Serum Levels of 25-Hydroxyvitamin D and Bone Turnover Markers in Female Basketball Players: Age-Related Differences

Bone is a metabolically active tissue with constant remodeling throughout the lifespan. Biochemical markers of bone turnover provide insight into dynamic process of bone formation and bone resorption (1). Previous studies (2, 3) observed almost constant levels of bone turnover markers between the age of 19 and 30 in the general population, suggesting that bone turnover is relatively constant in this period of life. On the other hand, rising levels of biochemical bone turnover markers have been observed in children during growth and puberty. While bone turnover markers mainly represent bone remodeling in adults, in children they are associated with the processes of bone modeling, remodeling and growth in length (4).

Although bone turnover markers have been identified as important determinants for the prediction of fracture risk and repair of microinjuries, as well as in monitoring the rate of linear growth in children (3), limited attention has been given to bone remodeling in female basketball players. Most of the available data give reference ranges for serum markers of bone formation and resorption in the general population, but only few studies were undertaken in basketball players [early pubertal male basketball players (5) and late adolescent to early adulthood female basketball players (18–26 years) (6)], with no reference data available in middle adolescent female basketball players (15–18 years). Data collected in the general population cannot be transferred to athletes due to the adaptations (BMD accretion) that occur in response to their high-impact loading during exercise (7). In this way, bone turnover screening may be of clinical utility in monitoring growth in children.

The rapid process of bone modeling and remodeling with progression through puberty could potentially increase the need for vitamin D in this accretive phase of life (8). Vitamin D receptors identified in human cells of the bone and skeletal muscles have been positively associated to bone mineralization and the synthesis of muscle proteins (9). Although vitamin D may originate from the diet, the principal source is obtained through sunlight ultraviolet-B (UVB) exposure of the skin. In this regard, seasonal variations are observed in vitamin D status, with low serum 25-hydroxyvitamin D [25(OH)D] levels in winter and high levels in summer (10). Therefore, it may be presumed that indoor athletes are more susceptible to vitamin D insufficiency in winter, and that this would adversely affect bone health.

The aims of this study were to 1) quantify 25(OH)D status during the midpoint of the in-season phase when the sun exposure is lowest (in late Fall); 2) provide age-specific reference data for serum markers of bone turnover in middle adolescent (15–18 years) and late adolescent to early adulthood (19–30 years) female basketball players. We hypothesized that a high prevalence of vitamin D inadequacy (deficiency and insufficiency) would be observed. Also, it was hypothesized that younger female basketball players posses higher bone turnover rate.

Median, minimum, maximum and corresponding interquartile range (25 and 75^th centiles) for 25(OH)D, osteocalcin, and CTx-I are presented in Figure 1. Mean, SD, geometric mean, and 95% CI for 25(OH)D, osteocalcin, and CTx-I are presented in Table 1. Three participants had Tanner stage 4, while all remaining participants had Tanner stage 5. Considering 25(OH)D concentration, 21 participants (80.7%; 15–18 years: n = 11; 19–30 years: n = 10) displayed vitamin D insufficiency (50–75 nmol/L), three participants (11.5%; 15–18 years: n = 1; 19–30 years: n = 2) displayed vitamin D deficiency (<50 nmol/L), and two participants (7.7%; 19–30 years: n = 2) were vitamin D sufficient (>75 nmol/L).

Figure 1.

Median, mainimum, maximum and corresponding inter-quartile (25 and 75^th centile) range for 25(OH)D, osteocalcin, and CTx-I in middle adolescent (n = 12) and late adolescent to early adulthood (n = 14) female basketball players. In the box plots, whiskers indicate the minimum and maximum values, the boundary of the box closest to zero indicates the 25^th percentile, a black line within the box marks the median, and the boundary of the box farthest from zero indicates the 75^th percentile.

