The Cooper Institute
 

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH

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One Fat, Two Fat, White Fat, Brown Fat

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Eat better

Thursday, Dec 18, 2014

If you are under the age of 40 or so, you might not understand the Dr. Seuss reference in the title of this article. I just couldn’t help myself.

Many years ago, it was assumed not only that all fat cells were alike, but also that fat cells were simply storage facilities for fat; and not active metabolically. More recently, we have learned that nothing could be further from the truth. Everybody’s probably familiar with white fat because that’s the type of fat cell we find ~98% of the time in the human body.  It’s also the type of fat cell that the 35% of American adults who are obese have way too much of!  What you might not know about white fat is that there are three separate storage depots for it. They include subcutaneous upper body fat, subcutaneous lower body fat, and abdominal visceral fat. The fat cells in each of these three depots do not ‘behave’ in exactly the same manner. In fact, both upper and lower body subcutaneous fat appear to be relatively benign when compared to visceral fat. Think of your abdomen in terms of layers.  Subcutaneous fat lies just under the skin; below the subcutaneous fat we find three layers of abdominal muscles. Visceral fat lies underneath the abdominal muscles among the internal organs. These fat cells have been shown to release a number of potentially harmful substances into the bloodstream. While not going into too much detail, these substances cause inflammation, insulin resistance, blood clotting, and constriction of blood vessels. So, the more visceral fat we have, the more of this toxic brew is released into the bloodstream. In turn, the risk of cardiovascular disease, diabetes, and some types of cancer is increased. Waist circumference provides us with an estimate of visceral fat stores; that’s the main reason why having a high waist circumference is a risk factor for many chronic diseases.

What is Brown Fat?

Brown adipose tissue (BAT) gets its name from its color. The color of these fat cells is due to the fact that they are absolutely loaded with mitochondria. When we took Biology class in high school, our teachers would typically refer to the mitochondria as the ‘powerhouses of the cell’. All of our cells require energy to function. What happens inside the mitochondria is that with the help of oxygen, food energy from carbohydrate and fat is converted into chemical energy called adenosine triphosphate. Because this process is only ~40% efficient, some heat is produced as a by-product. The two things that make BAT different than white fat cells are that BAT contains a tremendous number of mitochondria while white fat cells have relatively few mitochondria. More importantly, BAT bypasses the energy production aspect of carbohydrate and fat use in the mitochondria such that only heat is produced from the breakdown of these two fuels. You can think of brown fat cells as mini-furnaces that produce heat. Heat production by the body is the same thing as caloric expenditure. So, BAT is actually a ‘good guy’ in terms of helping to prevent obesity and type 2 diabetes. Not only does BAT have a great blood supply, but it can also take up large amounts of glucose and fat from the blood and convert them into heat very effectively.

It is well-established that animals exposed to a long cold winter have relatively high amounts of BAT to help them stay warm. We have also known for quite a while that newborns have a fair amount of BAT. It is speculated that the reason for this is to help keep the infant warm. For many years, it was thought that BAT pretty much disappeared by the time humans reached adulthood. More recent studies have shown that varying amounts of BAT are present in most adults (1). The most common locations for BAT are between the scapulae, above the clavicle (collarbone), as well as near the heart and kidneys. When individuals are in the process of developing obesity or type 2 diabetes, BAT shrinks and become less active. Conversely, lean individuals tend to have greater amounts of BAT that is very active. What obesity scientists are trying to find is a way to increase BAT and ‘flip the switch’ to make it more active. If this could be achieved, then rates of obesity and type 2 diabetes would most likely decrease because heat production (caloric expenditure) by the body would increase 24 hours a day. Even small increases in caloric expenditure at rest (i.e., resting metabolic rate) translates into greater total caloric expenditure over a 24 hour period!

A continuum of fat cells effects on health is shown below:

Adverse Effects on Health

Relatively Benign

Beneficial Effects on Health

White visceral fat

White subcutaneous fat

Brown adipose tissue (BAT)

To learn more about obesity, physical activity, healthy eating, and risk of chronic disease, take our Nutrition for Health and Fitness (NHF) course.

Reference:

Stephens, M., Ludgate, M., and Rees, D.R.  Brown fat and obesity: the next big thing? (2011)  Clin Endocrinol.  74(6):661-670.