Have you ever wondered why the first cold day of fall feels so much colder than a day the exact same temperature at the end of the winter? Its not just a matter of perspective: Your body really is more prepared for cold conditions at certain times of the yearthanks to a mysterious form of fat.

When you experience cold, your body responds in a few noticeable ways. Your blood vessels constricta process called vasoconstrictiontaking blood away from your extremities and keeping it near your core. And you shiver, meaning certain muscles start shaking to produce heat. But shivering also triggers the release of a hormone called irisin, which jump-starts a lesser-known cold weather response: the activation and buildup of brown fat.

Brown adipose tissue, or brown fat, is different from the white fat we might think of when discussing diet or weight loss. White fat lines our skin and muscles, cushioning our organs and bones. But brown fat appears only in specific areas around the neck, spine, aorta, and kidneys. It builds up in clumps around major blood vessels, warming the blood as it passes through the body. If vasoconstriction is closing the window, brown fat is turning on the heater, says Yossi Rathner, a physiologist at the University of Melbourne.

We dont create enough brown fat to cause noticeable weight gain, but the small deposits are still powerful thanks to their high concentration of energy-creating mitochondria. Instead of burning calories to produce energy to power the body, the mitochondria in brown fat burn calories to produce heat. By acting like little heating stations for blood vessels, these soft clumps of insulation help us deal with the cold more efficiently than shivering, which expends a lot of energy, and vasoconstriction, which puts us at risk of frostbite.

Your body goes from a rickety radiator to a smooth central heating system by the end of the winter, says Francesco S. Celi, a professor of medicine at Virginia Commonwealth University.

But the effect only lasts for as long as we need it. When the temperature warms up, brown fat fades away. If we are not exposed to the cold, the brown fat will atrophy, says Barbara Cannon, a physiologist at Stockholm University. There may be a few stem cells left in the area for later regeneration, but it will nearly disappear, Cannon says.

Even a one-month tropical vacation is enough to deplete ones brown fat reserves, Celi says, creating that extra cold sensation a traveler might feel upon returning to a cold climate.

You could say brown fat is winter's undercover hero, dropping in when we need it most, and disappearing once the job is done. So, the next time a chilly day leaves you shivering, just remember help is on the way.

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Brown fat warms your body in cold weather | NOVA - NOVA Next