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.
"Everyone has to work to stay alive," Murphy said. "But we can't store ATP. It's too unstable. But you can utilize fat to make ATP. So we store fat in case we miss a few meals."
Brown fat is different, Murphy said. "(Brown fat) burns fat just to make heat."
This process of making body heat is called thermogenesis, according to Dr. Sheng Bi, associate professor of neuroendocrinology at Johns Hopkins University School of Medicine.
"Brown adipose tissue is the thermogenesis organ. (It) will produce the heat and keep the body warm," Bi said. "White adipose tissue, that mainly stores energy. But the brown adipose tissue has a lot of mitochondria and many small lipids (and) produces heat."
It's good to be cool
Lots of things affect our metabolism, such as genetics and our level of physical activity. Another thing is the temperature outside the body.
"If we're cold, we crank up the internal furnace," Murphy said. "Shivering is another way to warm up. When we shiver, antagonistic muscles — pairs of opposing muscles, like biceps and triceps — contract at the same time."
Shivering warms up the body, but it has a downside. When muscles are contracting involuntarily, they are not fully available for other uses, like escaping from enemies, hunting wild beasts or gathering food.
"It puts the body more at risk," Murphy said.
But the body has a backup system to delay the onset of shivering. Bi cited the work of a research team from the University of Sherbrooke in Quebec, Canada. The researchers reported that when a group of test subjects were kept chilled, their brown fat burned stored fat to maintain body heat.
"This nonshivering thermogenesis, this produces heat, makes the body warm," Bi said. "The easy way (to think of it) is brown fat produces heat."
Burning fat means a reduction in stored fat, of course. So is chilling your body a potential technique to lose weight, a way to burn more stored fat? Or can brown fat cells be "turned on" without chilling the body?
Bi said more research is needed.
"I think they still have a long way to go," he said.
Crank up your metabolism
Much is already known about how our bodies' metabolism works. And it's no secret how to lose weight: Just like in the earlier example of your house and your heating fuel supply — consume only as much fuel as you need.
And you don't need to wait for more research on brown fat cells to get rid of your body's excess stored fuel.
Dr. Dan Sullivan, a physician with Parkway Neuroscience and Spine Institute, said knowing a bit about the details of metabolism helps when trying to lose weight. There are three stages to energy consumption — readily available ATP; quickly available stored energy; and oxygen-based slow-burning energy.
Take running, he said. Your body maintains a small supply of ATP — the molecule that fuels cell activity — ready for immediately action. So, Sullivan said, start fast.
"If you're running a race, the first few seconds is strictly (using) the ATP that's already made. It's like free energy," he said. "(After) you use up your ATP, now you've got two minutes of (stored) energy before your system will start breaking down and making lactic acid to stop you from running."
Lactic acid produces a feeling or achiness and tiredness in muscles. It's a signal that your body is switching to the third energy source. This Sullivan called the aerobic system — it uses oxygen and it's good for long-term activity.
"Say you're running a fast mile — you can't run that mile and go another mile. You've built up lactic acid," Sullivan said. "In longer events, you pace yourself (and use) the aerobic system. You really can go a long time before you're going to build up metabolites which stop you from running."
Use this knowledge to maximize the body burning stored fat, Sullivan said.
"If your goal is to lose weight, maybe you want to lose a dress size, do intervals," he said. "Rev up your system by doing interval training. Go really fast for a half-minute, then jog for two minutes, then fast, then slow. You rev up the metabolism."
Adjust this fast-slow-fast-slow pattern to suit your body, Sullivan said.
"If you can't do fast, intense stuff," he said, "the long, slow stuff is good. In the long run, (the point is) burning more calories."
The lesson is still basic: Eat only what you need and be physically active. And your body — your house — will be a good home.