Adult humans have heat-producing and energy-consuming brown adipose tissue in 
the clavicular region of the neck. There are two types of brown adipose cells, 
the so-called classic and beige adipose cells. Brown adipose cells produce heat 
by means of uncoupler protein 1 (UCP1) from fatty acids and sugar. By applying 
positron emission tomography (PET) measuring the utilization of sugar, the 
metabolism of brown fat has been shown to multiply in the cold, presumably 
influencing energy consumption. Active brown fat is most likely present in young 
adults, persons of normal weight and women, least likely in obese persons.