No significant difference in 25(OH)D (p = 0.09) was observed between middle adolescent and late adolescent to early adulthood female basketball players. Significant differences were observed in bone turnover markers (osteocalcin: p = 0.003 and CTx: p = 0.03) between groups, whereby middle adolescent female basketball players displayed significantly higher serum levels of osteocalcin and CTx-I compared to late adolescent to early adulthood female basketball players

In support of our hypothesis, a high prevalence of vitamin D inadequacy (insufficiency and deficiency) was observed in female basketball players (92.2%) examined in the present study. In addition, middle adolescent female basketball players possessed a higher bone turnover rate compared to late adolescent to early adulthood female basketball players.

The prevalence rate of vitamin D inadequacy we observed is congruent with results reported in other investigations examining basketball players (9, 15,16,17,18,19,20). Previously, similar prevalence rate of vitamin D inadequacy, ranging from 77% to 95% has been observed in female and male basketball players (9, 15,16,17,18,19,20). The limited sunlight exposure in late Fall (November) and within indoor environments from training and competition may negatively affect vitamin D status, underpinning high prevalence of vitamin D inadequacy in female basketball players we observed. In addition to sun exposure, the high percentage of vitamin D inadequacy seen in female basketball players is likely multifactorial and perhaps related to reduced vitamin D intake and lifestyle factors (eg, sunscreen and clothing) yet to be evaluated. Collectively, our results and those made previously (9, 15,16,17,18,19,20) suggest that female basketball players possess a high risk of vitamin inadequacy in late Fall.

Bone turnover markers complements the static measures of bone tissue (i.g. bone mineral density), while detecting the dynamics of bone remodeling with respect to bone formation and resorption. Our findings showed that middle adolescent female basketball players possess greater bone turnover rate than late adolescent to early adulthood female basketball players. Age-related differences align with findings from previous research (4), showing lower values of bone turnover markers (osteocalcin and CTx-I) in the transition to adulthood. Circulating concentrations of osteocalcin vary by age and pubertal stage, with the highest levels being found throughout puberty (females: 11–12 years; males: 13–14 years) (21). The beginning of pubertal growth spurt is characterized by increases in estrogens and androgens (22). Rising levels of estrogen and other sex hormones are accompanied by the secretion of growth hormone and insulin-like growth factor I, which stimulates greater bone turnover during puberty compared to other phases of life (3).

In addition to variations with chronological age, our geometrical mean concentrations for osteocalcin [15–18 years: 43.8 ng/ml (95% CI 32.9, 53.1) and 19–30 years: 29.5 ng/ml (95% CI 26.0, 34.8)] and CTx-I [15–18 years: 991.4 pg/ml (95% CI 846.4, 1191.2) and 19–30 years 737.7 pg/ml (95% CI 637.7, 915.7)] are higher than those observed in the general population. In females aged 20–24 years and 25–29 years, Hu et al. (23) observed lower geometrical mean concentrations of osteocalcin [21.2 ng/ml (95% CI 18.7–22.5) and 18.5 ng/ml (95% CI 17.2–19.8)] and CTx-I [412 pg/ml (95% CI 354–470) and 335 pg/ml (95% CI 304–367)]. Furthermore, XX (24) also demonstrated lower concentration of osteocalcin (geometrical mean ± SD: 13.0 ± 3.6) in females between the age of 15.5–18.4 years. Collectively, findings from our study support the notion that mechanical loading of bone through long-term basketball training and competition produce an increase in osteocalcin and CTx-I, and there may be a need to use specific reference limits in basketball players to from correct conclusions about exercise-induced bone remodeling.

First, variations in bone turnover markers are more dependent on pubertal stage than chronological age. Therefore, additional studies are warranted to gather further insight into puberty-related differences in bone remodeling. Second, disparities in training frequency were not accounted across comparison groups. Although the bone turnover response to training frequency suggests an overlap (osteocalcin: p = 0.07 and CTx-I: p = 0.14) between athletes completing <8 h or >8 h training sessions per week (25), future investigations should include a matched sample size across training frequency to ensure the generalizability of data.

A high prevalence (92.2%) of vitamin D inadequacy (insufficiency and deficiency) was observed in the players examined in our study. In addition, middle adolescent (15–18 years) female basketball players possess a higher bone turnover rate than late adolescent to early adulthood (19–30 years) female basketball players.