Ice baths, chilli-laced meals, cryotherapy chambers – if you listen to influencers, these are the keys to unlocking a healthier life.
The goal? Activating brown fat. Stimulating this so-called ‘good fat’ has become social media’s latest obsession, being credited with everything from melting pounds to slowing ageing.
Here’s the kicker: there may be some truth to these claims. Researchers hope that harnessing brown fat could genuinely improve health. But – as with most things in the body – it’s rarely as simple as plunging yourself into freezing water and hoping for the best.
So, is that icy morning plunge actually worth it for weight loss? Here’s what the science says.
Brown fat is a hidden heater
Have you ever cuddled a newborn baby and felt like they were radiating warmth? This isn’t solely due to their lovely nature – it’s brown fat at work. Newborns are packed with this special tissue, which acts like an internal heater.
But what actually is brown fat? Also known as brown adipose tissue, it’s one of three types found in mammals, alongside white and beige. While our white fat turns fuel into energy stores, our brown fat has an exceptional ability: it turns fuel into heat.
Beige fat sits somewhere in between and may help with weight loss – but more on that later.
One way brown fat is ‘activated’ is when cold triggers the release of hormones from the brain – specifically norepinephrine – and switches on the process that turns energy into heat instead of storing it, thus burning energy.
When norepinephrine binds to proteins (called beta-3-adrenergic receptors) at the surface of the cell, the cell starts breaking down fat stores into smaller molecules. These are known as free fatty acids, and they’re then released into the blood stream.
In small mammals, brown fat is crucial for survival. Mice, for example, can have up to 10 per cent of their body weight made up of brown fat, which keeps them toasty through winter hibernation.
Human babies, too delicate to shiver, rely on it during their early months before they build up the muscles and fat they need to regulate their temperature more actively.
But the benefits of brown fat go far beyond heat. Decades of research, mostly done in mice and rats, suggests brown fat can burn through vast amounts of energy, sucking up sugars and loose fatty molecules from the bloodstream. When scientists chill mice, their activated brown fat appears to ‘melt’ other fat away.
Brown fat also seems to protect against metabolic diseases linked with weight, like diabetes and heart disease.
“There’s very compelling evidence in mouse models that brown fat plays a really important role in energy metabolism and body weight,” says Dr Paul Cohen, a physician and scientist at The Rockefeller University in New York, in the US, who studies brown fat, obesity and metabolic disease.
When it comes to humans, however, the picture is a little more complicated. Adult humans have other ways to stave off a chill – we can shiver, stay near a fire, or slip on a sweater – so we don’t really need to hold on to brown fat beyond infancy.
That’s why brown fat dwindles to nearly nothing after early childhood. So much so that, until the mid-2000s, scientists studied it primarily for its heating properties in small rodents, believing that this good fat completely disappears in adulthood.
It all changed in 2009, when three papers published in The New England Journal of Medicine showed that humans not only carried brown fat into adulthood, but that the tissue could be activated when adults were exposed to cold.
The studies showed that in temperatures below 16°C (60.8°F), regions of the body known to contain brown fat increased their uptake of glucose – a sign the tissue was active. People with more detectable brown fat also had higher metabolic rates in the cold.
These findings sparked a surge of interest in brown fat’s potential for human health.
“Those three papers are viewed as heralding the rediscovery of brown fat in humans,” says Cohen.
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Brown fat in humans
On average, a healthy adult carries about 25–100g (0.9–3.5oz) of brown fat, mostly nestled in the neck, under the collarbones, and along the spine. That’s about as heavy as one to four AA batteries, representing less than 1 per cent of your total body mass.
Some adults have no detectable brown fat at all, suggesting that humans can function just fine without it.
So what role, if any, is this tiny stash of tissue playing in our health? In a landmark study for the field, published in 2021 in the journal Nature Medicine, Cohen and his colleagues offered major clues.
The team studied brown fat in PET scans of over 52,000 people. They found that among those who had detectable brown adipose tissue, conditions such as type 2 diabetes, heart disease, stroke, and high blood pressure (also known as hypertension) were much less common.
These people also had lower levels of glucose and triglycerides (the blood fat that acts as our energy source but that’s harmful in large quantities) circulating in their blood, and higher HDL, or ‘good’, cholesterol. These are “all the markers of more favourable health,” says Cohen.
The study also found that leaner people were more likely to carry brown fat than heavier people, and among those with obesity, people with more brown fat had fewer obesity-related health problems.
