DEAR DOCTOR K:
In a recent column about abdominal fat, you talked about two kinds of fat — brown fat and white fat. I’d like to hear more about them.
I’m glad you asked, because the discovery of these two types of fat could prove to be very important. In the column you’re referring to, I discussed how visceral, or abdominal, fat (which accumulates deep inside the abdomen) is more harmful to our health than subcutaneous fat (the fat just beneath the skin). But when it comes to fat, it’s not just location that matters. Color counts, too — and brown is better.
Humans and other mammals have two types of fat: white fat and brown fat. (There may also be a third kind that’s in between, sometimes called “beige fat.”) Until very recently, we thought that adult humans had only white fat. We knew some adult animals had brown fat, and that human babies did, too. But we thought babies lost their brown fat as they grew older.
Discoveries here at HarvardMedicalSchool found that all of us maintain some brown fat cells throughout our life. Why is this important? White fat cells store fat. When we get “fat” — when those love handles start to appear — that’s because we’re growing more and bigger white fat cells.
Brown fat cells, on the other hand, don’t store fat; they burn it. They are brown because they contain lots of mitochondria, the energy factories inside each cell. The mitochondria in brown fat cells burn fat to create one kind of energy: heat.
Animals with relatively more brown fat are less likely to develop insulin resistance, a precursor to diabetes. They’re also less fat, because they burn more fat and store less fat. In other words, it appears that brown fat wards off obesity.
These findings would be interesting — but not particularly relevant — if we couldn’t control the amount of white and brown fat in our bodies. But ongoing research suggests we can.
Colleagues here at Harvard have recently discovered natural chemicals in all of us that affect the number of white and brown fat cells. One is a molecule called irisin. This hormone is produced by muscle cells, particularly when those cells are regularly exercised. It circulates through the blood and transforms white fat cells into brown fat cells.
When obese mice with insulin resistance are given injections of irisin, they lose weight and have reduced insulin resistance. This discovery has excited scientists, for obvious reasons. If it worked the same way in humans, and without side effects, it could have huge health benefits.
A word of caution: Many “silver bullets” for obesity have come and gone. Still, I’d bet on this newly discovered molecule, and on other advances triggered by the discovery that we adult humans have brown fat within us.