Brown Fat: Your Body's Built-In Heater

Jono Blodgett
Oct 18
5 min read

Updated: Oct 18

Winter is fast approaching and that means many of you are unpacking the sweatshirts, beanies, and extra blankets. But have you ever had that friend that can step outside in shorts and t-shirt when it's 45 degrees as if it were a warm summer's day? What’s their secret? It might have something to do with brown fat, a natural, built-in furnace that helps regulate your body temperature.

Most of us don’t realize we have brown fat, let alone how it works. But this incredible biological feature does much more than keep you warm, it can improve your health in ways you might not expect. Let’s dive into the science, the stories, and how you can harness its power (with a little help from ice baths!).

What is Brown Fat?

When we think of fat, we usually picture the kind we’re always trying to lose, which is white fat. That’s the energy-storing fat that accumulates around your belly and thighs. But brown fat is different. Its main job isn’t to store energy, it’s to burn it, producing heat through a process called non-shivering thermogenesis.

Here’s how it works, stick with me, here comes the science: brown fat cells are packed with mitochondria, the tiny power plants of your cells. These mitochondria burn calories from glucose and other fats to generate heat, especially when you’re exposed to cold temperatures. That’s why brown fat is often referred to as your body’s “internal heater.”

Why Babies Don’t Shiver

Ever noticed that babies don’t seem to shiver when it’s cold? That’s because they’re born with large amounts of brown fat, concentrated around their neck, shoulders, and spine. It's not there just to make them look cute and want to pinch their cheeks. Without much muscle to help generate heat by shivering, brown fat is their first line of defense against the cold.

Years ago, Spanish researchers conducted a fascinating study: they placed one of a newborn’s feet in cool water (around 66°F) and used thermographic imaging to see what happened. Immediately, brown fat near the baby’s neck and shoulders kicked into action, generating heat. Even more interesting, babies exposed to cold on their first day of life showed an even stronger response the next day. It’s as if their bodies were learning and adapting to become better at staying warm.

And if you’re still not convinced that babies are tougher than they look, here’s a fun fact: in countries like Denmark and Finland, it’s not uncommon to see mothers leaving their babies bundled up in prams outside, yes, even in winter. The belief is that naps in crisp, cold air help babies sleep better, stay healthy, and boosts their immune systems. Thanks to all that brown fat, those babies aren’t shivering, they’re literally built for it. Makes you wonder if they’re on to something we’ve been missing!

Losing Brown Fat as We Age

Unfortunately, as we grow older, we lose much of this “magic fat.” By adulthood, most of us have very small amounts of brown fat, located in areas like the neck, shoulders, and upper chest. Scientists used to think adults didn’t have brown fat at all, until the early 2000s, when it was rediscovered during PET scans for cancer imaging.

The decline in brown fat is largely due to modern living. With central heating, warm clothing, and climate-controlled environments, our bodies don’t need to generate heat like they used to. Without regular exposure to cold temperatures, our brown fat becomes dormant. Research shows that while over 15% of people in their 20s have detectable brown fat, that number drops to less than 4% by the time they hit their 50s.

The Surprising Benefits of Brown Fat

When researchers rediscovered brown fat in adults, it sparked a wave of studies exploring its potential benefits. Spoiler alert: they’re impressive.

Improved Metabolism: People with more brown fat are less likely to develop metabolic diseases like type 2 diabetes. A study of 5,000 patients found those with active brown fat had lower blood sugar levels, lower triglycerides, and a reduced risk of high blood pressure and heart disease.
Body Composition: Since brown fat burns calories, it’s linked to lower body weight and better fat distribution. People with more brown fat tend to have less visceral fat (the dangerous kind around your organs) and more subcutaneous fat (the less harmful kind under your skin).
Hormone Production: Brown fat produces hormones, including those that regulate metabolism. According to metabolic researcher Ben Bikman, PhD, brown fat is responsible for producing more than half of the body’s active thyroid hormone.
Brain and Liver Health: Brown fat may even protect against cognitive decline and non-alcoholic fatty liver disease. Studies show it’s linked to lower levels of liver fat, which can prevent serious health complications.

One researcher described brown fat as a potential “fountain of youth,” given its strong connection to metabolic health and diseases of aging.

So what is the key to activating brown fat? Cold exposure!

I'm not gonna lie, when I first started taking ice baths, my skinny ass would shiver for a while after getting out! But now that I've been doing them for years, the water either has to be VERY cold, or I need to spend a long time in the ice bath before I get that shivering effect. Although I've never tested it, my guess is my body has developed more brown fat and is able to warm my body much quicker than when I first started.

When your body gets cold, your sympathetic nervous system kicks into gear. This releases norepinephrine, a hormone that tells your brown fat to start burning fuel for heat. Ice baths are one of the most effective ways to trigger this process.

Here’s how you can get started:

Start Small: You don’t need to dive into freezing water right away. Begin with a cold shower or dip your feet in icy water to get used to the sensation.
Know Your Temperature: For brown fat activation, the water should be cold enough to trigger your “gasp reflex”—that sharp intake of breath when you first get in. For most people, this means water below 60°F.
Short and Sweet: You don’t need to suffer for hours. Research shows that just 11 minutes of cold water immersion per week (broken into several short sessions) is enough to increase brown fat activity and improve cold tolerance.

And now that winter is almost here, for those of you that live in a cold climate, you don't need an ice bath to get these benefits. Next time the temperature drops, go take a 5-15 minute walk outside, but instead of bundling up, wear minimal layers so by the time you finish your walk, you're body has initiated that shiver response.

Brown Fat: Your Secret Weapon

Brown fat is an incredible tool hiding in plain sight, ready to improve your health and resilience. Whether it’s through ice baths, cold showers, or simply spending more time outdoors in chilly weather, reactivating this “lost” fat can help boost your metabolism and protect against aging-related diseases.

So, embrace the cold and let your body’s built-in heater work its magic. Who knows, you might even find yourself reaching for fewer sweaters this winter!

If you’re intrigued and want to experience the invigorating rush of an ice bath for yourself, click here to learn more about how I can bring the sauna and ice bath experience right to your doorstep!