Comparison of ectopic fat deposition between children with and without obesity: Influence of sex and physical activity level

Introduction: According to the theory of lipitoxicity, ectopic fat deposition (i.e., storage of triglycerides in non-adipose tissues, such as muscle, liver, pancreas) can lead to inflammation, insulin resistance and beta-cell dysfunction (1, 2). A previous study reported greater fat deposition in skeletal muscle and liver in children with obesity compared to their lean counterparts (3). However, the role of weight status (obesity vs. normal weight) on pancreatic fat deposition in children is yet unclear. In addition, little is known about the roles of sex and physical activity level in modulating ectopic fat deposition in children. We tested the hypotheses that children with obesity exhibit greater fat deposition in psoas muscle, liver, and pancreas compared to children with normal weight, matched for age and sex. We also investigated if sex and physical activity level modulate fat deposition in these organs.

Methods: 3T MR scanning with multi-echo Dixon imaging was used to acquire proton density fat fraction (PDFF) maps in 48 children (9-16 years; 33 boys, 15 girls) with obesity (body mass index, BMI > 90 percentile for age and sex) and in 30 age-matched children with normal weight (18 boys, 12 girls). From the PDFF maps, liver, psoas muscle and pancreas were outlined to obtain fat fraction in each specific organ. To assess physical activity level, participants wore a 3-axis accelerometer on the thigh for seven consecutive days. Daily minutes spent in sedentary, light, moderate and vigorous intensity were quantified. Fat fractions and physical activity categories were analyzed using 2-way (group, sex) repeated measures ANOVA. Associations between physical activity level and fat fractions were explored using Pearson’s correlation.

Results: Compared to normal weight, children with obesity showed higher fat fraction in liver (6.3±6.9% vs. 1.7±0.7; p<0.001), psoas (2.8±1.6% vs. 1.1±0.6%; p<0.001) and pancreas (4.9±2.8% vs. 1.7±0.7%; p<0.001) with no effects or interactions of sex. Daily minutes spent in moderate (29.0±13.7 vs. 43.1±19.5; p<0.001) and vigorous (8.2±7.6 vs. 21.9 ±13.2; p<0.001) intensity were lower in children with obesity compared with normal weight, with no effects or interactions of sex. Daily minutes of moderate-to-vigorous intensity was inversely associated with fat fraction in liver (r=-0.27, p<0.05), psoas (r=-0.34, p<0.01), and pancreas (r=-0.50, p<0.01).

Conclusion: Children with obesity exhibited greater fat deposition in liver, psoas muscle and pancreas compared to their lean counterparts. While sex did not play a role in modulating fat deposition, time spent in moderate-to-vigorous intensity activities were inversely associated with fat fraction in each organ. Future studies are required to 1) elucidate biomarkers that can identify children who display a critical amount of ectopic fat deposition, and 2) explore interventions effective in reducing ectopic fat deposition in children with obesity.