Function of Brown Fat

One of the mechanisms that allowed them to dominate was advanced thermogenesis-the ability of the body to maintain a constant temperature, not least of which is due to brown adipose tissue.

Thermogenesis comes in many forms. There are two types: contractile thermogenesis (chills, shivers, "can't get a tooth for a tooth"), in which heat generation is caused by contractions of skeletal muscles (cold muscle shiver is a special case), and noncontractile thermogenesis (brown adipose tissue work). In some diseases, the body itself raises the temperature to fight the sources of the disease more effectively. When a fever develops, the body's thermoregulatory system dynamically rebuilds, activates and works at a higher functional level, so if the first couple of hours the body temperature is not higher than 37.5 - it should not be knocked down at all.

Brown fatty tissue was first found in animals. Among animals, brown adipose tissue is best developed in those that hibernate in winter, since metabolism slows down during hibernation, making it impossible to maintain body temperature with skeletal muscle contractions. Brown adipose tissue is also important when animals awaken from hibernation: with the heat it generates, the body temperature rises considerably, so that the animal can come out of hibernation.

Who has brown fat?

It used to be thought that only children have brown fat. It allows them to adapt to the new world after leaving the womb. In newborns, brown fatty tissue makes up about 5% of body weight and is found around the neck, kidneys, along the upper back and on the shoulders. Also in infants, brown adipose tissue is often found mixed with white adipose tissue. For newborns, brown adipose tissue is very important because it helps avoid hypothermia, which is a frequent cause of death in premature infants. Because of brown fatty tissue, infants are less susceptible to cold than adults (it is helpful for parents to read this phrase twice)  

Brown fat cells have an exceptional feature - they contain a lot of mitochondria (organelles responsible for storing energy in the cell). This is actually the reason why it is "brown". In the mitochondria of brown fat cells there is a special protein UCP1, which instantly converts fatty acids into heat, bypassing the phase of ATP synthesis.

The lipids (triglycerides) contained in adipose tissue are a store of material from which energy (ATP) can be derived.

When a newborn needs a lot of energy (e.g., to keep warm) - fats undergo lipolysis, which results in fatty acids.

UCP1 in brown fat cells turn fatty acids into heat, causing the fat stores to melt. First, the triglycerides in brown fat itself are consumed, and when they are exhausted, lipids in the hated white fat surrounding the brown fat cells (BRITE - brown in white) begin to melt as well.

As a result, "the body loses weight. However, for this process to be effective, the newborn must breathe normally (conversion of fatty acids requires oxygen) and eat! (it also needs energy to start lipolysis).

Alas, this mechanism weakens in adults. As early as 2 weeks after birth, shivering, as a reaction to cold, begins to replace brown fat, especially if children are strongly wrapped up and kept in the heat

Now it turns out that "healthy" fat is also present (and working) in adults. For a long time, brown fat was thought to lose its importance as early as the end of the first year of life. However, relatively recently (in 2008!) It was discovered that brown fat is not only present in the adult body (this was already known in 1908), but can also be activated by cold.

We owe this discovery to a new method of imaging active metabolism in tissue, positron emission tomography combined with computed tomography (fused PET-CT), which showed that the adult person has about 20-30 grams (so little) of functional brown fat, mostly in the supraclavicular area.

PET-CT registers tissue metabolic activity, and in the right figure we see brown fat activity increasing in response to cold stress in an adult.

"We gave 24 young people a certain dose of radioactive glucose," says physiologist Wouter van Marken Lichtenbelt . - This was done in order to then be able to detect active brown adipose tissue with special equipment." The study participants were then led into a room where the temperature did not exceed 16°C. PET and CT scans showed that "healthy" fatty tissue was present under the skin of the 23 people's necks, breasts and abdomen, which worked by keeping people warm in the cold room.

"We were very surprised to find so much of it and so many people!" - exclaims van Marken Lichtenbelt. When the three participants were examined at room temperature, however, they found no traces of brown fat. This does not mean that the tissue disappeared, just that it stopped actively working, experts believe.

The efficiency of brown adipose tissue in humans

Brown fat makes up no more than 1-2% of body weight. Nevertheless, stimulation of this tissue by the sympathetic nervous system during cooling of animals pre-adapted to cold increases the heat production of brown fat to such an extent that it can reach one third of all additional heat formed in the body. When activated, brown fat can waste up to 300 Watts (that's another study's figure, some say 400) per kilogram of adult weight.

That's 21 Kilowatts per 70-kilogram person. By comparison, a person at rest burns about 1 Kilowatt of energy in an average weight person. By activating brown fat you can lie on the couch and burn twenty times more energy than before. More on this in the following articles (out tomorrow and the day after tomorrow).

Fat burning.

Brown fat allows you to burn fat by transferring fatty acids from white adipose tissue to brown fat. Unlike its more common counterpart, which is deposited under the skin, in the omentum and capsules of internal organs, brown fat, instead of storing energy, burns it in large quantities, releasing heat. This is thermogenesis due to excessive food consumption.

Н. Rotuel and M. Stock set up the following experiment. Adult rats were fed a restaurant diet, i.e. varied and tasty food. Consumption of this food by animals was 80% higher than in the control group, which received regular food. However, the weight of the animals increased by only 27% in three weeks. Measurement of gas exchange showed that the well-fed rats consumed 25% more oxygen than the control. This gain disappeared after administration of propranolol, a norepinephrine antagonist, to the animals. The mass of brown fat more than tripled during the same three weeks of the experiment, and the amount of thermogenin in the mitochondria increased.

Conclusions

The American Diabetes Association believes brown fat has very important potential for patients with obesity and diabetes. Activated brown fat tissue can actually burn huge amounts of glucose and fats and help control blood sugar levels. It is also noteworthy that in people who are overweight, the amount of brown fat is reduced and its activity is suppressed. Therefore, new medications and other methods of accumulating and activating brown fat in adults may soon be available, thanks to the advent of new methods of imaging active brown fat.