INS and obesity due to melanocortin 4 receptor deficiency: In addition, obese subjects have reduced fatty acid (using the PET tracer 18fluoro-6-thia-heptadecanoic acid) uptake by BAT during both warm and cold exposure in keeping with decreased BAT mass and activity (34), although greater insulin resistance may also contribute to the reduced glucose/fatty acid uptake by BAT in obesity (46).