Context: Brown adipose tissue (BAT) is particularly abundant in neonates but its association with measures of adiposity and metabolic health in early infancy is poorly delineated. Besides sustaining non-shivering thermogenesis, BAT secretes brown adipokines that act on systemic metabolism. The chemokine CXCL14 has been identified as a brown adipokine in experimental studies. Objective: To determine the relationships among BAT activity, adiposity and circulating CXCL14 levels in the first year of life in girls and boys. Design, setting and participants: Indices of fat accretion, circulating endocrine-metabolic parameters and serum CXCL14 levels were assessed longitudinally in a cohort of infants at birth and at 4 and 12 months. BAT activity was estimated using infrared thermography only at age 12 months. Main outcome measures: Weight and length Z-scores, total and abdominal fat content (by DXA), BAT activity at the posterior-cervical and supraclavicular regions, serum levels of glucose, insulin, insulin-like growth factor-I, high-molecular-weight adiponectin and CXCL14; CXCL14 transcript levels in neonatal BAT and liver. Results: Posterior-cervical BAT was more active in girls than in boys (p=0.02). BAT activity was negatively associated with adiposity parameters only in girls. CXCL14 levels were higher in girls than in boys at age 12 months and correlated positively with the area of active posterior-cervical BAT in girls. Neonatal BAT showed high CXCL14 gene expression levels. Conclusions: BAT activity and the levels of CXCL14 -a potential surrogate of BAT activity- are sex-specific in the first year of life. BAT activity associates negatively with indices of adiposity only in girls.

Here, we assessed for the first time brown adipose tissue (BAT) activity and the circulating levels of CXCL14 in a longitudinal cohort of infants in the first year of life, and determined their relationship with measures of adiposity (by DXA) and endocrine-metabolic parameters. The study cohort was composed of N= 43 infants (23 girls, 20 boys) who were recruited prenatally during the customary third-trimester visit among pregnant mothers consecutively seen in the outpatient clinics of Hospital Sant Joan de Déu and Hospital de Sant Boi – Parc Sanitari Sant Joan de Déu (Barcelona, Spain).