The purpose of the presented study was the examination of nutrition intake influence on bone metabolism in young male volleyball players. The study was performed in twenty four 16-18 years old male volleyball players in the competition period. Subjects were divided into 2 groups, depending on the calcium intake: more than 1300 mg/day in the first group (13 athletes); and less than 1300 mg/day in the second group (11 athletes). The nutrition mode assessment was based on the 24-hour dietary history using the recall method. In the blood serum concentrations of osteocalcin, alkaline phosphatase (bALP), C-terminal telopeptide of collagen I (ICTP), insulin-like growth factor-1 (IGF-1), insulin-like-growth factor-binding protein-3 (IGFBP-3), growth hormone (hGH), and ionized calcium and magnesium were determined. Statistical analysis showed significant differences between both groups investigated in respect to the calcium (p<0.01) and protein (p<0.05) intake, and the bALP and (IGFBP-3) concentrations (p<0.05). The results of the study led us to conclude that low calcium and protein intake together with systematic sport activity negatively influenced the bone formation level.

It is well recognized that bone mass is controlled by genetic factors [21], as well as largely by such environmental and lifestyle factors as nutrition and physical activity [1, 8]. Among nutrients calcium is the most important for attaining peak bone mass [25]. The calcium component of bone mineral increases from about 25g at birth to about 1000g by 15-20 years of age, which indicates that the inhibition in mineral accrual during the years of growth results in a deficit in the peak bone mineral content [20]. Dietary calcium intake and physical activity interact with each other, and their combined effects determine the extent to which the bone density genotype influences the peak bone mass achieved in young adulthood [23]. However, competitive sport requires intensive physical conditioning, and nutritional demands are increased in active youths.

The present study was aimed, therefore, at an examination of calcium and other nutrients intake influence on bone metabolism in young male volleyball players.

The study was performed in twenty four 16-18 years old male volleyball players in the competition period. The subjects, training for 2-9 years (mean 5.1 ± 2.77), were divided into two groups depending on the calcium intake per day. The intake of calcium in the first group (13 athletes) was more than 1300 mg/day, and in the second group (11 athletes) less than 1300 mg/day. The nutrition mode assessment was based on the 24-hour dietary history using the recall method [6].

During the competition period the volleyball players were training every day. The collective questionnaire for load recording and the record of training method groups were used [24]. During the competition period the athletes trained at intensity levels I-V. The total volume of training in this period was 32.4 hours on the average (3.6h at the intensity level I, 5.4h – level II, 7.9h – level III, 7.2h – level IV, and 8.3h – level V).

The venous blood was drawn for analysis in a fasting state, between 8 and 9 a.m. In the blood serum, concentrations of bone formation markers – osteocalcin and alkaline phosphatase (bALP) – were determined with the Metra Biosystem (USA) ELISA immunoenzymatic method, while concentrations of the bone resorption marker – C-terminal telopeptide of collagen I (ICTP) and growth hormone (GH) were determined with Orion Diagnostica (Finland) radioimmunological tests. Concentrations of insulin-like growth factor-1 (IGF-1) and insulin-like growth factor-binding protein-3 (IGFBP-3) were analysed using a radioimmunological test produced by BioSource Europe. Concentrations of ionized calcium and magnesium were measured with Cormay tests.

Mann Whitney test was used for statistical analysis of the obtained results. Spearman correlation coefficients were calculated and were used to determine the relationships between the variables. The study got the approval of the Local Committee of Ethics in Scientific Research.

Table 1 presents the mean values of anthropological parameters and biochemical indices for the two groups as well as their comparative analysis. No statistically significant differences were found with respect to age, age of puberty, body weight and BMI (Body Mass Index). The BMI values show regular body mass in all the subjects.

Table 1. Mean values (± SD) and comparative analysis of anthropological and biochemical parameters in both groups of volleyball players

From biochemical indices, significant differences between two groups were noted for activity of bALP (p<0.05) and concentration of IGFBP-3 (p<0.05). The mean values of bALP activity and IGFBP-3 concentration in the first group were higher by 17.78U/l and 387.33 ng/ml, respectively, in comparison with the mean values established in the second group.

Table 2 presents average intake of energy, protein, fat, carbohydrate, total calcium and calcium from dairy products, and magnesium for both groups under study. In the first group significantly higher values of the daily intake of protein (by 57.7g; p<0.05), protein calculated per 1kg of body weight (by 0.7 g/kg; p<0.05), total calcium (by 1553.0mg; p<0.01) and calcium from dairy products (by 1449.5mg; p<0.01) were observed as compared with the second group.

The negative correlation was noticed between the body weight and ICTP concentration and between BMI and osteocalcin concentration in both groups under study. Spearman's correlation coefficient for the relationship between the ICTP concentration and body weight was –0.609 (p<0.05) in the first group and –0.694 (p<0.05) in the second group. The values of this coefficient calculated in both groups for the osteocalcin concentration and BMI amounted to –0.842 (p<0.01) and –0.666 (p<0.05) respectively. In the second group a positive correlation was found between the osteocalcin concentration and protein intake per kg of body weight (r = 0.920; p<0.01).

