How to boost your calorie-crunching brown fat in the cold winter months

Brown fat burns energy when we are cold. Now scientists are trying to harness its powers to fight obesity and type 2 diabetes.

When the nights draw in, the leaves fall off the trees and the outside world becomes blanketed in frost, many of us will be tempted to reach for the thermostat and crank up the heating. When venturing outdoors, we'll likely wrap up in a scarf and hat, piling on the layers in a desperate attempt to keep warm. Yet being cold and uncomfortable could be the key to improving health and losing weight.

The reason is all down to a special type of fat called brown fat. Unlike white fat, which clings stubbornly to thighs, waists and hips, brown fat burns calories, converting them into heat when we are cold. Some believe that if harnessed properly it could make us thinner. Intriguingly, all we theoretically need to do to activate it is to hop into a cold shower or eat lots of spicy foods and caffeine.

But before you jump into an icy lake or down that fourth cup of coffee, it is worth exploring whether the hype behind brown fat is justified.

How is brown fat different?

When we are born, each of us has plentiful stores of brown fat which act like an internal heater. Babies don't have enough muscle mass to shiver, so they rely on brown fat to convert sugars and fats into heat.

Brown fat cells have a disproportionately high number of mitochondria – the energy producing factories inside cells. But unlike normal mitochondria which produce adenosine triphosphate (ATP) – a kind of universal energy currency the body can use – the mitochondria inside brown fat cells contain a protein called thermogenin, or UCP1, which allows them to convert calories from food directly into heat instead. 

"When stimulated, brown fat has the capacity to produce 300 times more heat per unit mass than any other tissue or organ in the body," says Michael Symonds, professor of developmental physiology at the University of Nottingham in the UK.

Getty Images

Most of what we know about this brown fat comes from studies on small mammals such as mice and rats. These rodents have large reserves of it, which helps them keep toasty and warm during the winter months when they go into hibernation.

"For our ancestors, or for small animals like mice, cold temperatures would have been a major threat to survival, so it was advantageous to have a tissue that could convert energy sources into heat," says Paul Cohen, an associate professor at The Rockefeller University, New York, who studies molecular metabolism.

Decades of research shows that, in mice at least, brown fat sucks up sugars and fats from the bloodstream. Brown fat also appears to protect mice from obesity and metabolic diseases linked with weight, such as diabetes and heart disease.

Brown fat in adults

Adult humans are, however, adept at finding other ways to keep warm – turning the heating on, snuggling under a thick duvet or pulling on an extra-warm coat. It was widely assumed that brown fat all but disappeared by puberty for this reason, with the traditional white thigh-hugging fat we are all familiar with taking its place.

And this also meant that most scientists believed it was impossible for brown fat to play a meaningful role in health beyond infanthood. This changed in 2009, however, when researchers from Finland and Sweden showed that human adults not only still have brown fat, but in temperatures below 16C (60.8F), the fat activated and started absorbing glucose and fats from the bloodstream.

Not only that, there was a clear correlation between a person's body weight and their brown fat reserves. Thinner, leaner individuals had more, and obese people less. Those with more brown fat also had higher metabolic rates in the cold. Together, this suggested that brown fat could be a target for the treatment of obesity.

The field received a boost in 2021, thanks to a study by Cohen and colleagues at The Rockefeller University in New York. Cohen and his team examined positron emission tomography (PET) scans of over 52,000 people. They compared people who had no discernible brown fat with individuals who did, finding that rates of type 2 diabetes, heart disease, stroke and high blood pressure were much less common among those with brown fat. They also found that leaner people were more likely to have brown fat than individuals with obesity.

What's more, people with larger reserves of brown fat also had lower levels of glucose and triglycerides (a type of fat) floating around in their blood stream, improved insulin sensitivity, as well as more of the "good" kind of cholesterol – high-density lipoprotein (HDL) – all signs of a healthy metabolism.

Can brown fat lead to weight loss?

It's not clear whether brown fat is actually responsible for these health benefits, however. The truth is that our bodies are probably too large, and contain too little brown fat, for it to burn through enough white fat to make a meaningful difference to our weight. Most adults have a vanishingly small amount of the stuff – just 0.02 to 300g, or less than 0.5% of the total human body mass.

"Whereas white fat has almost an unlimited capacity to expand – there are some people who have over 100kg (220lb) of white fat – brown fat probably measures in the hundreds of grams at most," says Cohen.

Getty Images

However, it's possible that, while brown fat might not help you shed the pounds, it could still boost your health in other ways. For instance, instead of burning calories, brown fat could help to improve metabolic health – the body’s ability to process and use energy from food effectively. People who have poor metabolic health are unable to clear glucose from their blood quickly, raising their risk of type 2 diabetes. Brown fat could act simply by taking up glucose from the bloodstream, or by secreting hormones that act on other tissues to regulate insulin sensitivity.

"In terms of whole-body energy expenditure, it [brown fat] probably does not play as large of a role in humans as it does in mice," says Cohen. "So my personal feeling, based on the data, is that brown fat activation is not going to be a good means to treat obesity or to lose significant amounts of weight." 

"However, the data is much stronger to suggest that activating brown fat might lower glucose levels and improve metabolic health. So, I think if there is going to be a therapeutic side to this, it's much more likely going to be in treating the complications of obesity and ageing than in treating obesity per se."

