Brown Fat - Thermogenesis, Hormonal Control and Health Effects

There are two types of fats in our bodies - the familiar white fat and the less common brown fat. White fat has important uses but has a bad reputation, and is dangerous if too much builds up in the body or if it’s deposited in the wrong place. Brown fat, however, has unique benefits compared to white fat, including helping to control body mass.

White fat cells (which form white adipose tissue, or WAT) store fat molecules, which can be used to produce energy if this is necessary. If it’s not, the fat remains in the cells. Brown fat cells (which form brown adipose tissue, or BAT) also contain fat molecules, but they break these molecules down in a process called non-shivering thermogenesis instead of acting as a long-term storage depot for fat. The thermogenesis process produces a large amount of heat. Researchers have discovered that slender people have more brown fat than overweight people.

Increasing the amount of brown fat in a person or stimulating the activity of the brown fat cells might be a way to reduce obesity, but at the moment there is still much that is unknown about brown fat and thermogenesis and about the safety of artificially triggering brown fat activity. New and exciting discoveries are being made, though, including the recent discovery of a hormone named irisin that appears in our blood after exercise and appears to increase the amount of brown fat, reduce weight and improve the sensitivity of cells to insulin. The latter effect may be especially helpful for people with Type 2 diabetes. Researchers caution that even if we find a way to safely increase brown fat activity and insulin sensitivity there will still be a need for a healthy diet and lifestyle to help decrease body mass and maintain health.

Until 2009 it was thought that brown fat was most abundant in newborn babies, who can’t shiver to keep warm, and in small mammals, which lose heat rapidly, but was either absent or present in only a tiny amount in adult humans. Researchers now know that adults actually have a significant amount of brown fat.

White fat is the kind that collects under the skin and sometimes - very dangerously - around organs, where it’s called visceral fat, or inside blood vessels. In adults, patches of brown fat are found in the front part of the lower neck and in the upper chest around the collar bone.

Fat cells are known as adipocytes. White fat adipocytes contain one large droplet of fat, which consists of triglyceride molecules. Brown fat adipocytes contain several smaller fat droplets of varying sizes, which are also composed of triglycerides. In addition, brown fat cells contain a much higher number of mitochondria, the structures that produce energy in the cell. Brown fat also contains many more capillaries than white fat and is darker in color.

White fat under the skin has several important functions. It insulates the body from heat loss, cushions organs from blows and stores energy. The triglyceride molecules in the adipocytes can be broken down to produce ATP molecules (adenosine triphosphate) which are able to quickly release energy when it’s needed. If we don’t need the the energy, though, the fat continues to be stored in the adipocytes.

In brown fat the triglyceride molecules are also broken down, but instead of ATP molecules being made, a large quantity of heat is produced during thermogenesis. Brown fat becomes more active when we are cold. In addition, people have more brown fat in their bodies in winter than in summer. Women seem to have more brown fat than men, perhaps because they generally have a lower percentage of muscle mass. Shivering of muscles is one of the body's mechanisms to produce heat.

Recent research has shown that when both mice and humans exercise a previously unknown hormone appears in their bloodstream. The researchers have named this hormone irisin after Iris, the ancient Greek messenger goddess. Like other hormones, irisin carries messages to body tissues and triggers specific effects. The researchers found that mice and humans who had been participating in exercise sessions for a greater number of weeks had a larger concentration of irisin in their blood.

The researchers discovered that significant effects of irisin - at least in obese, pre-diabetic and non-exercising mice - are to trigger white fat cells to turn into brown fat cells, to induce a small weight loss and to make the body's cells more sensitive to insulin. Insulin is a hormone made by the pancreas that joins to receptors on cell membranes. As a result of this union glucose is able to leave the blood and enter the cells. The researchers suggest that if they had used more irisin in their experiment or if they had treated the animals for a longer time the mice would have lost more weight, but this idea still has to be tested.

In people suffering from Type 2 diabetes the body is producing enough insulin, but the cells aren't responding properly to the hormone, a condition known as insulin resistance. Obese people have a higher chance of developing insulin resistance. Since it affects brown fat and body mass and also improves the insulin sensitivity of cells, irisin could be very helpful for people with Type 2 diabetes (if it has the same effects in humans as it does in mice).

Investigating the role of brown fat in adult humans is a relatively new area of research, but it could produce some very exciting health benefits. It seems that there's still a lot to be learned about the human body!