Over the last two decades, the rise in the prevalence of overweight and obesity may explain the number of risks for obesity-related metabolic and endocrine derangements, and have been leading to early incidence of obesity-related diseases that used to be exclusive to adulthood.
Nonalcoholic fatty liver disease (NAFLD) is one of the most frequent complications associated with excess adiposity (Papandreou et al., 2007) and has been identified as the leading cause of liver disease in pediatric populations worldwide (Barshop et al., 2008). It is characterized by pathological fat accumulation in the liver, which may lead to liver damage in the form of inflammation and fibrosis (Siegel and Zhu, 2009). Furthermore, NAFLD predisposes subjects to other pathological conditions, such as metabolic syndrome and type 2 diabetes (Targher et al., 2005; Yki-Jarvinen, 2005).
Weight loss via lifestyle modification remains the most common and well-established fundamental therapy for reducing hepatic fat (Dixon et al., 2004) since low levels of physical activity (PA) seems to be associated with many NAFLD risk factors, such as high cholesterol, diabetes, and metabolic syndrome (Muros Molina et al., 2011; Skaaby et al., 2012).
In a previous study we investigated whether cardiorespiratory fitness (CRF) was associated with fat liver (alanine aminotransferase--ALT) in obese children. Our findings suggested that there might be a potential protective effect of CRF against abnormal ALT values (Martins et al., 2013). Because CRF is related to PA levels, and increased PA plays a protective role against NAFLD risk factors, we hypothesized that it might be an association between PA and NAFLD in obese children and adolescents, independently of central adiposity
Recent studies suggested that increased PA might have a beneficial effect on NAFLD (Golbidi et al., 2012; Kwak et al., 2014). Nonetheless, to the best of our knowledge, no studies investigated this association in youth populations. Therefore, the aim of this study was to analyze the association between PA and a fatty liver marker (ALT) in obese children and adolescents, independently of central adiposity or CRF.
Participants and data collection
This is a cross-sectional baseline study, which is part of a school-based intervention PA program carried out in 5 primary and 2 middle-high public schools from a suburban setting from Oporto-Portugal. Over a period of 6 months, 159 students volunteered to attend a PA program, twice a week. For this cross-sectional study, the sample was comprised by 131 children (83 girls, 7-15 year-olds) involved in the program who: (1) did all the baseline testing procedures; (2) were not attending any other formal sports or PA program; and (3) were classified as obese, according to age-gender specific body mass index (BMI) cut-off points (Cole et al., 2000).
For the purpose of this study, the sample size was calculated using the G*Power software 220.127.116.11. Hypothesizing an effect size (Cohen's d) of 1.0 for a required power of 95% at p
The Regional Education Board approved the study protocol, and students, parents and schools agreed to participate. The nature, benefits, and risks of the study were explained to the volunteers, and a parent's written informed consent was obtained before the study, consistent with the Helsinki Declaration. The Scientific Board of the Research Unit from The Faculty of Sports of Oporto University approved the evaluation methods and procedures used.
One week before the evaluation week, all the subjects were familiarized with the testing protocols, procedures and equipments. Six Physical Education teachers, three pediatric nurses, and a medical doctor carried out all measures, which took three consecutive mornings. Participants were identified through an individual code number and a school code number, and divided in three groups, according to their convenience. One group was evaluated per day. Fasting blood samples were taken between 08:00am and 9:00am. The children and adolescents were then given breakfast, followed by the evaluation of the anthropometric, body composition, maturational, and cardiorespiratory fitness variables, between 09:30 am and 11:00 am. At the end of the evaluation protocol, the accelerometers were given to the students. Participants' parents were recommended to encourage their children to maintain the same standard meal.
Body height was measured to the nearest mm in bare or stocking feet with the adolescent standing upright against a Holtain Stadiometer. Weight was measured to the nearest 0.1 kg, lightly dressed and after having breakfast, using an electronic weight scale (Seca 708 portable digital beam scale). BMI was calculated from the ratio of body weight (kg) / body height ([m.sup.2]). Waist circumference (WC) was evaluated using the NHANES protocol (Kuczmarski, 1996). A bony landmark is first located and marked. The examiner, positioned to the right of the participant, palpated the upper hip bone to locate the right iliac crest. Just above the uppermost lateral border of the right iliac crest, a horizontal mark was drawn, and then crossed with a vertical mark on the mid-axillary line. The measuring tape was placed in a horizontal plane around the abdomen at the level of this marked pointed on the right side of the trunk. The plane of the tape was parallel to the floor and the tape was held snug, but did not compress the skin. The measurement was made at a normal minimal respiration.
Blood samples were collected by three pediatric nurses. A mean inter-observers and intra-observers coefficient of variation (CV) of 2.4 and 1.2% respectively, was observed. Blood was collected from the antecubital vein between 8:00 and 09:00 a.m., after at least ten hours of fasting. Blood samples were obtained on a fasting basis and processed within 2h of collection. Blood was obtained by venipuncture, to EDTA containing tubes. Following centrifugation, plasma was separated, aliquot and immediately stored at -80[degrees]C until the end of the week, when it was assayed. To determine ALT concentrations, a routine automated technology (ABX Diagnostics) was used. The biochemical evaluation of all participants was conducted in the same laboratory.
Physical activity (PA) measurement
PA was objectively assessed by accelerometers (wGT3x, Actigraph, Florida) during 7 consecutive days. Data was stored in raw mode in samplings of 30Hz. With a specific software (ActiLife, version 6.9, Actigraph, Florida), data was reduced into one-minute periods (epochs), organized into daily physical activity and analyzed after data collection. Wear and nonwear time was determined according to Choi et al. (2011) algorithm. Time periods with at least 10 consecutive minutes of zero counts recorded were excluded from analysis assuming that the monitor was not worn. A minimum recording of 8-hours/day (480minutes/day) was the criteria to accept daily PA data as valid. Individual's data were only accepted for analysis if at least three-week days and one weekend day were successfully assessed. The main outcomes of reduced accelerometer data were: total physical activity [total PA (counts/min/day)], time in sedentary behavior [SB...