Introduction

Moderate physical activity favours bone mineralisation and reduces the rate of bone loss. The majority of studies tend to demonstrate that people who exercise have higher bone mass than those who are sedentary (Maggioni et al, 1998; Sundgot-Borgen, 1998; Nickols-Richardson et al, 1999; Proctor et al, 2002).

However, some studies report slower growth rates and bone mineralisation in women who practise certain sports, particularly those associated with lower body weights (Sundgot-Borgen, 1998). Further, deficient energy and calcium intakes are common among athletes, especially women (Castillo et al, 1996; Manore, 2002; Ziegler et al, 2002). An excessive consumption of meat and phosphorus-providing drinks could further aggravate this problem by altering the Ca/P ratio (McArdle et al, 1991; Ortega, 1992).

The present study examines the influence of diet, anthropometric data and sport practised on the bone density of different groups of sportswomen.

Materials and methods

The study subjects were 74 sportswomen: 15 skiers (age 17.12.9 y), 26 basketball players (17.22.1 y) and 33 dancers (16.22.0 y). In total, 90 young women who led sedentary lifestyles (16.71.0 y) formed the control group.

A personal questionnaire was used to record the number of hours spent training, age at menarche, and other personal, medical and socioeconomic data.

All subjects kept a food record for 5 days (including a Sunday). Foods were transformed into energy and nutrients using the Institute of Nutrition's Food Composition Tables (Instituto de Nutrición, 1994).

General anthropometric data were recorded first thing in the morning. Subject height (without shoes) was determined using a digital stadiometer HARPENDER (range 70–205 cm). Body weight (in underwear) was recorded using a digital balance (model SECA ALPHA; range 0.1–150 kg). Standard techniques were employed throughout and WHO guidelines followed (FAO/UNICEF/WHO, 1976). Body mass index (BMI) was calculated thus: BMI=weight (kg)/height² (m²) (Durnin & Fidanza, 1985).

Bone mineral content (BMC; in g) and bone mineral density (BMD; in g/cm²) were evaluated by double photonic bone densitometry in the lumbar spine (L2, L3, L4, and total L2-L4), in the right hip (neck, greater trochanter, femoral intertrochanter, and Ward's triangle), and in the right forearm (first third, middle, and ultradistal). Bone densities were measured using dual energy X-ray absorptiometry (DEXA) (HOLOGIC QDR 1000 S/N 166) (Cullum et al, 1989).

The data shown are meanss.d. With respect to sport practised, the significance of differences between means was determined by one-way ANOVA using the Newman–Keuls test. Correlation coefficients were calculated to determine relationships between variables. Significance was set at P<0.05.

Results

Tables 1 2 show the results obtained. The anthropometric data reflect the body image required by the sport practised. The dancers therefore showed the lowest body weights and BMIs, the basketball players were the tallest and had the highest weights and BMIs, and the skiers showed anthropometric values close to those of the control group (thus showing that skiing requires no specific body type).

Table 1 - Personal and dietetic data of the women studied (values are means.d.).

Full table

Table 2 - BMC and BMD of the women studied (values are mean s.d.).

Full table

The dancers spent longer training/performing per week than the other groups; the basketball players spent the least time practising their sport.

The dietetic results showed the dancers to have a very inadequate mean energy intake, and that their protein and calcium intakes were higher than in the other groups. Their Ca/P ratios were also higher than those of the other groups.

With respect to bone density, the sportswomen had greater bone mineral densities, except for the dancers who showed values similar to, or lower than (forearm), controls.

Discussion

The beneficial effects of certain sports on bone mineralisation has been reported by several authors (Nickols-Richardson et al, 2000; Pettersson et al, 2000, Proctor et al, 2002), and physical activity is considered to have a positive influence on bone density by many others (Uusi-Rasi et al, 1997; Duncan et al, 2002).

Spinal BMD was greater among the skiers and basketball players than among the dancers and controls (who had similar values) (Table 2). Similar results were seen for the right hip, except for the BMD of the trochanter fraction: that of the dancers was greater than that of the controls, but still lower than values for the other two groups of athletes (ANOVA, P<0.001). It may be that dancers exercise this part of their bodies more than the others that were examined, and that the positive effect of physical activity is manifested more strongly in this area. This has been described for the spine and leg in gymnasts (Bass et al, 1995; Padro et al, 1995). In agreement with the latter authors, the dancers showed low right forearm BMD values compared to the other athletes and controls. This part of the body is less exercised by dancers, who are also characterised by their thin arms (Pozo, 1997). The skiers showed the highest BMD in all areas examined.

