The present study was carried out on obese children and adolescents, of both sexes, and identified that RHR has a significant relationship to dyslipidemia.
Previous studies have identified that, in children and adolescents, the chronological age is inversely related to RHR [6
]. Al-Qurashi et al. [5
] proposed age-specific reference values of RHR to Saudi children/adolescents, and identified that the RHR values were lower in adolescents than in children. A possible explanation for this is the alteration in the autonomic cardiac control, which is age dependent.
Previous studies carried out on subjects from birth to 24 years observed changes in the autonomic nervous system in accordance with nutritional status and advancing age [18
]. They observed that sympathetic and parasympathetic activity increase in infants but, in children and adolescents, there is a great decrease in sympathetic activity and only a slight decrease in parasympathetic activity. Therefore, the lower cardiac sympathetic activity in children and adolescents may explain the reduction in the RHR values [5
] observed in the present study, and support the necessity to adjust the statistical analyzes by age.
In this obese sample, both elevated occurrences of dyslipidemia (85.6%) and elevated blood pressure (17.8% [data not shown]) were observed. In fact, scientific literature has linked dyslipidemia and arterial hypertension to increased adiposity in children and adolescents [6
]. In pediatric obesity, the endothelial dysfunction occurs due to a state of increased oxidative stress and the action of the vascular cells adhesion molecules [20
]. Moreover, the above mentioned inflammatory mechanisms are strongly related to dyslipidemia [22
]. Our data agrees with previous research, in which there is an elevated occurrence of the components of metabolic syndrome in obese Brazilian youths [23
]. Caranti et al. [23
] identified that metabolic syndrome has a higher occurrence in obese Brazilian youths (34.8% in boys and 15.6% in girls) than in obese Italian youths (23.6% in boys and 12.5% in girls). The above mentioned data reinforces the dramatic necessity to implement effective public health action, targeting the prevention of pediatric obesity in developing nations.
It is well established that the practice of regular physical activity improves the production of superoxide dismutase and nitric oxide [24
] and, in turn, that regular physical activity from an early age, prevents the development of cardio-metabolic and cardiovascular diseases in adulthood [26
]. In the present study, the sedentarism of the sample participants could be a factor in justifying the elevated occurrence of dyslipidemia and elevated blood pressure.
Research has shown an increased sympathetic activity in obese individuals [27
]. Similarly, even in healthy normal weight subjects, the venous infusion of non-esterified fatty acids increases central sympathetic activation [30
], while weight loss decreases sympathetic activity [31
]. Our findings indicate the potential of RHR to screen dyslipidemia in obese children and adolescents. On the other hand, the observed relationship between tachycardia and dyslipidemia is not as simple to explain, because it is affected by many pathways and the causality in these biological mechanisms is still not clear [2
The actual function of some adipokines that affect the insulin binding by blocking the insulin receptor substrates-1 activation, stimulate the lipolysis and contribute to development of dyslipidemia, was recently described [2
]. These adipokines increase the production of reactive oxygen species in the brain, through activation of the nicotine adenine dinucleotide hydrogen phosphatase oxidase, increasing the oxidative stress in rostral ventrolateral medulla, which determinates the basal sympathetic activity [33
]. In fact, recent studies have reported that the status of oxidative stress affects, positively, the sympathetic nervous system activation, which is responsible for the increase of RHR [33
In our study, fasting glucose was not related to RHR. Oda and Kawai [10
] identified, in a large sample of Japanese adults, increased fasting glucose in subjects with a higher RHR. Likewise, our results do not support these results, because our sample was composed exclusively of obese children and adolescents and further studies are necessary to clarify this issue.
A positive aspect of the present study is the analysis of TFM by DEXA. The inclusion of TFM in the multivariate model was important because this adipose tissue is related to the increased release of adipokines related to many pro-inflammatory mechanisms [2
]. Moreover, our data indicated an important relationship between sexual maturity and higher TC, lower RHR and higher body fatness and, therefore, to take into account the pubertal stage in the analysis (instead of only chronological age) makes the findings more consistent, because sexual maturity is strongly related to factors that directly affect the RHR and lipid profile (e.g. hypertrophy/hyperplasia of adipose tissue, increased release of hormones and adipokines) [35
On the other hand, some limitations must be pointed out. The cross-sectional design does not offer support to causality statements and, therefore, prospective studies from childhood to adolescence are necessary to describe more accurately the longitudinal relationship between RHR and dyslipidemia. The absence of inflammatory markers related to oxidative stress and, the absence of insulin measures to screen more clearly the relationship between RHR and glucose metabolism should be considered in future research.