Now think of your body. Just like your house, you want to keep it well-maintained, keep fuel and water coming in and keep invaders out.
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Now, suppose your housemate obsessed about being warm and started bringing home a steady supply of fuel for the furnace. Not a lot. Maybe two gallons a day. Every day. In winter, that would be great. Keep that fuel tank topped up.
But in summer, when you don't use the furnace, you'd need to store that extra fuel. So you build a second storage tank. And then a third. And more and more.
This is essentially how our body's metabolism works. If there's more fuel brought in than we need, we store it. A house can't get obese, but you can, if you store too much fuel in your body.
One key to having a healthy size is knowing about your body's metabolism.
The skinny on fat
According to Bernard Murphy, associate professor of biology at Hagerstown Community College, fuel comes into the body in three forms — carbohydrates, protein and fats.
Protein is mostly used for construction and repair in our bodies, but it can also be converted to fuel when needed, Murphy said in his office in the new HCC Science, technology, Engineering and Mathematics Building. But protein can't be stored. Excess protein is converted to fat and stored.
Carbohydrates, he said, are consumed by the body fairly quickly. But extra carbs are stored.
"(They are) stored as glycogen in the liver and muscles — for short-term use between meals," Murphy said.
The third form of fuel, fats, can also be stored for future use.
There are two kinds of fat cells — also called adipose tissue — in a human body. (See chart on this page.) White fat cells are plentiful and typically found around organs and in a layer beneath the skin. Brown fat cells are thinly scattered in the upper chest and neck and along the spine and function like tiny furnaces. They literally create heat.
'Work to stay alive'
Brown fat cells — also called brown adipose tissue, or BAT — are common in hibernating mammals and in human infants, but, until recently, researchers believed brown fat cells were absent in adult humans.
"The brown fat cells are interesting. They are tissues used by true hibernators, such as squirrels and groundhogs," Murphy said. "These hibernators' body temperature is just a few degrees above freezing. Every now and then, the brown fat cells warm up to regular body temperature and then the animals go back down deep."
Human infants don't hibernate, but they still need brown fat cells to survive, Murphy said.
"They are very susceptible to hypothermia," he said. "It was known they had brown fat. But it was thought that adult humans had very, very little."
White fat cells store fuel that can be converted into ATP, an energy molecule needed by virtually every cell in the body to accomplish its work.