Brown fat is different, Murphy said. "(Brown fat) burns fat just to make heat."
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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.