Jarek Mäestu, University of Tartu
Jaak Jürimäe, University of Tartu
Toivo Jürimäe, University of Tartu
Arunas Emeljanovas, Lithuanian Sport University
Puberty is one of the most dynamic periods of growth and development, and is characterized by rapid changes in body morphology and composition. Different studies have found that both positive and negative changes take place, which might have significant impact for the future life.
Pubertal years are also an important period for linear growth and bone mineral accrual. Bone mineralization increases with age, height, and body mass throughout childhood, with a most significant gain during pubertal development. Studies have indicated that approximately 40-50% increase in bone mass take place during the puberty, reaching the highest bone mass at approximately 20-22 years of age from where ongoing the bone mass starts to decrease. Osteoporosis can be characterized as a state with decreased bone mineral content that may result in fractures. Therefore, the higher is the achieved bone mass, the later the osteoporosis will probably occur. This indicates the high importance of the puberty period where the bone mineral parameters can be modified on great extent.
It has been shown that the increase in bone mineral parameters is highly related to mechanical loading to bones. One of these factors is the size of the body, with subjects who have higher body mass have also higher bone mineral density. In contrast, it has been argued that the main factor regarding bone mineral accrual is not the body mass per se, but the amount of lean body mass. Further, it has been found that physical activity, especially high impact physical activity has a positive influence on bone mineral parameters. However, it is not known what is the contribution of body composition in subjects with different physical activity levels. So far the majority of the studies have been mainly cross-sectional, while it is known that during puberty physical activity decreases significantly. Therefore, cross-sectional design might not be very reliable in describing the changes that occur in physical activity during puberty.
The general aim of the present study was to evaluate bone mineral parameters in normal and overweight boys aged 11-13 years in associations with body composition and physical activity.
According to the general aim, the specific aims of the present investigation were to:
1) investigate different bone mineral density values between overweight and normal weight boys (Study I);
2) investigate the relationships of different body composition parameters on whole body and areal bone mineral density values in overweight and normal weight boys (Study I);
3) investigate the relationships of different physical activity levels on whole body and areal bone mineral density values in overweight and normal weight boys (Study II);
4) investigate the physical activity changes to whole body and areal bone mineral accrual during one year period in 11-13-year-old boys (Study III).
264 boys, aged 11-13 years from Tartu and its surroundigs took part in this investigation. They were divided into normal and overweight categories according to cut-off points by McCarthy et al (2006). Their body composition was measured using DXA methodology, physical activity was measured by 7-day accelerometry and biological age by X-ray of the wrist. Similar testing was conducted in one-year period. Only those subjects were included into analysis who had accumulated all data from both measurements.
1) Overweight boys, aged 11-13 years had higher whole body, lumbar spine and femoral neck BMC values compared with normal weight boys. In contrast, no differences in lumbar spine and femoral neck body apparent (volumetric) BMD were seen in both groups. Despite higher whole body BMD, overweight boys had lower whole body apparent (volumetric) BMD compared with normal weight boys;
2) Fat free mass characterizes better than fat mass and BMI bone mineral values in normal weight boys, while fat mass and BMI are better determinants of bone mineral values in overweight boys;
3) Physical activity is more associated with bone mineral values in overweight boys compared with normal weight boys, and in addition to vigorous physical activity, already moderate physical activity has the significant impact on bone mineral values in overweight boys;
4) The increase in sedentary time emerged as one of the main predictors from physical activity to have the negative influence on bone mineral acquisition during a one-year observation period in 11-13-year-old boys.