Cold exposure activates brown fat, a type of fat that burns energy to produce heat. Regular cold exposure can increase brown fat activity and improve insulin sensitivity in some studies. The extra calories burned are modest, so cold is a small helper rather than a weight-loss method.
- Adults have small amounts of brown fat, mainly around the neck and collarbones.
- Mild cold exposure may raise daily energy use by roughly 100–200 calories at most.
- Cold plunges carry real risks for people with heart conditions.
Cold plunges, ice baths and winter swims have become popular “biohacks”, with claims that cold melts fat. Canadians have plenty of cold to work with. Here is what the science actually supports.
What brown fat is
Most body fat is white fat, which stores energy. Brown fat contains many mitochondria and a protein called UCP1 that lets it burn fuel to produce heat instead of storing it. Babies have a lot of brown fat to stay warm. Adults have much less, mainly around the neck, collarbones and spine, but PET scan studies since 2009 confirmed that most adults have active brown fat, especially when cold.
White fat can also develop some brown-like “beige” cells with repeated cold exposure.
How cold burns energy
- Non-shivering thermogenesis: brown fat activation in mild cold.
- Shivering: muscles contract involuntarily to generate heat, which burns more energy but is uncomfortable.
What studies show
- Spending a few hours a day in mild cold (about 15–17 °C) for several weeks increased brown fat activity and reduced the urge to shiver in several studies.
- In a 2014 study, men who slept in rooms kept at 19 °C for a month increased brown fat activity and improved insulin sensitivity.
- A small 2015 study of people with type 2 diabetes found 10 days of mild cold exposure improved insulin sensitivity by about 43%.
- Estimates of extra energy burned from cold-induced thermogenesis range from small amounts up to roughly 100–200 calories a day under sustained mild cold, with large individual variation.
Weight-loss studies in people are limited, and cold can increase appetite, offsetting some of the extra energy used.
Cold plunges and ice baths
Short cold plunges (a few minutes in very cold water) activate brown fat and raise noradrenaline, which many people find energising. Their contribution to fat loss is small. After strength training, cold plunges may also reduce muscle growth; see cold exposure and muscle recovery.
Simple, safe ways to use cold
- Keep your home a little cooler, especially the bedroom (around 17–19 °C).
- Walk outdoors in winter without overdressing, while staying comfortable and safe.
- End showers with 30–60 seconds of cold water.
- Try cold plunges only if healthy, starting with short exposures and never alone in open water.
Safety
Sudden immersion in cold water triggers cold shock: gasping, rapid breathing and a spike in heart rate and blood pressure in the first minute. This can cause drowning or heart rhythm problems. Hypothermia can follow if exposure lasts too long. The Lifesaving Society notes that cold water incidents are a major cause of drowning in Canada.
Cold and performance drugs
Stimulants, clenbuterol and thyroid hormone raise heart rate and body temperature, and anabolic steroids can raise blood pressure. Combining these with cold shock adds strain to the heart. Anyone using them should be particularly cautious with cold plunges.
Who seems to have more brown fat
Research suggests younger people, women and leaner people tend to have more active brown fat, and people living in colder climates or exposed to cold regularly may develop more. Brown fat activity also appears to drop with age and with obesity, though cold acclimation can increase it at any age. These findings have led to interest in brown fat as a target for treating obesity and type 2 diabetes, but practical treatments are still in research.
Cold, appetite and food
Cold exposure tends to increase hunger, which may offset the extra calories burned. After a winter swim or long time outdoors, people often eat more. Planning a balanced, protein-rich meal afterwards helps prevent overeating.
Where cold fits
Cold exposure may support metabolic health and is enjoyable for some people. For fat loss, the calorie deficit, protein, strength training and daily movement remain far more important. See winter weight loss and insulin sensitivity.
Frequently asked questions
Does cold exposure burn fat?
It activates brown fat and slightly increases energy use, but the effect on body weight is small.
How many calories does cold exposure burn?
Under sustained mild cold, estimates range up to roughly 100–200 extra calories a day, with large individual differences.
Are cold plunges safe?
For healthy people with short, gradual exposure and supervision, generally yes. People with heart conditions should get medical advice first.
Photo: Elliott Chau, StockSnap (public domain).
General information, not medical advice.
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