Babies typically have a constitution of 5% brown fat. It is one of their main mechanisms for staying warm.

Many people report that they feel warmer throughout the day after taking a cold shower or bath. This could partly be due to enhanced circulation, but the stimulation of brown fat also seems a likely cofactor.

Brown fat is one of two types of fat in the body – the other being white fat, or what we normally just refer to as “body fat”. While white fat should be kept to a minimum, brown fat is beneficial because it helps burn calories and generate body heat. A study at the University of Maastricht in the Netherlands revealed that obese people have less brown fat than lean people [1].

A cold shower could be a good way to stimulate the body’s supply of brown fat. A branch of the aforementioned university found that in men, exposure to chilly temperatures increased the metabolic rate of brown fat 15-fold. Sustaining this rate alone could help a person shed 9 pounds a year. Women typically have twice as much brown fat as men [1].

In a Chinese experiment involving voles exposed to a 5° C environment, they found that the “thermogenic capacity” of brown fat increased; the cold-exposed voles had a higher content of uncoupling protein 1 in their brown fat, increasing their ability to convert stored calories into heat [2].

However, in one small study of six men, the researchers did not find that their adaption to cold was facilitated by brown fat [3]. Hopefully future studies will find the most favorable conditions for activating brown fat and the precise benefits we can expect.

[1] Brown Fat: Don’t Try To Burn It
[2] Cold exposure does not decrease serum leptin concentration, but increases energy intake and thermogenic capacity in pregnant Brandt’s voles (Lasiopodomys brandtii).
[3] Effects of acclimitazation to cold baths on men’s responses to whole-body cooling in air.