Body fat, also known as adipose tissue, is oftentimes thought of as a “bad” thing and something we must all strive to get rid of. For the most part, adipose tissue is metabolically inactive and is our bodies’ means for storing energy. However, what most people don’t know is that there are three types of adipose tissue: white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue. When we refer to fat, we are typically referring to WAT, which is most abundant in our bodies. The main functions of WAT are to store energy and cushion our organs. BAT on the other hand, is metabolically active and involved in heat generation in our bodies. It is only found in small quantities, mostly in newborns and in the supraclavicular and thoracic regions of adults. Beige adipose tissue is a combination of WAT and BAT. Although beige adipose tissue is developmentally WAT, when activated by “browning” or “beiging”, it has the characteristics and functions of BAT. 

In 2012, a research team at Harvard Medical School published a study in the journal Nature with findings about a new hormone called irisin that could stimulate brown-fat-like development in WAT. This research opened up new doors to potential ways we could “hack” our metabolism into burning more calories. Since publication of these findings, interest in BAT and “beiging” or “browning” of WAT has sparked and made its way to mainstream media outlets. In research, BAT has garnered attention for its potential in combating the obesity epidemic due to its contributions to energy expenditure regulation

BAT is a heat-producing tissue characterized by small fat molecules that use large amounts of the carbohydrates and fats we consume to generate heat. However, it has been most notably studied because of its high number of mitochondria and abundant expression of a protein called uncoupling protein 1 (UCP1). UCP1 can generate heat in a process called thermogenesis by diverting the electrochemical gradient in mitochondria from energy producing processes to the production of heat. 

Many scientists have looked into the possibility of increasing the amount and the activity of BAT because of the potential use of BAT to increase basal metabolic rate (BMR) and to exert metabolic effects on obesity, type 2 diabetes, and heart disease. Several mechanisms for activating BAT or “beiging” of WAT have been studied, but two of the most prominent and accessible methods have been through exercise and cold exposure. 

While reports in the literature regarding the effects of cold on activating BAT have been consistent, studies looking at the effects of exercise on BAT have shown mixed results. Current evidence suggests that exercise triggers a series of reactions in our cells that stimulates the secretion of the hormone irisin. Irisin, in turn, increases UCP1 gene expression and thermogenesis. The diversion of the energy producing process in mitochondria to heat production by UCP1 results in a “beiging” of white adipocytes (WAT cells) to beige adipocytes, which resemble brown adipocytes (BAT cells) in morphology and function. In other words, exercise induces a physiological process that makes WAT more similar to thermogenic BAT. 

In cold exposure, the sympathetic nervous system (responsible for activating the fight or flight response) is activated, causing norepinephrine to be released and triggering a series of reactions that activate UCP1 in BAT to increase the use of dietary carbohydrates and fats in heat production rather than energy storage. While one mechanism of WAT beiging has been highlighted here, it is likely that more than one pathway from WAT to BAT may exist. Additionally, because of the overlap in WAT beiging mechanisms in exercise and cold exposure, there may also be a synergistic effect on BAT between the two, although this has not been proven. 

So, could exercising in the cold help you lose more weight? 

Even if exercise and cold exposure did in fact increase BAT activation and beiging, both individually and synergistically, the overall contribution to energy metabolism caused by BAT is unlikely to cause weight loss. In cold exposed adults, the estimated contribution BAT makes to whole body energy expenditure is relatively small. Estimates of a 10-20% increase in energy expenditure have been reported by some studies, while others have reported a 1-7% increase in resting metabolic rate. Using these estimates, Marlatt and Ravussin speculated that fully stimulated BAT would only provide approximately 100 kcal/day of additional energy expenditure. At this rate, it would take at least 35 days of 2+ hours of cold exposure and/or exercise to lose one pound, assuming energy consumption is not changed. Thus, the use of prolonged exercise and cold exposure may not prove to increase brown fat and basal metabolic rate (BMR) as significantly as once thought.  

While BAT activation and WAT beiging showed promising results in the potential to increase energy expenditure through exercise and cold exposure independently, there is currently not enough evidence to support that the two may work together to increase BMR. Additionally, the degree to which BAT activation and WAT beiging increases BMR needs to be further investigated to understand the practicality of this method for weight loss or weight maintenance. 

Although exercising in the cold may not provide the fantastic weight loss benefits promised in magazines, it shouldn’t stop you from exercising outside in the winter. Exercising alone has many benefits including improved brain health, reducing disease, and strengthening muscles and bones. And being outside, especially in the winter, can help to improve your vitamin D levels, immune function, and mood.

Peer edited by Rachel Haake and Gabrielle Dardis

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