Brown adipose tissue (called brown fat) helps babies, young children, and other small mammals stay warm by burning calories when activated by low temperatures. The more familiar fat — known as white fat, although it’s actually yellowish — stores up calories and stubbornly accumulates around waistlines, thighs and bottoms.

Three new studies show that most adults have unexpectedly large and active deposits of a calorie-burning type of fat that biologists once thought disappeared after infancy.

In one of the new studies, researchers at Maastricht University Medical Center, in the Netherlands, examined 24 young men, about half of them lean and half overweight. Each was kept in a cool room (61 degrees Fahrenheit) for two hours and then given a PET scan, which lights up any tissue that is using a lot of glucose, showing that it is highly active on a metabolic level.

After the men had been kept cold, activated brown fat deposits were seen in 23 of 24 of them. (The one in which there was no activation was the heaviest.) Lean men had about four times as much brown fat activity as the overweight men. When several were retested without first being cold, there was no brown fat activity.