Lack of sex difference in cerebrospinal fluid (CSF) leptin levels and contribution of CSF/plasma ratios to variations in body mass index in children

In adults, leptin seems to cross the blood-brain barrier by a saturable tra nsporter. This may contribute to the development of obesity. The present st udy in healthy children investigates leptin levels in plasma and cerebrospi nal fluid (CSF) in relation to body constitution. This prospective study an alyzed leptin levels in plasma and CSF samples (stored at -80 C) of patient s without CNS infection or blood-brain barrier dysfunction. Inclusion crite ria included temperature less than 38.5 C, C-reactive protein levels below 10 mg/L, CSF leukocyte levels less than 10(7)/L, no need for neurosurgical or oncological treatment, and no history of trauma. Four groups were design ated according to body mass index. Sixty-five children (28 girls and 37 boy s) entered the study. Plasma leptin (median) was 7.4 in girls and 2.6 ng/mL in boys., CSF leptin was 0.273 and 0.204 ng/mL, respectively, leading to C SF/plasma ratios of 0.045 and 0.071, respectively. Ratios were clearly depe ndent on body mass index percentiles (r = -0.484; P < 0.01, significant dif ferences between groups by ANOVA). Median plasma leptin levels in the 4 gro ups (body mass index, <10th, 10th-50th, 50th-90th, and >90th percentile) we re 2.0, 2.3, 4.1, and 8.8 ng/mL; CSF/plasma ratios were inversely related: 8.2%, 7.6%, 5.5% and 3.6%. In healthy children, CSF leptin levels account for approximately 5% of plas ma levels. CSF/plasma ratios in girls are lower than those in boys, explain ing why calorie intake and energy expenditure are not grossly different des pite large differences in circulating plasma leptin concentrations. CSF/pla sma ratios of lean children are higher than those in obese children. The dy namic changes in the CSF/plasma ratios are more pronounced in lean children , i.e. the nonlinear transport characteristics of the leptin system amplifi es the information about changes in body energy stores in this population, indicating that leptin is part of a mechanism to protect the body from crit ical weight loss rather than to avoid obesity.