Believe it or not, there’s actually a simple, non-scammy way to “hack” your metabolism. And not only is it free, but it takes around two seconds to do: just walk over to your thermostat and tap a button.
Humans are warm-blooded. You are human. Hence, you need to keep your blood warm to stay alive. And what keeps your blood and other tissues warm? Burning fuel!
To keep your tissues warm during cold exposure, you need to burn even more fuel — more calories — than normal. But how much more: a negligible amount, or enough to impact weight loss? How low do you have to set your thermostat, and will you get uncomfortably cold? And what exactly is going on in your body?
These questions were answered in studies conducted from 2001 to 2016. Here’s a quick summary.
Nearly 50% of the calories you burn can go toward maintaining your core temperature (about 37°C or 98.6°F). You might have known that, but what you probably don’t know is that obese people tend to have lower core temperatures during waking hours than do non-obese people.
Why is it important? Theoretically, that lower core temperature could lead to a weight increase of five pounds per year. It’s never been directly studied, however, so it’s hard to know what the real-life impact is.
Thermoregulation is totally different in modern life from what it was in centuries past. You seldom depend only on your body to keep you feeling warm, unless you’re on a TV survival show and do jumping jacks in the cold of the night. Rather, you rely on a plentiful supply of clothing and on central heating. Not only are indoor temperatures fairly toasty, but they appear to be increasing a bit each decade.
People seem to be more and more willing to crank up the heat in their bedrooms. Which might not be a good idea, since cooler temperatures at night could help the body fall asleep.
A surprisingly large amount of energy goes toward keeping warm-blooded animals like us … warm. But not always equally warm: a recent study showed that obese people have lower core temperatures during waking hours than non-obese people, which could theoretically impact bodyweight regulation. Regardless, indoor heating is ubiquitous, and indoor temperatures are creeping up each decade.
When it gets too cold, the body creates heat by shivering. This shivering can multiply our heat production fivefold and hence burn a ton of calories. Which is great, except for one thing: shivering totally sucks.
Luckily, once you get accustomed to a colder environment, you stop shivering so much, but you continue burning calories to produce heat. This is known as “non-shivering thermogenesis”. How does this process occur? Through BAT: brown adipose tissue. There may also be some non-shivering thermogenesis through muscle, but the evidence is mixed.
Whereas normal fat (white adipose tissue) is used for energy storage, brown fat is used for heat production. Up until around 2009, researchers didn’t even know that adult humans still had any notable amount of brown fat. They knew that babies had brown fat, to keep their delicate little bodies warm. And rodents have brown fat, so there was a bunch of rodent research already. But with the advent of PET scanners, which can highlight highly-metabolically-active tissue (such as tumors, of course, but also brown fat), research really got cooking, and brown fat is now a popular therapeutic target for weight loss drugs and supplements.
A quick note on how brown fat works: it hosts a multitude of mitochondria that use something called uncoupling protein (UCP1) to skip the energy-producing step (the production of ATP) and instead use the fuel to produce heat. Most of our brown fat is located around the shoulder blades and neck. It only shows up on PET scans when it’s been activated — as shown on the left below — which is to say, when we’ve been exposed to the cold.
There’s also something called “beige fat” (sometimes called “brite fat”), which is regular white fat that gets converted to a type of brown-ish fat that also produces heat. This fat isn’t as prevalent as the brown type, but it’s being heavily researched.
One important thing about brown fat is that, whereas it is activated during cold exposure, it is also produced after cold exposure. This doesn’t mean you can develop brown fat after a cold shower, but it does mean that long-duration cold exposure can build up your brown fat stores. In trials, only a very few participants couldn’t develop (much) brown fat after cold exposure.
The first step to cold adaptation is shivering, along with drawing heat away from the skin toward the core. With consistent cold exposure, shivering lessens and brown fat can help warm the body.
When it comes to developing brown fat, details are important. Some trials used temperatures as low as 59°F for multiple hours a day, which would be uncomfortably cold for most people in real life. But once you bump the temperature up too much, brown fat is no longer produced in any significant quantity. Researchers estimate that around 65°F is the highest temperature that would accrue benefit, albeit with a longer exposure time needed than temperatures in the lower 60s.
Ten days of cold exposure (six hours per day at 60°F) led to a 37% increase in the volume of brown fat. This trial, though, didn’t show an increase in energy expenditure. A shorter trial (three days at a continuous 61°F) showed an added calorie burn of around 140 kcal/day. It’s possible that switching to normal room temperature (or a warm outdoor climate) for too long may negate some of the benefits of cold exposure, but there aren’t any trials looking at this. As it stands, the evidence is mixed, as a previous trial showed an added calorie burn of around 400 kcal in a day from just two hours of cold exposure. The comparison was a bit wonky though, as it was two hours at 66°F along with putting legs on an ice block intermittently, compared to the same time without ice at 81°F.
Because there’s no standard cold exposure protocol used in every trial, there’s no definite answer to how much weight you can lose: 5–10 lb/year is the current estimate, but long-term trials are needed for confirmation.
There’s a caveat here. As usual.
Some people eat more when they get cold. In one trial, office workers at 68°F ate 100 kcal more, on average, than workers at 79°F. It would be interesting to see that trial replicated with colder exposure (such as 65°F) and comparison to room temperature (72°F).
When the temperature gets in the low 60s, eating more food could help produce body heat in two ways: in the short term, digestion produces heat (it’s the thermogenic effect of food); in the long run, an increase in fat stores means an increase in energy reserves, in addition to better insulation.
Animal evidence also suggests that cold exposure might make you particularly susceptible to overeating highly-palatable foods (notably comfort food or junk food). But the balance of evidence suggests that eating more after cold exposure doesn’t totally negate the benefits of increased energy consumption through brown fat.
Brown fat is only net beneficial if cold temperatures don’t cause you to overeat.
Some people feel cold very easily, whereas others can wear shorts and t-shirt outside in the winter. Everyone should check with their physician before doing any type of cold therapy, especially people who get cold easily (as circulation problems or Raynaud’s phenomenon may be undiagnosed).
Older people should also be careful, as they’re more liable to experience hypothermia and blood pressure issues. Moreover, they tend to have less brown fat.
That being said, cold exposure around 65°F is not that extreme. Further, studies have shown that after a few days, subjects find the mild cold exposure less uncomfortable and shiver less or not at all. Most adults, including the elderly, appear to find periods of eight hours at 63–65°F acceptable.
Despite all these studies, it’s impossible to predict the effects of cold exposure on any given individual. One study showed that energy expenditure after very mild cold exposure (a full day at 68°F rather than 82°F) ranged from a 12% increase down to a 5% decrease.
Spending extended periods in the lower-to-mid 60s (°F) could lead to a weight loss of 5–10 lb/year through the production of brown fat to maintain the body’s core temperature. Controlled trials have only lasted days or weeks, though, so long-term weight loss is not known for sure.
Overeating in response to mild cold exposure can lead to weight gain.
Mild cold exposure may be a simple option for weight loss, but cold is not something to be trifled with. Don’t do something stupid, such as sleeping outside in your underwear when it’s freezing, or fainting in an ice bath.
Turning down the heater can also save you money. But if you really hate the cold, don’t feel forced to forswear your comfy room temperature. Cold exposure may have a multitude of potential benefits, but normal room temperatures haven’t been shown to be inherently harmful. Cold exposure is just one potential tool in the health-improvement tool chest.
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