Cohen is cautious not to overstate the findings. These are associations, not proof of cause and effect – meaning it’s not yet clear whether brown fat is directly responsible for these health benefits.
Still, the data suggest that in humans, as in mice, having more brown fat may be a sign of better health.
Brown fat doesn't mean much for our body weight
Despite what your social feed might suggest, activating brown fat isn’t a magic bullet for weight loss. Mice rely heavily on brown fat to generate heat, so it’s constantly switched on.
But in adult humans, the story is different: we’re simply too large, with too little brown fat, for it to meaningfully burn off body fat.
“Although there is some debate, I personally think that brown fat has almost zero of a role in terms of body weight loss,” says brown fat researcher Prof Shingo Kajimura, a principal investigator at Beth Israel Deaconess Medical Center and a professor of medicine at Harvard Medical School, both in the US.
Many of the promising weight loss responses to brown fat activation seen in mice haven’t really been reproduced in humans, he adds.
How brown fat may actually benefit our health
But it’s not a lost cause. Instead, brown fat might be acting more indirectly to boost your health.
Rather than torching calories, it could be helping regulate metabolism; some scientists believe it behaves more like an endocrine organ, releasing hormones that influence how our body processes energy.
“The idea is still that once you activate brown adipose tissue, then you can secrete those molecules to regulate the whole metabolism,” says Prof Yu-Hua Tseng, who studies physiology and metabolism at Joslin Diabetes Center, in the US, and at Harvard Medical School.
Another theory is that brown fat could work as a cellular clean-up crew.
It not only uses sugar as fuel, but also mops up harmful byproducts in the blood, such as free fatty acids and branched-chain amino acids. While good in small quantities, these molecules when left to build up are linked to insulin resistance and other metabolic diseases.
Crucially, this effect doesn’t seem to depend entirely on the cells’ ability to warm up the body.
In a recent mouse study published in Nature Medicine, Kajimura and his team showed that, when they genetically disabled the heat-producing function of brown fat in mice, the rodents were still protected against cardiometabolic disease.
All of this means that even if brown fat might not be a major weight-buster, it could still support better health – potentially even at a higher weight.
“The idea might be that if you activate brown fat in someone who’s obese, they may not lose much weight but you may be able to improve a lot of the comorbidities (the related conditions) that are often seen in people with obesity,” says Cohen. “I would argue that if we can do that, that would be incredibly valuable.”
Hacking white fat to make it act like brown fat
But what if we could learn to make brown fat? Well, of those two other fat types – white and beige – scientists are especially interested in beige fat, which forms when white fat is stimulated to take on brown fat’s heat-producing qualities. They call this process ‘beigeing’.
Mild cold activation is one very efficient way to trigger beigeing. In a 2014 study published in the journal Diabetes, five healthy men around the age of 21 were asked to spend one month sleeping in a room heated at 19°C (66°F) overnight, with just hospital clothes and light bed sheets for warmth.
By the end of the trial, they’d increased their brown fat volume by 42 per cent, and its metabolic activity by 10 per cent. These effects disappeared as soon as the scientists cranked up the heat again – but the study nevertheless showed how powerfully cold can affect brown fat.
The benefit of this cold exposure might even be passed on to future generations. A more recent study, published in the journal Nature Metabolism in 2025, has suggested that people who were conceived during colder months have more active brown fat.
Still, cold exposure isn’t exactly user-friendly.
“It’s really unpleasant to get that much cold exposure,” says Michael D. Jensen, professor of medicine at the Mayo Clinic College of Medicine and Science, in the US, and editor-in-chief of the journal Obesity.
A temperature of 19°C (66°F) might not sound that chilly, but when you have to do it for hours in little clothing to feel any benefits, the discomfort adds up.
“Unless that’s your job, most people don’t want to do it.”
For some people, that is a part of their job. That’s the case, for instance, for people who work outdoors in sub-zero temperatures like construction workers or cold-water swimmers.
But for most people, cold exposure isn’t only impractical – it could also be dangerous and shouldn’t be undertaken without prior medical advice.
Cold exposure without the cold
That’s why researchers are now looking for alternatives that trick the body into thinking it’s cold, without actually lowering your core temperature and putting your body (especially your heart) under stress. These alternatives are called ‘cold mimetics’.