But how can we activate brown fat, and harness the superpowers it may – or may not – contain? One way is to expose yourself to cold, as uncomfortable as it may be. Think ice baths, cold water swimming, or cryotherapy chambers – a specialised device that exposes the body to extremely low temperatures for one to three minutes to promote healing and wellness. The shock of cold-water immersion causes the body to think it's in danger, activating the fight-or-flight response. This releases the neurotransmitter noradrenalin, which binds to and "activates" brown fat cells.

Susanna Søberg, a metabolic scientist who founded the Søberg Institute in Denmark to promote the health benefits of cold and heat therapy, has spent years studying the effects of cold-water swimming on the human body. In one study, her team compared men who regularly plunge themselves into icy Scandinavian lakes during the winter with men who do not perform this seemingly insane practice.

The participants were given a glass of sugar water, and then two hours later their glucose and insulin levels were measured. The cold-water swimmers were able to clear the glucose from their blood much faster and had better insulin sensitivity. The research showed that just a couple of minutes of exposure to freezing cold temperatures two-to-three times a week was enough to activate brown fat, improve insulin sensitivity, and lower stress.

However, it could be reasonably assumed that people who regularly take icy plunges are also more likely to swim or exercise generally, which could explain their better health. 

Cohen's group are also looking into whether the same effects can be found in people who regularly swim in the icy lakes of Minneapolis, Minnesota, in the winter.

"In the winter it's cold enough there that the lakes freeze, so they [the swimmers] often have to cut a hole in the ice to get in," says Cohen.

Boosting brown fat levels

However, it's possible that the same effects could be achieved with cool – but not freezing – temperatures. For instance in a 2014 study, five healthy men spent one month sleeping in a cool room heated to 19C (66F), with just thin hospital clothes and bed linen to keep them warm. After the trial had ended, the men had increased their brown fat volume by 42%. The men's insulin resistance, a key measure of health, also improved. However once the researchers cranked up the thermostat to 24C (75F), the brown fat disappeared again.

In 2013, meanwhile, researchers in the Netherlands subjected 17 healthy adults to temperatures of 15-16C (59-60.8F) for six hours a day. Just 10 days of this treatment was enough to increase brown fat activity and reduce shivering, with the participants reporting that after the trial they felt better able to tolerate cold.

Getty Images

Cohen's group, meanwhile, are currently looking into whether wearing a "cold vest" for three hours a day could have the same effects.

"The cold vest looks almost like a life jacket and it has cold water circulating through it," says Cohen. "The goal is not to have people shivering, but to get to the temperature when they start shivering and then raise the temperature by about two degrees Celsius."

It's possible that turning down the thermostat or jumping in a cold shower could also improve health in a similar way. One study showed that living in a room heated at 14-15C (57.2-59F) was enough to increase the insulin sensitivity of eight type 2 diabetes patients, while a randomised control study from the Netherlands found that people who took a cold shower every day had fewer sick days at work – with just 30 seconds of standing under freezing water enough to provide a benefit.

"Small studies suggest that things like exposure to cold showers may have beneficial effects, although the type of person that might undergo that may be a healthier person anyway," says Symonds. "Most people consider it quite a severe challenge to have a cold shower, even for a short period of time."

So, are there other ways to boost brown fat without becoming unbearably cold?

Symonds' research suggest that caffeine, and specifically coffee, can stimulate brown fat, and encourage it to start burning glucose and converting it into heat. However, other scientists argue that to reap the benefits, you'd need to down 100 cups of coffee a day, an unrealistic amount even for the most dedicated of coffee fans.

Capsaicin, a compound found in chilli peppers, has been shown to activate brown fat in mice, and there are signs it could do so in humans too. In one study, 10 men took capsaicin pills every day for six weeks. At the end of the trial, the men who took the pills had greater brown fat activation when subjected to cold temperatures than they had done at the beginning of the trial.

More like this:

• Why some people can deal with the cold

• How living in a cold home affects your health

• Five myths about staying warm in cold weather

So, should we all be leaving the thermostat alone and embracing the cold weather a little more? Possibly, although if you want a sure-fire proven way to boost your health, there are probably other things to try first. Cohen points to eating a heart-healthy diet; doing regular exercise; making sure your cholesterol, blood pressure and blood glucose are well controlled; and being at or close to a healthy body weight as being most important.

It's also vital to consult a doctor before trying cold-water swimming, as it may not be safe and suitable for everyone. Symonds believes that activating brown fat could play a part in improving health and losing weight, but only alongside other health-boosting lifestyle changes.

"When we published our study on the benefits of caffeine for activating brown fat, I had people writing to me saying I have 10 cups of coffee a day, and I still have a weight problem," says Symonds.

"It's not that the more caffeine you have, the more likely you are to lose weight. Caffeine is one potential factor that can simulate brown fat. But ultimately, it's about having a healthy, balanced diet and having a healthy lifestyle, keeping active, avoiding being sedentary for too long. And I think avoiding processed foods."

Disclaimer

All content within this column is provided for general information only and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. The BBC is not responsible or liable for any diagnosis made by a user based on the content of this site. The BBC is not liable for the contents of any external internet sites listed, nor does it endorse any commercial product or service mentioned or advised on any of the sites. Always consult your own GP if you're in any way concerned about your health.

--

For trusted insights into better health and wellbeing rooted in science, sign up to the Health Fix newsletter, while The Essential List delivers a handpicked selection of features and insights.

For more science, technology, environment and health stories from the BBC, follow us on Facebook and Instagram.