What is brown fat?

In one line

Brown fat is a type of fat that, unlike ordinary fat that stores energy, burns its stored fat to make heat and keep the body warm.

What is brown fat?
The fat that is said to make heat
When people think of fat, they picture white grease stored in the body. But the body also has 'brown fat,' which looks different and does a different job. What is this fat, and how is it different from ordinary fat?
In one line
Fat that burns fat to make heat
Brown fat is fat that burns its stored fat to make heat. It keeps body temperature up when the body is cold. Its job is different from ordinary fat, which stores extra energy.
Ordinary fat is white fat
A storeroom holding one big oil droplet
Fat is kept as droplets of oil inside cells called 'fat cells.' Most body fat is 'white fat'; a white fat cell holds one big oil droplet and acts as a storeroom for extra energy. A brown fat cell, by contrast, holds many small oil droplets.
Why it is brown
Packed with mitochondria
Cells contain tiny parts that burn nutrients and turn them into energy the cell can use; these are called 'mitochondria.' Brown fat cells are densely packed with mitochondria. This is what gives the cells their dark brown color.
How it makes heat
Heat instead of ATP
The energy substance a cell uses right away to do work is called ATP. Mitochondria normally use the energy from burning fat to make ATP. The mitochondria in brown fat have a protein called 'UCP1,' which lets this energy flow out as heat instead of being captured as ATP.
A heater for newborns
Making heat without shivering
Newborn babies have especially large amounts of brown fat. The body also makes heat in the cold by shivering its muscles, but newborns use brown fat to make heat and stay warm without shivering. Animals that hibernate also have a lot of brown fat.
Where it is in adults
Around the neck and collarbones
Adults have less brown fat left than babies. It sits mainly in an area running from the front of the neck, over the collarbones, to the upper chest. Brown fat lying along the spine has also been found.
When it switches on
It works in the cold
Brown fat switches on in the cold. After 2 hours in a cool room at 19 degrees Celsius, body scans showed active fat in the neck and back areas in just over half of the younger people. In a warm room at 27 degrees, the same people showed no such activity.
It differs between people
Less with older age and more body fat
The amount and activity of brown fat differ from person to person. It was seen less often in older people and was lower in people with more body fat. It was seen more often, and in larger amounts, in women than in men.
A related term: beige fat
Brown-like cells inside white fat
Cells that carry UCP1 and make heat like brown fat can also appear within white fat. These are called 'beige fat.' Beige fat cells taken from the white fat of mice had almost no UCP1 normally, but their UCP1 rose sharply when they received a signal.
Common misconception
Is brown fat found only in babies?
No. For a long time, brown fat was thought to exist only in newborns and to be nearly absent in adults. Then body scans found working brown fat around the neck and collarbones of adults, and tissue samples taken from there also contained UCP1.
Summary
The fat that makes heat
Brown fat is fat rich in mitochondria that burns its stored fat to make heat. It is not found only in babies; adults have it around the neck and collarbones, and it works in the cold. Related terms: visceral and subcutaneous fat, ATP, body fat percentage.