and it's finally cold enough to wear my favorite winter sweater and jacket and beanie and gloves.

And I drink pumpkin spice lattes to stay warm.

But I also love the feeling when it's the middle of winter and out of nowhere it's 55 degrees.

So I leave my parka home and go outside in my shorts and t-shirt

and I feel the warmth of the sun on my skin.

And I consider going to the beach to tan because it's so warm.

The temperature is the same, but my reaction to it is vastly different.

Part of the reason why is that there's more of a substance in my body in January as compared to September.

It's the same substance that allows kids and teens to handle the cold weather better than adults.

When you're hot, you sweat.

When you're cold, you shiver.

But just before you start to shiver, your body actually does something else.

It starts to burn fat.

But not the fat we usually think of.

There are two categories of fat that we like to think about.

There's the white fat.

White fat is the one that we tend to just think of as "fat".

It's called white because it looks white.

White fat exists all throughout the body,

cushioning our bones and organs.

That's Dr. Aaron Cypess, by the way.

White fat cells have many functions.

One of the most important being that they're the body's primary energy source.

Every single day we are using the fuel that's inside the white fat cells.

But there's another type of fat that has a totally different function:

Brown fat.

These cells are much smaller,

and because of that, resemble muscle more than fat.

Instead of one large lipid droplet in a white fat cell,

the brown fat cell is one tightly packed bag of mitochondria.

This is what gives it its brown appearance.

And if you remember anything from high school biology class,

you know that the mitochondria are the powerhouse of the cell.

Brown fat, instead of serving as an energy reserve for organs throughout the body to burn,

uses its power to burn energy on its own.

And unlike white fat, it's only located in a few tactically positioned areas.

It is in the neck, the shoulders, in the upper arms.

It's down the spine and then some places within the belly.

Lots of overlap with major blood vessels

because warming the blood effectively warms the whole body.

The brown fat around the neck vessels heats up the blood, goes to the brain.

That's good.

The brown fat depots in the shoulders.

That blood, once it's been heated by the brown fat goes right — very quickly, into the heart

and then can get pumped to the rest of the body.

And that's the first line of defense against the cold.

If it's not enough to raise the body temperature sufficiently...

then the body shivers.

But one of the craziest things that I learned while researching this story is that...

babies can't shiver.

When babies are born, they do not have the muscle mass

and perhaps not even so much of the coordination with the brain and the muscle to shiver effectively.

So they need other ways of generating heat,

and brown fat is part of that.

Babies are born with a lot of brown fat.

According to the Cleveland Clinic,

about 2 to 5% of a baby's body weight is made up of this stuff.

It's necessary for our survival.

The peak age for brown fat actually seems to be in the teen years,

which explains why a lot of the guys in my high school wore shorts in the winter.

But as we get older, our body makes less of it.

And this could give us some insight into why kids seem to run hot

and our parents or grandparents seem to run cold.

Older people, they certainly have much less brown fat in terms of absolute and relative amounts.

There's also less muscle,

which is also important for generating heat.

The similarities between brown fat and muscle don't end there, though.

If you work your brown fat out, it also increases volume.

And the way you do this is by exposing yourself to the cold for extended periods of time

as this 2014 study shows.

Look at how much more brown fat these men from Maryland had after a month of exposure to cold temperatures.

And look how much less they had when exposed to warm temperatures.

Which gives us some insight into why 55 degrees in the winter can feel so much warmer than 55 degrees in the fall.

Brown fat doesn't just keep you warm, though.

Scientists have figured out that having more of it correlates to lower risk of diabetes and heart disease.

Studying the effects of brown fat using cold exposure is challenging, though.

One challenge is that it's hard to dose cold.

How much... I gave you this amount of cold,

I mean, you know the temperature,

but it's hard to know what you're doing

and therefore it's very hard to design a study where it's reproducible.

So instead,

they're trying to activate brown fat with medicine to study how burning it affects us biologically.

A medication called Mirabegron,

which is approved to treat overactive bladder,

was able to activate the brown fat in a way that was very similar to the effectiveness of the cold exposure.

There's a lot we still don't know about brown fat,

but we do know that our bodies use it to adapt to cold weather.

And when a kid doesn't want to put a coat on in the winter,

it's not necessarily that they're just stubborn.

It might just be because they have a little more of something in their body than you do.