The Cooper Institute
 

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

 
 
 

One Fat, Two Fat, White Fat, Brown Fat

Tuesday, Dec 14, 2021

Hope you liked the Dr. Seuss reference in the title! 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. In recent years, we have learned that nothing could be further from the truth. Everyone knows about white fat because that is the type of fat cell we find ~98% of the time in the human body. It is also the type of fat that 40% of American adults who are obese have excessive amounts 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 release a number of potentially harmful substances into the bloodstream. While a detailed explanation of these substances is beyond the scope of this article, they 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; which is 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 phosphate (ATP).  Because this process is only ~40% efficient, some heat is produced as a byproduct. The two things that make BAT different from 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. 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. Thus, 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 who are 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. At this point, you may be wondering where in the body these BAT depots are found. The most common locations for BAT are between the shoulder blades and above the 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 becomes 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 24-7. Even small increases in caloric expenditure at rest (i.e. resting metabolic rate) translates into greater total caloric expenditure over a 24-hour period.

It goes without saying that increased levels of physical activity are also necessary in order to achieve optimal health and long-term weight control.

 

To summarize, a continuum of fat cells effects on health is shown below:

Adverse Effects on Health          White visceral fat

Relatively Neutral                    White subcutaneous fat

Beneficial Effects on Health       Brown adipose tissue (BAT)