(NaturalNews) Sitting for extended periods of time causes the body to turn off its fat-burning mechanisms, according to a new study published in the journaland presented at the Second International Congress on Physical Activity and Public Health.Researchers injected rats and pigs with a small amount of radioactive fat, then traced where that fat went in their bodies. Among animals that were kept stationary, the fat was removed from the blood vessels in the muscles and instead stored in adipose tissue in places such as the kidneys. Unlike fat in the blood vessels of muscles, fat in adipose tissue is not easily burned.The researchers also found that after animals were kept stationary for several hours, an enzyme called lipase "was suppressed down to 10 percent of normal," according to lead researcher Marc Hamilton. "It's just virtually shut off."Lipase is critical to the body's ability to split apart and break down fat.The researchers then followed up with a study on human participants. They found that when humans were sitting, activity of lipase was suppressed. This led to raised levels of HDL ("bad") cholesterol, reduction in metabolism and increased retention of fat.According to the researchers, the new findings suggest that merely exercising more is not enough to lose weight among people who spend most of their day seated, such as in an office. Nor does using the arms at a computer or video game console provide enough activity to keep lipase active. According to Hamilton, it is essential to actually stand up and move around."When we think about the postural muscles that are mostly in the legs and back, these are big, powerful muscles," Hamilton said. "We're talking probably 20 pounds of muscle in each leg."People who spend much of their day seated need to get up periodically and "putter," Hamilton recommended.Researchers still do not know how long the lipase-suppressing effect of sitting last after activity resumes.