Brown and Beige Fat Functions and Possible Health Benefits
White, Brown, and Beige Fat
When most people hear the word "fat" in reference to body tissues, they think of the soft material under the skin that is responsible for weight gain. This picture is not completely accurate, however. There are actually three types of fat in our bodies—the familiar white fat and the less common brown and beige fat.
White fat has important uses but can also be harmful. The fat molecules inside its cells can be used to produce energy. If energy isn't needed, the molecules are stored inside the cells and form part of the body fat. Brown fat has special benefits compared to white fat, including helping to control our weight. Once beige fat has been activated, it has the same benefits as brown fat. The cells of brown and activated beige fat break down fat molecules to produce heat instead of storing them.
Researchers have discovered that slender people have more brown fat than overweight people. Increasing the amount or activity of brown and beige fat might be a way to reduce obesity and the incidence of some diseases. Obesity is a risk factor for heart disease, high blood pressure, type 2 diabetes, and some types of cancer.
Scientists warn us that finding a safe way to increase brown or beige fat activity is unlikely to be a perfect solution for a weight problem. We will probably still need to follow a healthy diet and lifestyle in order to lose an adequate amount of weight.
Fat Distribution in Our Bodies
White fat is the kind that collects under the skin, where it's known as subcutaneous fat. It also collects around organs. In this location it's known as visceral fat. Excess subcutaneous or visceral fat is dangerous. Fat deposited inside blood vessels is very dangerous and increases the risk of a heart attack or stroke.
Until 2009, it was thought that brown fat was most abundant in newborn babies and small mammals. Very young babies are unable to shiver to keep warm and therefore need the heat produced by the fat. Small mammals also need brown fat since they lose heat from their bodies very quickly. Adult humans were believed to have no or very little brown fat. Researchers now know that adults have a significant amount of the fat and that it's located in patches around the body.
Beige fat cells are always found within white fat. Like brown fat, beige fat is found in patches in various parts of the body. Its cells have many of the characteristics of brown fat cells, including the ability to produce heat from fat once they are activated. Activation by an appropriate stimulus causes changes in the cells that enable them to produce heat.
Differences in Adipose Tissue
There are some important differences between white, brown, and beige fat and their cells. The cells that store fat molecules are called adipocytes.
- White fat is also known white adipose tissue or WAT.
- Brown fat is known as brown adipose tissue or BAT.
- There is no standard abbreviation for beige fat since it has been identified quite recently. (It was known that beige fat cells are located in white fat, but it was thought that the cells were classical brown fat cells.)
- One abbreviation that's used for beige fat is iBAT (induced brown adipose tissue).
- Beige fat cells are sometimes called brite cells (brown in white cells).
- White fat cells contain one large droplet of fat, which consists of triglyceride molecules.
- Brown and beige fat cells contain several smaller fat droplets of varying sizes, which are also composed of triglycerides.
- Brown and beige fat cells contain a much higher number of mitochondria than white fat cells. Mitochondria are the structures that produce energy in a cell. They contain iron, which gives brown and beige fat their color.
White fat is white or pale yellow because its cells contain a relatively low number of mitochondria. Brown fat is brown because it consists of mitochondria-rich cells. Beige fat is a lighter shade of brown because it contains a mixture of mitochondria-rich cells and cells that contain fewer mitochondria (the white fat cells).
Differences Between Brown and Beige Fat Cells
Although brown and beige adipocytes are similar, they aren't identical.
- Brown fat cells produce heat from fat molecules while beige fat cells do this only when activated.
- Brown fat cells are derived from cells that also produce muscle cells.
- The development of a beige fat cell is related to that of a white fat cell.
- Brown and beige fat cells differ genetically.
- Despite their different life histories and genetics, a brown fat cell and an activated beige one perform the same job. Beige cells may not produce as much energy as brown cells, however.
The lack of distinction between brown and beige fat cells in some research reports—especially those completed before beige fat was identified in 2012—can be confusing. Some descriptions of brown fat may really be referring to the beige type. Hopefully future discoveries will clarify the research.
Functions of Fat
White Fat Functions
White fat under the skin has several important functions. For example, it insulates the body from heat loss, cushions organs from blows, and stores energy. It also produces hormones. The triglyceride molecules in the adipocytes can be broken down to produce ATP (adenosine triphosphate) molecules. These are able to quickly release energy when it’s needed. If we don’t need the the energy, though, fat continues to be stored in the cells.
Brown and Beige Fat Functions
In brown fat and activated beige fat, the triglyceride molecules are also broken down. The amount of ATP that is made is greatly reduced, however. Instead, a large quantity of heat is produced from the triglycerides during a process called non-shivering thermogenesis.
Beige fat cells normally have a low level of a protein known as UCP1. Mitochondria need this protein in order to produce heat. Activation increases the amount of UCP1 in a beige fat cell and enables it to act like a brown fat cell.
