Finn: Brown fat! Let's find out more about this from Dr Wouter van Marken Lichtenbelt, who's been speaking to the BBC's Health Check programme. What happens to brown fat when it is activated?Dr Wouter van Marken Lichtenbelt, Biologist, Maastricht University Medical Centre:Brown fat cells are not really filled with fat, but with other particles that can increase energy metabolism a lot. So if brown fat gets activated it produces heat so it warms up the body.