daily servings of dairy products
* younger age at menarche
* lower lean body mass and lower weight
* younger age
* shorter height
* lack of previous oral contraceptive use
* history of menstrual irregularity.
Perhaps the most significant finding from this study is that a history of previous stress fracture is an independent marker of susceptibility to fracture, above and beyond other linked factors that a woman may have, such as bone mineral content. Therapists and coaches need to ensure their female athletes are aware of their increased risk if they have already fractured, and should try to identify potential contributory factors in order to reduce that risk.
Positive impact for girls
Stress fractures are an overuse injury caused when the body is unable to repair bone in response to a repetitive strain. So it’s not surprising that many people suspect high training volumes as a potential contributory risk factor. But what if higher training loads had a protective role to play, particularly in the development of strong bones during adolescence?
Recent research has shown that increased training loads actually have a positive impact on the musculoskeletal system (‘Assessment of bone strength at differentially loaded skeletal regions in adolescent middle distance runners’, Journal of Science and Medicine in Sport2006: 9 221 230). Scientists in Australia compared a group of adolescent middledistance runners against their less active peers to see if physical activity actually helped develop bone strength. Using MRI and DXA (bone density) technology, the researchers compared the differences in bone strength between boys and girls. They showed that weightbearing activity (running) did in fact have a positive impact, helping to increase bone strength in female athletes compared to their non-athletic peers. Good news for the girls. Despite similar exposure to training loads, however, the male athletes did not appear to be advantaged in bone strength compared with non-athletic male peers.
In conclusion, rather than being the main cause of stress fractures, physical activity may have a protective mechanism to play in the development of bone strength, particularly for females. It is important to remember that our musculoskeletal systems require stress for adaptations to take place. Physical activity is normal – the body needs it to function. The key is to make sure that overload is applied in a controlled, gradual and progressive way.