“Cold mimetics mean your brain doesn’t think it’s cold, your heart doesn’t think it’s cold, but only brown adipose tissue feels cold. That’s the ideal situation,” says Kajimura. “I personally think it’s very feasible, but let’s see.”
One promising avenue is mirabegron, a drug already used to treat overactive bladder. Some early studies suggest it might help activate brown fat in humans – though results so far have been mixed, it is showing some promise, says Cohen.
Other studies are exploring whether GLP-1 receptor agonist drugs like Ozempic and Wegovy – already blockbuster treatments for weight loss – could also trigger brown fat activity. In mice, these drugs appear to switch it on, though more research is needed to confirm.
Another intriguing possibility might be brown fat transplantation. In mice, transplanting brown fat cells to other mice reduced their weight gain and white fat levels.
“One approach might be that we can do that in humans using either bona fide transplants or stem cells, or some other engineered approach,” says Cohen.
Dietary hacks for brown fat activation
And what about the coffee and spicy foods that influencers would have us believe can tip the odds towards brown fat?
While some research supports coffee’s influence on brown fat, other scientists have said that to generate enough of an effect to reap the benefits, you’d need to knock back about 100 cups of coffee a day. Not even the most dedicated coffee fans are drinking that much.
As for capsaicinoids – the molecules present in spicy foods – there’s some indication that they’re linked to weight loss and higher energy expenditure in patients with obesity. And, crucially, there’s evidence they could trigger that much sought-after brown fat activation.
But on their own, Tseng warns, none of these dietary hacks are a strong enough dose to make a difference.
“I think it’s a promising target, but it’s not just simply about eating some spicy food or whatever other regimen that’s been shared on social media,” says Tseng.
Nevertheless, even if cold exposure and spicy foods might not have a dramatic effect on brown fat, that doesn’t mean they’re useless.
“Even though they’re not necessarily acting on brown fat, they still could provide some beneficial effect in terms of calorie burning,” says Jensen.
As long as you don’t end up eating more to make up for the discomfort of these procedures, and your doctor says you have no health risks associated with them, it may do you some good, he adds.
What to do about brown fat
Let’s be clear – at this stage, none of this is conclusive. Brown fat remains a frontier of science, and what it actually does in humans is still uncertain.
“The human studies are extremely hard to interpret, because the people who are already healthy are the ones who are most likely to have brown fat,” says Jensen. “My joke is that we’ve probably expended more calories doing research on brown fat than we’ve actually expended by brown fat.”
Still, it may also be that we don’t yet have the right tools to understand the full extent of its effects. Tseng hopes to change this. She’s developing a blood test biomarker that could reveal not only how much brown fat a person has, but also how much of their white fat is beigeable.
“I think what’s important is to know whether my white adipose tissue is beigeable. And hopefully the biomarker can provide us with some indication of that,” she says.
In the meantime, Cohen says the best advice for weight loss remains the tried and tested: talk with your doctor, eat a heart-healthy diet, exercise, and take GLP-1 agonist weight loss drugs if you need them – all in moderation and slowly.
Even if you don’t lose much weight, your body will thank you for the better overall health.
Kajimura notes that endurance training has also been linked to brown fat activation in mice, though results are more mixed in humans.
“Exercise, don’t stay too warm, go play outside, walk… A healthy lifestyle, as far as we are aware, is also good for brown fat,” he says.
As for brown fat-specific treatments, keep your eye on the horizon, says Cohen. “I do think there’s reason for hope in the future.”
About our experts
Dr Paul Cohen is a physician and scientist at The Rockefeller University in New York, in the US. He is published in various scientific journals such as American Journal of Physiology: Endocrinology and Metabolism, Cell Biology and Circulation Research.
Prof Shingo Kajimura is a principal investigator at Beth Israel Deaconess Medical Center and a professor of medicine at Harvard Medical School, both in the US. He has been published in the likes of Physiological Reviews, Cell Metabolism and Nature Metabolism.
Prof Yu-Hua Tseng is a physiology and metabolism researcher at Joslin Diabetes Center, in the US, and a professor at Harvard Medical School. She is published in New England Journal of Medicine, Cell and The Journal of Clinical Investigation, to name a few scientific journals.
Michael D. Jensen is a professor of medicine at the Mayo Clinic College of Medicine and Science, and editor-in-chief of the journal Obesity, in the US. He has also been published in Physiological Reviews, American Journal of Physiology: Endocrinology and Metabolism and The Journal of Clinical Investigation.
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