Serotonin transporter binding and acquired obesity -- an imaging study of monozygotic twin pairs.

Serotonin (5-HT) has been implicated as one factor controlling body weight and feeding behaviour. We studied the association between obesity and 5-HT by investigating the brain serotonin transporter (SERT) binding in 16 monozygotic twin pairs with varying body mass index (BMI) differences. The radioligand [(123)I]nor-beta-CIT was used for single photon emission computed tomography (SPECT) imaging of SERT binding. SERT genotype was also identified for each subject. We hypothesized reduced SERT binding in twins with higher BMI as compared to their leaner co-twins, and increased SERT binding in subjects with LL homozygotes compared to LS heterozygotes and SS homozygotes. In pairwise analyses, twins with higher BMI had higher SERT binding than their leaner co-twins in the hypothalamus/thalamus (specific binding ratios 1.21+/-0.23 vs. 1.12+/-0.16, p=0.04). The difference was striking in women (1.17+/-0.24 vs. 1.04+/-0.16, p=0.01), but not in men (1.26+/-0.22 vs. 1.22+/-0.08, p=0.61). In individuals, no correlation between SERT binding and BMI was evident, and no differences were found in SERT binding between the three SERT genotypes. Our finding suggests an association between acquired obesity and the 5-HT system, particularly in women. However, this association was seen only in twin data, where genetic effects and many shared environmental factors are eliminated.