Sitting for extended periods of time causes the body
to turn off its fat-burning mechanisms, according to
a new study published in the journal Diabetes
and 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
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.