Amount and Activity of Brown Fat
Researchers have found that brown fat becomes more active when we're cold. In addition, people have more brown fat in their bodies in winter than in summer. Women seem to have a larger amount of the fat than men. This may be because they have less muscle and therefore shiver less effectively. Shivering is one of the body's methods to produce heat.
The Irisin Hormone, Weight Loss, and Insulin Sensitivity
Research has shown that when both mice and humans exercise, a hormone that was unknown until quite recently appears in their bloodstream. This hormone has been named irisin after Iris, the Ancient Greek messenger goddess.
Like other hormones, irisin carries messages to body tissues and triggers specific effects. Researchers have found that mice and humans who have been participating in exercise sessions for a greater number of weeks than other members of their test groups have a larger concentration of irisin in their blood.
In an experiment with mice, researchers have discovered that some significant effects of irisin—at least in obese, pre-diabetic, and non-exercising mice—are to cause more beige fat cells to appear in white fat (a process known as browning), to activate the beige 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 helps to control the blood sugar level.
Researchers at Washington State University have found that a chemical in fruit known as resveratrol also causes browning of white fat in mice. In addition, researchers have discovered that beige fat cells are activated by cold as well as irisin.
Irisin and Insulin Resistance
Insulin travels from the pancreas to the body's cells via the bloodstream. The insulin molecules join to receptors on cell membranes. As a result of this union, glucose is able to leave the blood and enter the cells. Glucose is an energy source for the cells.
In people suffering from Type 2 diabetes, the pancreas is producing insulin, but the cells aren't responding properly to the hormone. This condition is known as insulin resistance. Insulin resistance is often a precursor to full-blown diabetes.
Obese people have a higher chance of developing insulin resistance. Since irisin affects beige fat and body mass and also improves the insulin sensitivity of cells, it could be very helpful for people with Type 2 diabetes (if it has the same effects in humans as it does in mice).
In early 2015, a group of scientists claimed that irisin doesn't exist in the body and that the experiments that showed its presence were flawed. Since then, additional research has discovered that the chemical does exist, although it's produced in small quantities. The detection method that's used appears to be critical for obtaining the correct result.
Other Potential Benefits of Irisin
The irisin level in humans rises after regular and moderately intense aerobic exercise that requires endurance. It certaInly seems like a hormone that we would want to increase. Research suggests that it has a range of benefits.
Much of the irisin research has been done by Dr. Bruce Spiegelman and his colleagues at the Harvard Medical School and the Dana-Farbar Cancer Institute. In 2013, the team found that irisin acts on the brain of mice as well as on their fat. The hormone not only improves the cognitive abilities of mice but also stimulates the growth of new nerve cells, or neurons.
Irisin is made by skeletal muscles. In 2017, scientists at Tufts University found that it's also made in bone, at least in mice. After two weeks of "voluntary wheel running", mice had an increased level of irisin in their bones as well as increased bone mass. Administration of irisin to mice who didn't exercise also increased bone mass.
We need to be careful when reading about irisin benefits. Some discoveries have been made only in lab animals. These discoveries may or may not apply to humans. In addition, sometimes a discovery is made by one scientist but either can't be duplicated by others or is shown to be wrong.
The video below talks about the transformation of adipocytes from one type to another. This process might eventually be beneficial.
Potential Ways to Change White Fat into Brown Fat
Research in mice has shown that enzymes called sirtuins speed up metabolic activity (the rate of chemical reactions in the body) and stimulate the conversion of white fat into brown fat. We have sirtuins in our bodies too, so researchers want to discover if enhancing the activity of these enzymes will lead to weight loss in us.
Researchers have also found that a group of chemicals called thiazolidazines, or TZDs, can change white fat to brown fat and increase the body's sensitivity to insulin. However, TZDs have dangerous side effects. They poison the liver, cause bone loss, and cause weight gain instead of weight loss. Researchers think that there might be a way to safely use these chemicals to produce more brown fat in the body and stimulate weight loss, though.
Investigating the role of brown and beige fat in adult humans is a relatively new area of research, but it could produce some very exciting health benefits. Interest in the fat types is escalating, especially since they offer hope for the increasingly common problems of obesity and type 2 diabetes.
Having more brown and beige fat in our bodies could be very advantageous, but artificially triggering their production or activity could be dangerous. More research needs to be done to determine the differences between brown and beige fat, to understand their effects on our body in more detail, and to determine the methods, effects, and safety of changing one type of adipocyte to another. Brown and beige fat might be very helpful for us in the future, however.
- Brown Fat Benefits from the BBC
- Irisin Benefits from Psychology Today
- Beige Fat Benefits from the Harvard Gazette
- Scientists turn white fat into beige fat from Washington State University
- Exercise-induced hormone irisin is not a 'myth' from the Medical Xpress news service
- Irisin and Fat Loss from the University of Florida
- Bone Formation After Irisin Administration from Tufts University
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2012 Linda Crampton