Sitting is one of the causes of being overweight :

The radioactive tracer revealed that when the animals were sitting down, the fat did not remain in the blood vessels that pass through the muscles, where it could be burned. Instead, it was captured by the adipose tissue, a type of connective tissue where globules of fat are stored. That tissue is found around organs such as the kidneys, so it's not really where you want to see the fat end up.

The researchers also took a close look at a fat-splitting enzyme, called lipase, that is critical to the body's ability to break down fat.

After the animals remained seated for several hours, "the enzyme was suppressed down to 10 percent of normal," Hamilton said. "It's just virtually shut off."

The results from the animal studies were very convincing, he said, and human experiments were just as compelling. The researchers injected a small needle into the muscles of the human volunteers and extracted a small sample for biopsy. Once again, the enzyme was suppressed while the humans remained seated. That resulted in retention of fat, and it also resulted in lower HDL, the "good cholesterol," and an overall reduction in the metabolic rate.

original article : http://abcnews.go.com/print?id=3922069