Excluding single cases, all indices of trace element status were within the reference range at the first measurement point (Table 5). Excluding serum zinc, all other variables were within the reference range also at the second measurement. At the third measurement, several subjects in all groups had subnormal serum zinc concentrations. In addition, five athlete boys and six control girls had low serum ferritin concentration, and five athlete girls had low serum copper concentration. Only two group differences were identified: the prevalence of low-serum zinc concentrations at the second measurement, and low-serum copper concentrations at the third measurement, was higher in athlete girls.
Compared with control boys, the athlete boys were significantly taller, had greater upper arm muscle girth, and less subcutaneous fat throughout the entire study (Fig. 2). The apparent weight difference did not reach significance. The mean height increment was 14 cm in both athletes and controls. The general change in weight, height, and muscle girth was significant and linear, but without any differences between the groups.
Weight, height, and muscle girth in athlete girls were not different from controls’ results (Fig. 3), but the athletes had less subcutaneous fat. The change in all of the above anthropometric variables was linear, without any differences between the groups. The mean increase in height was 11 cm in athlete and 10 cm in control girls.
Regarding pubic hair stage, the athlete boys were more mature at baseline, but no other differences in self-reported maturation between athlete and control boys was found (Table 6). In contrast, girl athletes were significantly less mature than the controls throughout the follow-up (Table 6). The proportion of girls who had reached menarche was also smaller in athletes at the 1-yr and 2-yr follow-ups (Table 5). “Weight before menarche” was obtained for those athletes (N = 6) and controls (N = 18) who reached menarche during the study, that is, after the first but before the last measurement. Weight before menarche was 44.8 (95% CI: 40.7–48.9) kg in athletes and 44.1 (95% CI: 41.6–46.7) kg in controls.
The change in weight (from the first to the third measurement) was positively related to changes in height, skinfolds, and muscle girth (both sexes), and also to dietary copper (positive in boys), iron (positive in girls), and weight-related energy (negative in girls) intake (Table 7). Height increment was positively related to increase in weight and negatively to the increase in skinfolds (both sexes). In addition, the dietary intakes of weight-related energy (positive) and zinc (negative) were associated with the change in height in girls. PAL did not enter any of the multivariate models explaining changes in weight, height, muscle girth, or skinfolds.
The change in upper-arm muscle girth was positively related to change in weight in both sexes and independently also to the mean blood hemoglobin (positive association in both sexes) and serum ferritin concentrations (negative association in boys) (Table 7). The sum of four skinfolds increased similarly to change in weight (both sexes, P < 0.001), but in opposite direction to the change in height (both sexes). In girls, also the dietary intake of iron and blood hemoglobin concentrations were negatively associated with skinfolds. Similar to boys, PAL did not enter any of the multivariate models.
Nutritional requirements are generally increased during periods of rapid growth, such as during puberty, and potentially are also affected by athletic training (19). The present study is the first to study prospectively dietary energy intake, indices of trace element status, and growth in pubescent children engaged in club-level sports training. Club level in this context meant children who, in the beginning of the study, were less selected than and who did not train as seriously as junior elite athletes (e.g., gymnasts participating in international contests).
Interests in the selection of the boy and girl athletes were different: restriction of energy intake, clinical eating disorders, and even growth retardation are special concerns for some young female athletes, particularly gymnasts and figure skaters (4,5). The female participants in the present study belonged to sports with emphasis on low body weight. Hence, we speculated that at least some of the girl athletes might have inadequate nutritional status because of the independent or combined effects of dieting and physical exercise. Because of earlier maturation, the selected girls were 1 yr younger than the boys.
In boys, only wrestling has been identified as a potential sport with problems related to dietary restriction (16). However, because the participation in wrestling among children in our study areas was very rare, we recruited ice hockey players. Compared with many other sports, ice hockey players in Finland are involved in more regular and intense training (personal observations). The ice hockey players of the present study were elite athletes at the national level at their age group, and several of the 15-yr-old players were chosen to the national team in 1994. Therefore, the athlete boys were used to describe the possible interaction of rapid growth and high energy need on dietary intake and nutritional status.
The boy athletes trained three to four times weekly in the beginning, and five to six times weekly in the end of the study. The duration of one training session was approximately 1.5 h. Most of the training was on ice, but some running, light weight training, and calisthenics were performed during the summer. The playing season was from September to April. Hence, the data for the present study were collected in the end of the competition season. The girl athletes trained three to five times weekly throughout the entire study period. Each training session lasted 1–1.5 h. The data were collected in the end of the gymnasts’ and figure skaters’ competition period, but about 1 month before the beginning of the runners’ competition period.
Physical activity and dietary intakes.
At the first measurement point, the control children’s self-reported PAL was not different from the athletes’ PAL. However, the PAL in the control children (both sexes) decreased with advancing puberty, which was consistent to earlier reports (10,29). In the same measurements among athletes, the girls’ PAL remained stable, whereas the boys’ PAL increased. These findings indicate that the girls’ training compensated for, and the boys’ training even exceeded the age-associated decrease in PAL. Therefore, participation in junior sports during puberty seem to maintain a physically active lifestyle which is viewed by many as a major component of preventive medicine (20).
