A photo of a person in sports wear sitting and holding their head, with measuring tape and scales on the floor.

The fat we all want: brown fat

Turun yliopisto
A photo of Johanna Örling.
Johanna Örling

When we talk about body fat, we often associate it with bad things, such as unhealthy foods, obesity, eating disorders and strict diets. Especially nowadays most of us struggle with the issue of non-intended weight gain and are forced to make many different decisions every day to avoid it. Should I eat an apple or a doughnut? Should I go running outside when there’s a snowstorm or should I just lie on the sofa and watch “Friends” for the thirtieth time?

But did you know that not all fat is bad and there is a type of fat that we would like to have even more of? Most of the fat in our body is called white fat, which specializes in storing excess energy. However, we also have another type of fat called brown fat which, instead of storing energy, consumes energy for producing heat. Doesn’t this sound amazing? Especially in babies, brown fat has a significant role in heat production. Babies cannot move much due to their undeveloped muscles, so they need help in maintaining body temperature. Here is where brown fat sets in. Unfortunately, however, we lose most of our brown fat as we get older.

Luckily, not all hope is lost. Until 2009 it was thought that the remaining brown fat adults have is not active, but then researchers, including our research group, found that also adults have metabolically active brown fat. Since then, brown fat has gained a lot of interest not only among researchers but also the general public. Currently we know that higher brown fat activity is associated with healthier glucose and lipid metabolism. Vigorous research in the field has also shown that white fat can turn into more brown-like fat under certain stimuli, such as cold exposure like winter swimming. In Finland winter swimming is an increasingly popular hobby, for instance due to its relaxing and stress relieving effect. Many winter swimmers also say that they never get sick due to this extreme habit. However, there is no data available on whether the immunological profile in winter swimmers is different when compared to non-swimmers. In addition, the role of brown fat in this picture remains a mystery.

Many obesity-related issues, such as type 2 diabetes, are known to relate to so called chronic low-grade inflammation in the body. Inflammation in general is an important part of the body’s immune defense, but when it gets chronic, problems arise. Chronic low-grade inflammation is characterized by the infiltration of immune cells into white fat. These immune cells recognize the excess fat as a foreign material in the body and attack it. If the obesity persists, the inflammation gets chronic and may eventually lead to several different health issues.

Despite the current knowledge about low-grade inflammation in white fat, hardly anything is known about the immune cell profile of brown fat. Studies done on mice suggest that immune cells in brown fat could even have a protective role against inflammation, but there is no human data available to support this.

My research aims to shed light on the previously listed issues. If winter swimmers indeed are more resistant to catching a flu than non-swimmers, is that somehow related to brown fat activity? What is the “normal” immune cell profile of brown fat, and does it relate to the low-grade inflammation that is seen in white fat? To investigate this fascinating type of tissue, we use state-of-art methods such as single-cell RNA sequencing to understand the composition of human brown fat at the level of an individual cell. Using positron emission tomography, we can study brown fat activity and its association to cardiometabolic health. Understanding the role of brown fat in humans more deeply could provide us new tools in the battle against obesity.

Johanna Örling

The writer is a doctoral researcher at the Turku PET Centre. She is interested in the complexity of brown fat and its significance for human metabolic health, especially from the immunological point of view.

References

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