Human newborns and hibernating mammals have high levels of brown fat. The other type of fat is white or yellow fat.
Much remains unknown about brown fat. Scientists are just starting to understand what brown fat does. If they can find out how white fat converts to brown fat, it could lead to a treatment for obesity.
Brown versus white fat
Fat cells can be white or brown.
White adipocytes, or white fat cells, have a single lipid droplet, but brown adipocytes contain many small lipid droplets, and a high number of iron-containing mitochondria. It is this high iron content that gives brown fat its dark red to tan color.
Brown fat has more capillaries than white fat, because of its higher oxygen consumption. Brown fat also has many unmyelinated nerves, providing sympathetic stimulation to the fat cells.
A lipid is another name for fat. Lipids are substances that dissolve in alcohol but not in water, such as fat. Oil and wax are also lipids. Lipids contain oxygen, hydrogen, and carbon. They have a lower oxygen content than carbohydrates.
The fat that builds up around a person's waist and thighs is the white type. Brown fat mainly accumulates around the neck.
The functions of brown fat have only recently started to become clear. The main differences between the two appear to be as follows:
White adipose tissue (WAT), or white fat is the result of storing excess calories. When we consume too many calories, the body converts them into an energy reserve in the form of white fat.
WAT distribution affects metabolic risk. Large amounts of white fat around the abdominal area is associated with a higher risk of metabolic disease, while fat in the hips and thighs does not.
Brown adipose tissue (BAT) or brown fat generates heat by burning calories. When it is cold, brown fat's lipid reserves are depleted, and its color gets darker.
Humans and mammals with higher levels of brown fat take longer to start shivering from the cold, than those with lower levels. Newborns do not shiver in the cold, because their brown adipose tissue levels are higher than in older humans.
Experiments have shown that adding more brown fat to mice has been found to increase the rate at which they burn energy, reduce the amount of fat on their bodies, and protect them from diet-induced obesity.
Newborns have a higher proportion of brown fat than adults, and this gradually drops with age.
Brown fat can be detected in adults using a positron-emission tomography (PET) scan. This is easier to detect when a person has been exposed to cold temperatures. Most brown fat can be found in the lower neck of an adult, and the area above the collarbone.
A person who is overweight has proportionally less brown fat than a person who is not overweight. Brown fat may play a key role in keeping people lean.
Brown fat in newborns
In newborn humans, around 5 percent of body weight is made up of brown fat. It tends to be located on the upper half of the spine and towards the shoulders.
Extra supplies of brown fat in young infants helps to keep them warm.
Brown adipose tissue helps to produce heat. It is activated to do so when an organism, or body, needs extra heat. This can happen when a person is starting to have a fever, or when an animal awakens from hibernation. This heat generation, also called thermogenesis, is also triggered by feeding.
The function of the brown fat in newborn infants is to protect them from hypothermia, which is a drop in core body temperature. Hypothermia is a serious problem for premature newborns, and a major cause of death.
Older humans have a larger surface area, more muscle, a lower proportional surface area of the head, the ability to shiver, and the ability to move from cold areas. Newborns do not have this. High levels of brown fat in newborns provides an alternative way of regulating heat.
A solution for obesity?
With obesity currently at high levels, experts are looking for ways in which people can lose weight, either by cutting food intake or by burning more energy.
One way to burn energy is through exercise, but BAT also contributes to energy use. By dispelling energy as heat, it may be able to counter weight gain.
Experts say they do not yet know how humans could increase their brown fat content, but there is some thought that if they can find out how to turn more white fat to brown fat, this could help people to lose weight or prevent additional weight gain.
However, whether this is possible or useful remains unknown.
A study carried out by scientists from the Universite de Sherbrooke, Canada, found that volunteers with higher brown fat levels started shivering at lower temperatures compared to those with lower levels.
In addition, when the brown fat cells were active, the volunteers burned an extra 250 calories, an increase of 1.8 times in the calorie-burning rate.
The researchers also found that lean people have more brown fat than obese individuals, but they concluded that it is too early to draw any conclusions about the potential benefits of brown fat.
A number of pathways have been established that could be used to develop treatments for obesity. In December 2016, researchers announced they had found a signaling pathway that can start the process of turning white fat cells into brown fat cells, making them more likely to burn fat.
However, many challenges remain. For example, even if scientists can work out how to turn white fat to brown, how can it be activated to burn more energy?
Another concern is that even if brown fat can be increased to boost calorie consumption, the human body may still compensate by increasing hunger, leading to a higher calorie intake through food.
This research is still in the early stages, and it will be a long time before findings like these can be put to practical use.