Compared with the weight-related recommendations for energy intake (33), the controls’ energy intake was a little lower, whereas the athletes had slightly higher energy intake. However, because dietary records tend to underestimate energy intake even in children (11), the actual intakes were probably somewhat higher than the reported results. The estimated daily energy expenditures (clearly higher than reported intakes) are in line with suspected underreporting of energy intake. The qualitative and quantitative components of underreporting are not fully understood. However, we hypothesize that the likelihood for underreported micronutrient intake increases with the magnitude of energy underreporting.
Other studies with data on energy intake show a range of mean values between 180 and 220 kJ·kg−1 in girl gymnasts (31), figure skaters (12) and runners (6). Corresponding data in boys show a similar range of energy intake (175 to 230 kJ·kg−1) in swimmers (14,31) and in American football players (15). Hence, especially at the second and third measurement points, the present athletes fit very well within the above reported ranges.
Daily protein requirements (expressed as g·kg−1) decrease gradually after age 11 in girls and after age 13 in boys (33), that is, the highest recommendations (1.0 g·kg−1) coincide with the peak height increment. The protein intakes in the present subjects were high (all mean values ≥ 1.5 g·kg−1 daily regardless of the measurement point), agreeing with the view that inadequate protein intake is not a problem in pubescent athletes eating a mixed Western diet (6,12,14,15,31).
Similar to other studies (6,12,14), the reported iron intake in both girl athletes and controls was very close to the recommended intake (12–18 mg·d−1 for girls, 12 mg·d−1 for boys) in Nordic countries (23). Compared with the recommendations (8 mg·d−1 for girls and 11 mg·d−1 for boys) (23), zinc intake seemed marginal in control boys and adequate in all other groups.
Iron, zinc, and copper status.
Both red blood cell mass and blood hemoglobin concentration increase during puberty which concomitantly increases daily iron requirements (2). The changes associated with hemoglobin are usually more pronounced in boys, but the start of menstruation has at least equally important effects on iron needs in girls (34). The present weak declining trend in serum ferritin concentration agrees with the view that iron from stores (as reflected by serum ferritin) is redistributed to red blood cell mass during puberty (2). In addition, the increasing prevalence of low serum ferritin concentration in control girls was probably also associated with start of menstruation. The overall increase in blood hemoglobin concentration indicates that, despite a stress on iron stores, iron intake was adequate to cover iron losses and to increase red blood cell mass.
A clear age-related declining trend of serum zinc and copper concentration was found in all subjects, without an interaction with athletic training. A similar decrease in serum copper concentration by age-group was found in the cross-sectional study by Laitinen et al. (18), but, in contrast to the present results, there was no evidence that serum zinc concentration was affected by age.
We interpret the third year’s high prevalence of subnormal serum zinc concentrations in all groups to show the increased need of zinc for the growing tissues. A post hoc analysis revealed that girls with serum zinc concentration below the reference cut-off point at the end of the study also had grown more during the study (mean height increment: 12.2 vs 9.4 cm in girls with low concentration vs concentration within the reference range, P = 0.005). The same trend was not significant in boys (height increment: 14.6 (low serum zinc) vs 13.6 cm, P = 0.14). Because low serum zinc was associated with rapid (rather than slower) growth, it is unlikely that the deteriorating zinc status would have restricted growth. Hence, these data do not support the use of trace element supplementation to ensure growth in pubescent athletes.
Growth and biological maturation.
Although there are certain concerns regarding growth in elite adolescent athletes (19), the present study confirms the conclusions of others that athletic training at a nonelite level does not affect growth (3,7,25). Later sexual maturation in some girl athletes, such as gymnasts, is thought to reflect self-selection (24). Although the change of our technician after the first study year is a concern, we consider it very unlikely that the between-measurer variation would have affected the group comparisons.
Warren (32) has observed that the onset of menstruation occurred at a higher weight in ballet dancers than in less physically active girls. The author speculated that the large “energy drain” in the ballet dancers increases the weight that triggers menarche. The “weight before menarche” could be calculated in a subsample of the present subjects. Although the number of observations was limited, similar “weight before menarche” in athletes and controls agree with the view that club-level training did not affect maturation.
As one might expect, the changes in anthropometric variables were interrelated. In addition, it was especially interesting to find independent associations between serum ferritin and change in muscle girth (negative in boys), between blood hemoglobin and muscle girth (positive in both girl and boys), and between blood hemoglobin and skinfold thicknesses (negative in girls). We speculate that androgens, by increasing muscle and red blood cell mass, might be the link between iron status and body composition. Unfortunately, we have no biochemical data to support our suggestion.
Dietary intakes, trace element status, and growth was studied prospectively in pubescent athletes and nonathletic schoolchildren. The athletes’ dietary intakes were similar to or higher than the respective intakes in the control groups. In addition, no marked differences in chemical indices of trace element status was observed between the athlete and control children. These results suggest that club-level sports training does not cause a threat to the trace element status during puberty.
These data did not indicate any negative associations between sports training and growth. However, the present results do not elucidate whether very strenuous (elite) training or aberrant eating habits affect growth in child athletes.
The chemical analyses were done by Antero Julkunen (University of Kuopio) and Jari Lehto (University of Helsinki). The study was financially supported by the Ministry of Education.
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Keywords:© 2000 Lippincott Williams & Wilkins, Inc.
COPPER; IRON; BIOLOGICAL MATURATION; PHYSICAL ACTIVITY; PUBERTY; ZINC