The main aim of this work concerns the answer a question whether the general regularity of seasonal differences in changes of height, weight, and body components during ontogeny in tropics exist. If so, whether this phenomenon depends on cyclic changes in nature or this rather is an adjustment to local conditions and mode of life. To answer this question the studies took place in the tropical climate of Yucatan and included 49 boys and 47 girls aged 11-12 years and being of the Maya, Mestizo, and Creole origin. The youths were attending two schools which were located in rather poor districts of Merida (the capital city of the Yucatan State, Mexico). The investigations started in February 2002 and ended in November 2003 and were continued monthly. The standard anthropometric methodology was applied to measure body height, weight, arm, waist, hip and calf circumferences, and five subcutaneous fat folds (biceps and triceps brachii, subscapular, suprailiac and calf). Bioimpedance techniques were used to estimate fat mass (FM ), fat-free mass (FFM ), and total body water (TBW). The results show that general regularities in monthly or longer rates of stature increments and increments or declines of body mass do not exist. There are not similar regularities of changes even in groups of coevals of the same gender and within the youths coming from the same district. Each variable shows a quite specific rate of changes.
In the present, the third publication included to this volume, the main topic considers the explanation of the existence of any relations between studied variables of body build during the period of 22 months, e.g. from the beginning till the end of investigations. At that time the considerable changes related to the pubertal period took place. The results of the correlation matrix between studied variables and factor analysis for the whole material and for each gender separately are presented. It is seen that there are differences between boys and girls in the interrelation concerning studied variables and in separated latent factors. It is rather related to differences in the phase of puberty (more advanced girls than boys in this process), although the same calendar age. Girls who mature earlier are characterized by greater initial stature and greater body weight, BM I fat mass (including subcutaneous fat tissue), and fat-free mass at the beginning and end of the study than girls who mature later. However, later matured girls show, in the whole investigated period, greater increments in stature and weight than earlier matured ones (of the same calendar age). At the earlier stage of puberty (boys under study) the factor one (F I) representing body mass and its increments, is associated with final body height (at the end of study), whereas F3 is associated with initial height (at the beginning of the study). This association is reverse at the latter phase of puberty (girls under study), while F I (also representing body mass but not its increments) is associated with the initial height, whereas final height represents the separate factor (F3).
In the first report, the specific monthly rate of changes of body build measures, typical for a studied group of youths and /or seasonal changes according to climatic conditions were not observed (Siniarska et al., 2005). The preliminary results suggested that observed changes rather depend on an adjustment to conditions and mode of life than to changes in nature. In the present report changes in particularly studied individuals were analyzed to verify the previous hypothesis suggesting that the rate of development has an immediate (causal) sense. The problem of whether the pubertal spurt is a single developmental effort or series of changes with dominance of intensified increments was considered. To explain the phenomenon of difference in onset of the pubertal spurt and its intensity in various populations, monthly measurements were conducted in individuals being at the age of puberty. The results show that each individual and its variable show a different rate of changes. It suggests that the rate of development and changes in body mass and fat mass are occasional. It rather depends on living conditions, mostly on nutrition and mode of life (physical activity and leisure). The only regularity shows that rather short (1-3 months) periods of rapid growth (saltations) are divided by slower growth periods (stasis), but their duration and time of occurrence have a very individual character. The rate and rhythm of each body build variables are also different. These phenomena depend rather on condition s in which the development of each individual occurs, as well as on its genetic predispositions and eco-sensitivity. It is probable, that differences in onset and intensity of pubertal spurt depend on alternations between periods of saltations and stasis of growth processes and on changes in body mass. These studies need to be repeated during a longer period of time (at least within a 5-year period ), in different climatic conditions and social groups.
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