What is brown fat?

What do Antarctic explorers and newborns have in common? A nice layer of brown fat to keep them from hypothermia.

“I’m envious of my chilled Melbourne friends,” laments Paul Lee, a brown fat researcher based at Sydney’s Garvan institute. It’s not just that it’s harder to do his research in a subtropical climate; it turns out brown fat might also stave off diabetes and obesity.

But what exactly is brown fat and why is it good for us?

We have two types of fat. The vast majority is white; less than 1% is brown and it’s mostly distributed like a collar around the neck. Why here? Possibly to keep the blood supply to the brain well-warmed, Lee suggests.

Until 2009 it was thought that brown fat, though important for infants and rodents, largely disappeared in adults. But a 2009 study in the New England Journal of Medicine found that adults still carry brown fat. Proportional deposits were heftier in colder climates, women carried twice as much as men, and generally slim people carried more than the obese, especially as they aged.

These findings spurred a wave of new research aimed at discovering exactly what brown fat does, and whether raising its levels might be a way to fight the obesity epidemic.

White fat gets deposited all over the body but most commonly accumulates around the belly. It’s called white fat because the deposits, stored in fat cells, are white.

Brown fat bears more resemblance to muscle than fat. Like muscle, its brownish tinge comes from mitochondria – the fuel-burning engines of the cell – that are dotted through the fat cells. The entire tissue is threaded with blood vessels to keep up the oxygen supply. If white fat is a fuel store, Lee says, then “brown fat is the furnace”.

So if brown fat is burning fat, why don’t the deposits just disappear? In fact they do. But new fat is supplied from the circulation when the supply runs out. Lee knows this from autopsy studies of drowning victims who experienced severe hypothermia. The brown fat furnaces are empty, having burnt their stores trying to keep the victims warm.

Because brown fat is so good at burning glucose, Lee’s team wondered if there would also be immediate health benefits. In particular, would it benefit people at risk of developing diabetes where blood glucose levels rise to dangerous levels that poison the blood vessels? To find out, his team carried out two studies, one in 2014 and one in 2016.

In the first study, healthy volunteers spent their nights sleeping in a room fixed at 19 ˚C wearing a standard hospital gown. Not snug enough for me, but Lee says they had no trouble sleeping. After one month, the activity of the volunteers’ brown fat was measured by seeing how much radioactive glucose (a harmless dose) was burned in their neck and comparing that to how much they had burned a month before. The increased activity corresponded to a 44% increase in their brown fat deposits. Interestingly, brown fat burn also hewed to a circadian rhythm as shown in the 2016 study; it was most active in the hour before dawn, presumably helping the volunteers gear up for the day.

The volunteers with the highest levels of brown fat also performed best at controlling their blood glucose levels – a sign they were at less risk of developing type 2 diabetes.

Lee followed up this finding by studying how people handle sugar at different temperatures, using a New Zealand database of some 65,500 patients. The study looked at the year-long results of the so-called HbA1c test, which measures what percentage of haemoglobin proteins are sugar-coated – a long-term proxy of how high blood sugar levels have been. Those living at lower outdoor temperatures have lower levels of HbA1c, indicating better glucose control.

Brown fat may do more than just burn glucose; it may also spread its good influence by releasing factors that “brown up” the white fat.

Researchers haven’t pinpointed this factor but, for now, Lee has shown robust “fat browning” action by fibroblast growth factor 21 (FGF-21) in the laboratory. Treatment of white fat cells with FGF-21 transformed them into fat-burning brown fat cells.

Once researchers have nailed it, we might be in for a new type of fat-burning pill.

Until then, Lee’s advice is to not just rely on staying cool. “ Brown fat is hot and exciting but it is not the solution for obesity, at least not based on what we know now,” he says. “The commonsense things are true: diet and exercise.”