For many years it as mistakenly believed that brown fat was only present in babies still in the womb and disappeared after birth.
It is now known that the majority of humans have brown fat.
So they should not be utilized to routinely screen for activated brown fat, which is a transient condition anyway.
To prove the presence of brown fat in healthy human adults a group of 162 volunteers underwent PET scan after 2 hours of exposure to cold temperature.
Over the past decade when PET scans began to be used to visualize the location, size and growth of tumors by the infusion of radioactive sugar, areas around the neck and collarbone exhibited active metabolic activity and uptake of sugar, evidence of heat and energy expenditure that was identified as brown fat.
Modest biological threats such as radiation and food deprivation activate an internal guardian of genes called heme oxygenase-1 via the Nrf2 gene activator.
This body core temperature difference would account for the accumulation of ~4.4 lbs. [Chronobiology International 2015] Just a few pounds of weight difference in a year between those who have activated brown fat in their body and others who do not may sound almost trivial to a person who is 50, 75, even 100 pounds overweight.
However, in one study it was determined that brown fat activity explains 64% of the variance in body mass and 60% of the variance in body fat from individual to individual. In one study only 3 out of 15 morbidly obese subjects exhibited cold-temperature induced brown fat activity [PLo S One Feb 24, 2011] The current turn away from fat phobia and towards avoidance of refined sugars and carbohydrate intake (bread, rice, pasta) is now well substantiated to produce leaner and healthier humans.
In the animal lab, mice whose brown fat has been increased or activated are lean and protected from obesity.
It is the age-related change in brown fat activation that likely explains why the young are generally lean and fat mass increases with advancing age in adulthood.