Table 2. Mean values (± SD) and comparative analysis of dietary nutrient contents in both groups of volleyball players

There is strong evidence that physical activity in young age contributes largely to the higher peak bone mass [12]. Some results indicate that resistance and high-impact exercise are the most beneficial [26]. However, exercise-specific, local strain environment, systemic factors and diet play an interactive role in modulating the response of immature bone to exercise. In our study all the volleyball players trained in one club and all of them had an identical training regime.

A balanced diet, adequate calories and nutrients intake such as calcium and protein are the most important for attaining peak bone mass. Calcium and protein needs are greater during preadolescence and puberty (ages 9-17) than in childhood or adulthood. This period is characterized by the accelerated muscular, skeletal, endocrine, and emotional development [16]. The Institute of Medicine recommends calcium intake of 1300mg per day for children and adolescents aged between 9 and 17 years [9]. From our study only 54.2% of the volleyball players met these recommendations. The factor contributing to the low calcium intake in the second group, among other things, was dairy products restriction. The bad nutrition in this group may be associated with the fact that the majority of them lived in a boarding-schools and prepared food by themselves.

The statistical analysis showed that the group of volleyball players consuming diet with calcium content above its recommended level obtained higher concentrations of both bone formation markers. However, a significant difference between the groups was found only in respect to the bALP activity. Several previous studies [17] showed that osteocalcin values were not influenced by dietary calcium intake, which can be confirmed by our results.

It is apparent from biochemical studies that the bALP activity depends on the presence of magnesium and zinc, and may be decreased when deficits of these elements occur [28]. In our study we determined the blood serum concentration of magnesium as well as the content of this mineral in the diet of volleyball players, however we found no difference between both groups in respect to these measurements.

The higher level of bone synthesis in the first group as compared to the second group may be a result of higher calcium intake as well as the higher protein consumption. In the first group the average level of daily protein intake was 2.05 g per 1 kg of body mass and in the second group only 1.32g/kg. The current recommended allowances for high quality dietary protein range from about 1.0 g/kg/d in adolescents to 0.75 g/kg/d in adults [22]. There is a proposal to increase the protein intake in sport activity even by about 100%. Williams [27] recommends the protein intake at the level from 1.4 – 1.8 g/kg/d in adult athletes. Based on these recommendations we may conclude that protein nutrition is sufficient in the first group and deficient in the second group. Dietary protein intake clearly determines the rates at which tissues synthesize and metabolize endogenous proteins [10]. When protein nutrition is restricted in experimental animals, skeletal morphology is affected [19]. Chan et al. [5] reported that protein as well as calcium intake were correlated with bone mineral density even in well-nourished groups. In our study we also found a positive correlation between protein intake and the osteocalcin concentration but only in the group whose food protein contents was very poor.

The mechanism of regulating bone metabolism by nutrients may be multifactorial. The insulin-like growth factor system can play a role in this mechanism. The IGF-1 blood concentration is modulated, among other things, by dietary protein contents [4]. The effects of IGF-1 on bone are anabolic, and stimulate longitudinal growth and bone mass [18]. IGF-1 has both systemic and autocrine actions in bone [7]. In spite of insignificant differences between the groups under study in relation to IGF-1 concentrations we found tendency to its higher level in the first group. However, the concentration of IGFBP-3 was significantly higher in this group as compared to the second group. The synthesis of IGFBPs is regulated by IGFs and other growth factors [15]. IGFBP-3 levels best reflect the total IGF concentration, as IGFBP-3 in adults carries 80% of IGFs [2]. Hayden et al. [11] showed that the bioactivity of IGFs in bone tissue is modulated by several IGFBPs, mainly IGFBP-3, -4, -5. Therefore, the higher bone synthesis rate in the first group of volleyball players could be associated with a higher IGFBP-3 concentration and it cannot be excluded that it may be a result of dietary protein intake. IGFBP-3 is the predominant binding protein in serum and is a marker of growth hormone action [13]. The growth hormone effect on bone formation may be mediated via regulation of the local production of IGFBPs in osteoblasts [11]. However, in our study we observed no significant differences between GH concentrations in both groups.

Bone turnover markers levels are variable depending on the age. Its level peak at an approximate age of 14 years in boys represents the average age of peak linear growth in children [3]. In our study this finding is confirmed by the negative correlation between the concentration of bone turnover markers, body weight and BMI. This is probably due to the decrease in the rate of anabolic processes in boys with higher values of body weight, caused by the completion of intensive growing up. Several studies showed that the gain in bone mass at adolescence was more a function of pubertal stage than chronological age [14]. Because no differences between groups under study in respect to the chronological and biological age as well as the height and weight of subjects were found, we can suggest that anthropological factors did not contribute to different levels of bone biomarkers.

The results described in this paper show that low calcium and protein intake together with systematic sport activity negatively influence the bone formation level. The decrease of the bone formation rate during human maturation may lead to lower peak bone mass.

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