Bone is a body tissue that undergoes continual reformation and modification. The dense outer portions of bones do so very slowly, whereas the 'cancellous' portions of bones (found mainly in the vertebrae, pelvis, flat bones and the ends of long bones) do so more rapidly. Calcium compounds form about 60 per cent of bone matter, so it not surprising that dietary intakes of calcium can affect bone mass.
Normal bone growth produces a peak in bone mass at around the age of 20, and a decline with age occurs beyond this causing women to lose about 50 per cent of cancellous bone in a lifetime-almost twice as much as men. Bone mass and density are related to bone strength, and a critical fracture threshold is believed to exist. Stress fractures interrupt many athletic careers, and are related to the trauma or stress load experienced by the bones. A reduced bone mass will increase the likelihood of such a fracture.
In general, the greatest stimulus for increasing or maintaining bone mass in adults is weight-bearing exercise. However, a condition of 'osteopenia' has been identified in many amenorrhoeic athletes, ie, a reduced bone mass in female athletes who have menstrual dysfunction (Myburgh and co-workers, 'Low Bone Mineral Density at Axial and Appendicular Sites in Amenorrhoeic Athletes, Medicine and Science in Sports and Exercise, vol. 25, no. 11, ppl 197-1202). The beneficial effects of weight-bearing exercise are apparently outweighed by other factors which are related to amenorrhoea.
Menstrual irregularities
Amenorrhoea has a specific definition regarding the absence of a menstrual cycle for at least three months, while the term oligomenorrhoea describes an irregular cycle which does not fit this definition exactly. So 'menstrual irregularities' will be the term used in this report. Young, intensively training female athletes are at much greater risk (at least 15 per cent) of developing menstrual irregularities than sedentary women (2-5 per cent. This suggests a relationship between strenuous training, body type and dietary patterns. This distinction in risk status is polarised when low bodyweights, body fat levels and nutritional intakes are considered in the type of female athlete found to be particularly at risk-that is, distance runners, gymnasts and ballet dancers.
Some research has suggested that menstrual irregularities are related to training volume, especially in distance runners, although the evidence is not yet convincing. More research is needed with a careful control on training intensity before conclusions can be reached in confidence. A more consistent relationship has been shown which links body composition to menstrual irregularities. Every individual may have a personal set-point of body fat, below which she may go only at the expense of normal menstrual function. A tentative parallel in the psychological profiles of some female athletes with those of females with anorexia is made by Bale ('Body Composition and Menstrual Irregularities of Female Athletes: Are They Precursors of Anorexia?', Sports Medicine, 1994, vol. 17, no. 7, pp347-352) with intensive training replacing some of the personal control achieved through dietary extremism. Nevertheless, a very low level of body fat will still be the result, with menstrual irregularity a distinct possibility.
How does irregular menstruation cause osteopenia?
The most likely link between the occurrence of menstrual irregularity and corresponding osteopenia in an athlete is the former's effect on hormonal function. Reduced oestrogen levels are common findings in studies on athletes with menstrual irregularities, thereby producing a similar situation to that experienced by post-menopausal women. The reduced bone mass and bone strength (osteoporosis) and increase of bone fractures for these individuals is well-known and feared. This undoubted link between low oestrogen levels and loss of bone mass means that the athlete with menstrual irregularities (or the athlete who trains intensely but has not yet observed such an effect) can no longer be complacent.
One research study found that the occurrence of stress fractures in female distance runners with menstrual irregularities was almost twice that of similar runners with normal cycles (Barrow and Saha, 'Menstrual Irregularity and Stress Fractures in Collegiate Female Distance Runners', American Journal of Sports Medicine, 1988, vol. 18, pp209-216). Low bone mass produced in athletes when younger may also lead to premature and increased severity of post- menopausal osteoporosis later in life.
What are the options?
Considering that menstrual irregularities can cause a rapid initial loss in bone mass of about 4 per cent a year for the first three years, prevention or early intervention is crucial. In addition, resumption of normal menstruation does not produce a recovery of full bone mass, and the microscopic structure of any regained bone mass may be suboptimal as well. Ideal treatment outcome would be a resumption of normal menstruation to correct the oestrogen deficiency. However, the necessary treatment would vary considerably between individuals, while consisting generally of weight gain and reduced training which no goal-oriented athlete would consider a serious option. An alternative may be oestrogen replacement therapy, although sports medicine practitioners are divided as to the safety and effectiveness of this approach, and they should obviously be consulted. It is certainly not suitable for young athletes (American Academy of Paediatrics Committee on Sports Medicine, 'Ameonorrhoea in Young Athletes', Paediatrics, 1989, vol. 84, pp394-5).