The impact of body fat excess on bone densitometry and bone metabolims biomarkers: a study with overweight, obese and superobese adolescents

Abstract

Considering that obesity and osteoporosis are pathologies that affect the general population, that they are severe public health problems, that obesity has been observed at increasingly earlier ages and that, with survival increase, population ageing results in increased prevalence of osteopenia/osteoporosis and their consequent fractures, this project aims at evaluating the relationship between fat mass excess on bone mineral densitometry and on bone formation and resorption biomarkers in adolescents of both genders. To that end, an evaluation of Bone Mineral Density (BMD), conducted on the Lumbar Spine (L1-L4), Proximal Femur and Total Body, will be used to clarify the possible effect of body fat excess on bone mass gain in adolescents of both genders aged 10 to 20 years (incomplete) by relating the results to bone formation biomarker, such as Osteocalcin (OC), and to bone resorption biomarkers (S-CTx).Methods: Adolescents aged 10 to 19 years and registered as new cases at the Adolescents' Health Outpatient Unit of the Botucatu School of Medicine - UNESP will be clinically and nutritionally evaluated. Their weight and stature will be measured, and their Body Mass Index (BMI) will be estimated. The obtained data will be compared to the Curves of the National Center for Health Statistics (CDC, 2002). Those showing BMI between 85o and <95o percentile will be regarded as overweight, and those equal to or above the 95o percentile will be regarded as obese. The evaluation of secondary sex characteristics will be performed by a qualified professional, and the result will be classified according to Tanner stages (Marshall & Tanner, 1969). A food enquiry will be conducted by using a 3-day recall. Next, those considered to be overweight or obese will be invited to undergo a Bone Mineral Density evaluation (BMD g/cm2) by dual energy X-ray absormetry (DXA) bone densitometry obtained on the Lumbar Spine (L1-L4), Proximal Femur and Total Body, and to Bone Age determination. Additionally, by the subjects' fasting for at least 10 hours and as long as they agree to the procedure, venipuncture will be performed for blood collection. Blood will be centrifuged according to recommended guidelines for the collection of serum which will be immediately stored in a freezer at -70o for the later dosage of specified bone markers. Such procedures will only be performed after agreement by the adolescents and their guardians, following the reading and signature of an Informed Consent Statement. The project has been submitted to and approved by the institution's Research Ethics Committee. Sample size estimation determined that 300 adolescents, 150 for each gender, must be evaluated. The data will be analyzed by age range groups and characterized by descriptive statistics, means, medians, standard deviations, quartiles and maximum and minimum limits of each variable. Sequentially, Analysis of Variance will be performed for comparison between all age ranges, according to the nutritional classification based on BMI. Also, the Tukey test will be performed in order to find significant differences, with p<0.05. Spearman's correlation coefficients will be obtained between bone biomarkers and the BMD results in the three evaluated sites. (AU)