Continuing coverage: Brown fat is found in babies on their upper spine and shoulders. "When we are born, and for the first few months or years of life, humans have significant amounts of brown fat," says Labros Sidossis, a professor of internal medicine at UTMB. Babies are unable to shiver to stay warm, so when the temperature drops, brown fat jumps into action through a process called nonshivering thermogenesis, using "significant amounts of blood sugar and lipids to produce heat," Sidossis explains. As we age, most brown fat is lost and replaced by white fat. But it's still there, albeit in small deposits, around the neck, shoulders and spinal cord.