In this study the investigators aim to investigate whether the sympathetic stimulation of BAT, as assessed with a 123I-metaiodobenzylguanidine single-photon emission computed tomography computed tomography scan, differs between lean and obese individuals, as an explanation for the diminished metabolic brown adipose tissue activity in obese subjects.

The investigators hypothesis is that sympathetic nervous system activity in the obese is diminished as compared to their leaner counterparts as an explanation for the diminished metabolic brown adipose tissue activity in the obese.

The sympathetic nervous system is the primary activator of Brown Adipose Tissue.

Obese humans are known to have less metabolically active brown adipose tissue as compared to their leaner counterparts.

The reason for this diminished metabolic brown adipose tissue activity is unknown.

However, a lower sympathetic nervous system activation to brown adipose tissue in the obese might explain a diminished metabolic brown adipose tissue.

Therefore, in this study, both the sympathetic nervous system activation to brown adipose tissue and the metabolic activity will be measured in lean and obese subjects.

sympathetic nervous system activity will be measured using 123I-metaiodobenzylguanidine single-photon emission computed tomography scans, metabolic activity will be measured using 18F- fluorodeoxyglucose positron emission tomography CT scans.

Inclusion Criteria:

• Male
• Caucasian origin
• Subjects should be able and willing to give informed consent
• 18-40 years old
• BMI range of 19-25 kg/m2 (lean study subjects) or 28-40 kg/m2 (obese study subjects)

Exclusion Criteria:

• Renal failure (creatinine>135mmol/l)
• Daily use of prescription medication