Significant correlations were seen between different dietetic and anthropometric parameters and both the BMD and BMC for the different areas studied (Table 3). The association between low body weight and poorer bone mineralisation has been reported by several authors (Van Marken Lichtenbelt et al, 1995; Wynn & Wynn, 1995; Young et al, 1995). In agreement, the dancers showed the lowest bodyweights and poorest bone densities. The results obtained show a clear relationship between bodyweight and BMI and the BMD and BMC of all the areas studied (Table 3).

Table 3 - Significant correlation coefficients (P<0.05) between BMD and BMC and dietetic and anthropometric parameters.

Full table

Total energy intake was found to be an important dietary factor with respect to BMD and BMC. Significant, positive correlations were found between this parameter and BMD in the spine and with BMC in all three areas investigated. Low energy diets had a negative impact on bone mineralisation parameters, especially in the dancers, who had the lowest calorie intake.

All these results support the idea that sports associated with an adequate body weight and good muscular development stimulate growth and skeletal mineralisation (Maggioni, 1998; Duncan et al, 2002), while those associated with extremely low body weight and insufficient energy intake, the case of the present dancers, lead to poorer bone mineralisation (Sundgot-Borgen, 1998; Valentino et al, 2001).

Dietary calcium intake is considered to be a decisive factor in bone formation (Uusi-Rasi et al, 1997), especially during the time of growth (Tegarden & Weaver, 1994). The present results show that there is a positive and significant relationship between lumbar and hip BMC and calcium intake. However, no correlation was observed between calcium intake and BMD. This is in agreement with the work of Uusi-Rasi et al (1997), who studied adolescents with different activity levels.

Calcium intake among the dancers was greater than in the other groups as a consequence of their greater consumption of milk products (Table 1). As described by Welten et al (1995), intense physical activity, low energy intake and the aesthetic requirement of a low body weight in ballet dancers may have more influence on mineralisation than calcium intake itself. Nevertheless, the high calcium intake might help prevent the bone status of the population from being any worse.

Evidently, calcium intake is essential to bone mineralisation, but it is not the only nutrient required. Zinc and magnesium showed significant, positive correlations with BMC and BMD in different areas (Table 3). Zinc is clearly important, both in stimulating bone formation (Yamaguchi & Hashizume, 1994) and in inhibiting reabsorption (Moonga & Dempster, 1995). A greater zinc intake increases the activity of bone alkaline phosphatase and bone collagen content (Yamaguchi & Matsui, 1989), while magnesium is important in bone metabolism and acts as a cofactor for bone alkaline phosphatase (Ciancaglini et al, 1990).

Vitamin D also has a positive effect on bone mineralisation, maintaining adequate proportions of calcium and phosphorus in the serum and extracellular spaces (Holick, 1996). In the present work, significant, positive correlations BMC were found between vitamin D intake and forearm BMD and lumbar and hip BMC.

Excess protein is reported to negatively affect (New et al, 1997). However, the present results show positive correlations between protein intake and the BMD and BMC of the lumbar spine and hip. This may be because the protein intake recorded was below 2 g/kg body weight/day, the figure recommended for adolescents who practise sport (National Association for Sport and Physical Education, 1984); it was therefore not excessive.

Another parameter to bear in mind is the Ca/P ratio. An excess of phosphorus is a known risk factor for osteoporosis, and a Ca/P ratio equal to or greater than 1 is recommended (Levenson & Bockman, 1994; Anderson, 1996). Generally, the Ca/P ratio observed for the studied population was below this value. The highest values were seen among the dancers, although no clear association was found between this ratio and bone mineralisation data.

Conclusions

Physical activity has a positive influence on bone mineralisation. The dancers were the group most negatively affected in terms of BMD because of their specific characteristics that influence this for the worse: low energy intake and low body weight. This group should therefore be particularly vigilant with respect to fractures and skeleto-muscular lesions that could negatively affect their health and physical performance.

Possibly, the greater intake of milk products and Ca plus the more adequate Ca/P ratio of the dancers help them avoid worse bone deterioration.

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