New research shows drinking a cup of coffee can stimulate what is known as "brown fat". That's the fat in your body that keeps you warm by burning calories.

Dr. David Agus, a CBS News Medical Contributor, says "Normal fat stores energy. It stores calories and brown fat has lots of mitochondria which have iron, which is why it's brown and its purpose is to generate heat. The goal is to stimulate brown fat. Exercise stimulates brown fat, good sleep stimulates brown fat, and now we know caffeine or coffee can do the same."

The study published in Scientific Reports is one of the first to be carried out in people.

Previous research shows drinking coffee may have many health benefits, including a lower risk of diabetes and cardiovascular disease.

But too much may not be a good thing. Dr. Agus says, "Up to three cups a day may have a health benefit, no detriment. Don't do more. More isn't better.” He suggests avoiding coffee after one or two in the afternoon. Dr. Agus says, "caffeine's half-life is six hours, so if you have a 1pm, still it's only going to be half the level by seven o'clock at night. And so even if you sleep, it's not going to be the deep restful sleep with caffeine on board."

Researchers are now studying caffeine supplements to see whether the effect on brown fat is similar.

Experts say people who drink more than 4 cups of coffee a day may want to cut back if they are suffering side effects such as headaches, insomnia and nervousness.