In this blog post, we will explore the physiological mechanism by which newborns utilise brown adipose tissue to maintain their body temperature, as well as its medical potential.
When adults feel cold, they generate heat through actions such as shivering to maintain their body temperature. This is a natural response of the body. When the external temperature drops, the nervous system contracts the muscles to induce shivering, which generates heat to maintain body temperature. Mitochondria, which are the building blocks of cells, play an important role in this process. Mitochondria convert glucose obtained from food into adenosine triphosphate (ATP) and store it. When the body temperature drops, ATP is broken down through muscle shivering, generating heat.
However, newborns do not have as developed muscles as adults, so they cannot shiver enough to maintain their body temperature. Instead, newborns maintain their body temperature through brown adipose tissue distributed around the spine and kidneys. Unlike other adipose tissue, brown adipose tissue has an excellent ability to generate heat. This is because brown adipocytes are composed of oil droplets containing fatty acids and a large amount of mitochondria.
Mitochondria in normal cells consist of a matrix, an inner membrane, an outer membrane, and a space between the inner and outer membranes. Mitochondria in brown fat cells have the same structure, but they differ in that a specific protein in the inner membrane plays a role in heat generation. The process by which this protein helps newborns maintain their body temperature is as follows.
When the body temperature of a newborn baby drops, a signal is sent from the hypothalamus in the brain, causing noradrenaline to be secreted from the sympathetic nerve terminals. This stimulates the β receptors in the cell membranes of brown adipose cells, causing the fatty acids present in the brown adipose cells to be transported through the outer and inner membranes of the mitochondria to the matrix. The electrons generated by the breakdown of fatty acids are then transferred by coenzymes to membrane-bound proteins in the inner membrane. These membrane-bound proteins act as channels, transporting hydrogen ions (H+) already present in the matrix into the intermembrane space.
When hydrogen ions move from the matrix to the intermembrane space, a concentration gradient of hydrogen ions between the intermembrane space and the matrix is created. This concentration difference creates energy called proton motility, and this force causes hydrogen ions to return to the substrate where their concentration is lower. However, since the inner membrane is impermeable to protons such as hydrogen ions, they must pass through specific protein channels in the inner membrane in the same way as when they move into the intermembrane space.
The important point here is that hydrogen ions pass through a protein channel called ‘thermogenin,’ which is found only in the mitochondria of brown fat cells. In the mitochondria of normal cells, hydrogen ions pass through ATP synthase when they return to the substrate, and the proton motive force is used for ATP synthesis. In contrast, in brown fat cells, thermogenin does not use the proton motive force for ATP synthesis. Therefore, hydrogen ions are accelerated by proton transport and collide with the water in the substrate, generating heat.
Through this process, newborns are able to maintain their body temperature in a different way than adults. Thanks to the unique heat generation mechanism of brown fat cells, newborns are able to maintain a relatively stable body temperature even when the external environment changes, which plays a very important role in their survival. This body temperature regulation mechanism is not only observed in newborns, but also in animals that hibernate. These animals utilise brown adipose tissue to produce the heat necessary for survival in order to maintain their body temperature during winter.
The function of brown adipose tissue not only regulates body temperature, but also has an important effect on energy metabolism in the body. Recent studies have shown that brown adipose tissue has the potential to contribute to the prevention of metabolic diseases such as obesity and diabetes. This is because brown adipose tissue efficiently consumes fat in the process of generating heat. Therefore, the activation of brown adipose tissue can have a positive effect on weight control and metabolic health, and the possibility of developing new treatments utilising this has been suggested.
In conclusion, the mechanism of body temperature maintenance in newborns is achieved through the unique heat generation process of brown adipose tissue. This process plays an essential role in the survival of newborns and has an important effect on energy metabolism in the body. Further research on the functions and roles of brown adipose tissue is expected to lead to various medical applications.
