On a recent podcast I did to promote my upcoming book ‘Cold Water’ I was asked about brown fat (or, ‘brown adipose tissue’), which is a type of fat that generates heat and warms up the body. I realised I knew what brown fat did, due to swimmers talking about it in a certain way – like it dons a cape and soars around our insides, with one arm outstretched. I didn’t really know what brown fat was in any real detail though.

So, I’ve done my own nosedive into some scientific jargon and a few popular studies to see what I could find. Before we plunge in headfirst, there are a few key points to keep in mind. We know brown fat is activated when your body descends to colder temperatures. It stores energy and enables your body to burn calories. When you swim, blood flow increases to your muscles and stored fat is burned to enable you to expend more energy. Brown fat helps to regulate your body heat, glucose and fat metabolism. That’s mostly why cold swimmers tend to bring it up in reverent murmurs.

If you’re a cold-loving swimmer who relies on brown fat for heat production, this blog is for you…

What are the different types of fat?

The various types of fat in your body are commonly identified by their colour and function. Lots of folk have heard of white and brown fat, but did you know there was a beige fat too? Here’s what they each do in more detail:

White Fat – This is the fat that insulates your organs. Most of the fat in your body is white and used to store energy throughout your body. Too much of this white stuff will lead to obesity.

Brown Fat – Compared to white fat the brown stuff takes up less space to store energy. It then burns that energy to regulate your temperature and that’s why it’s so crucial to cold water swimmers. The term thermogenesis refers to brown fat generating heat and helping you burn calories before the shivers set in. It also helps to regulate glucose and fat metabolism.

Beige Fat – Last up, as the colour mixture suggests, beige fat is a blend of white and brown fat cells. These cells convert white fat cells to brown, helping us to burn calories and regulate our temperature as well.

 What does brown fat do?

There was a time when cold exposure seemed like a druidic pursuit for liberated Scandinavians. Now, everyone is in on the action, from the surf-sloshed Icebergs pool in Sydney to the tidal pools of Northern Ireland. All over social media you see folk sprinting to icy waters, trailed by wings of sauna steam as they clamber to that soothing fix, like lobsters leaping to freedom from a pot of boiling water. We know it’s good for us from how it makes us feel. What’s less known is what it does for us internally – how it affects the metabolism of mitochondria-rich brown fat, allowing us to better adapt to extreme heat and cold. Let’s say brown fat (back to this superhero analogy) is Batman and white fat is the Joker. White fat gathers energy. Brown fat use it. They are opposites. White fat also seems to collect in your bum and belly and isn’t very helpful if it builds up to excess.

When we enter the cold, brown fat is activated by noradrenalin and quickly increases up to 250%. This superhero of ours uses sugar and fat from your bloodstream to fuel your body and fend off hypothermia. It’s like an internal radiator – some ice swimmers call it a ‘furnace’. Meanwhile, brown fat uses suspended calories and increases our insulin sensitivity, enabling cells to use blood glucose more effectively. It's really described and works more like a muscle than a fat. Of course, it can also be strengthened like a muscle if we go full Scandinavian and expose our bodies to the cold for prolonged periods.

Why do we need brown fat?

Any swimmer who immerses themselves in cold water relies heavily on brown fat. It produces heat to warm the blood in our bodies and it does so when fat is activated by something shiver-inducing – say, a cold hole of glacial water you’re just drilled out of a frozen lake somewhere. Brown fat dons its little cape before we start to shudder. It warms us up by breaking down glucose (also called ‘blood sugar’) and fat molecules. This is the process we call thermogenesis, which is needed to maintain your body temperature at healthy levels, even when you’re somewhere most folk call you crazy for being, like squatting in said frozen lake.

So, if you want to attribute some powers to this little hero of ours, brown fat maintains a healthy temperature, burns calories, produces and stores energy and also helps to control your blood sugar and levels of insulin.

What does brown fat look like?

The anatomy of brown fat is more like muscle than other body fat. Brown fat is made of molecules of fatty acids and glycerol. These molecules are fat cells and the ones that store brown fat in tissues are smaller, wadded together in lumps to form a brown, oval-shaped mass. The reason this fat is brown has to do with it being full of mitochondria, which contains a lot of iron and gives the fat its colour. In newborns, you’d find this fat wadded in their back, neck and shoulders. Then brown fat starts whipping around our body as we grow older. Until, when we’re adults, it’s all been packed around our neck, kidneys, heart, adrenal glands and chest. Like all good superheroes it goes where it’s needed, cosying up to our vital organs.

How does brown fat vary from person to person?

In different folk you’ll find varying amounts of brown fat, but there’s always more of the white stuff. Newborns have the most brown fat with it making up 2-5% of their body weight. Then it reduces until we’re adults with only a small amount of brown fat left. Lean athletes, who exercise more often, tend to have more brown fat than most. This also depends on the food you eat and how much fat you’re taking in.

Fat is needed to produce energy and some sources of fat can be more healthy for us, like avocados, nuts, fish and yoghurt. This ‘good’ fat can be combined with vegetables, whole grains, protein and dairy for a well-balanced diet. While saturated fats come from processed foods and unhealthy but intoxicatingly tasty treats, like white chocolate. These are foods we should generally avoid as they fill our bodies with unhealthy fat.

How do we strengthen our stores of brown fat?

There are ways to increase the amount of brown fat you store – one way to lose weight and burn calories as well – like exposing yourself to colder temperatures. Cold showers, ice baths and outdoor swims are activities that produce brown fat and enable you to burn more calories. Brown fat is also rich in iron, so a supplement or food that is iron-rich (say, seafood, whole grains, vegetables and beans) helps your fat cells to stay healthy. In short, a well-balanced diet gives armour to your caped crusaders. Try to avoid processed foods or bouts of overeating, plonked in front of Netflix. Seek out foods with ursolic acid, like apples and dried fruit, which activate brown fat. Also, inactivity encourages your brown fat to hang up its cape and descend into sedentary life as a pudgy retiree. When you exercise your body activates the blood hormone, irisin, which burns white fat like brown fat and kicks beige fat into gear as well.

In short, brown fat has all kinds of positive influences on your body and your ability to maintain your core temperature and produce energy. If you’re ever wondered how ice swimmers swim long distance in sub-5C waters (without that trusty neoprene suit) – brown fat is one of the factors that gets us closer to an understanding. To reiterate, if we want to build up this defence we need a have a well-balanced, exercise regularly and condition ourselves to get close to shivers in a controlled environment, like a cold shower or a safe body of wintry water.

We hope you found this article helpful. You can pledge for copies of Jack’s new book ‘Cold Water’ to learn more about cold immersion and follow a four-year adventure of shudders from Guernsey to Iceland – it’s funding right now on the award-winning Unbound platform!