Fat. That one part of the body that everyone wants to get rid of.

But fat—or adipose tissue—is more complicated. And while too much of some types of fat can contribute to health issues, other types are critical for life.

How do fat cells work?

Adipose tissue is composed of adipocytes, cells whose main function is to store and release energy in the form of triglycerides (fat). Triglycerides are molecules made up of fatty acids and glycerol and are the major lipid part of fat obtained from food. In situations of energy surplus, circulating free fatty acids are taken up by adipocytes and converted and stored as triglycerides in a process called lipogenesis. In the reverse process, called lipolysis, triglycerides are broken down into fatty acids that are supplied to the body as an energy source during times of high energy demand (fasting, exercise, etc). In this way, fat cells are modulators of the body’s energy stores.

Fat cells—and the balance of fat storing and fat burning—are sensitive to insulin, a hormone released by the pancreas following a meal. Increased insulin levels turn off fat burning (lipolysis) and increase fat storing (lipogenesis). In contrast, in a state of fasting, low insulin levels stimulate fat breakdown and the release of fatty acids for energy.  In the context of diabetes or insulin resistance, insulin loses its ability to regulate triglyceride breakdown, resulting in unchecked lipolysis and an increase in the amount of free fatty acids in the circulation.

Fat cells, or adipose cells, 3D illustration
When you eat, adipocytes store fat in the form of triglycerides. When you fast, they release it to be used for energy. Both processes are controlled by insulin.

Fat cells also have an endocrine function and the main adipocyte-derived hormone in the body is called leptin. Leptin is produced by white adipose tissue and regulates energy homeostasis in the body by acting as an energy sensor, alerting the rest of the body to the status of adipocyte energy stores. Leptin levels are correlated with fat mass in humans, with high leptin levels indicating high energy stores, which blocks food intake and increases energy use.  Low leptin levels may occur during fasting or starvation and signal for reduced energy use and increased food intake. In obese individuals, leptin resistance may occur, disrupting the ability to respond to leptin, even if circulating levels are high due to high fat reserves.

What are the different types of fat in humans?

Fat tissue (and function) is defined by its color as well as where it is distributed in the body.

White fat makes up the majority of fat in mammals and is largely responsible for energy storage, insulin response, and endocrine signaling functions (described above). This is the type of fat that we think about when we refer to fat in general. 

Brown fat acts differently than white fat. Its main function is to burn energy and produce heat, and therefore, brown fat is important in the regulation of body temperature. Brown fat is most commonly found in infants and hibernating mammals and was recently confirmed to be present in adult humans who were thought to have very little amounts of brown fat. 

The third fat hue is beige. Beige fat functions like a white-brown fat hybrid and most often develops within white fat. Given the role of brown fat in increasing energy expenditure compared with the role of white fat in energy storage, there is interest in understanding how white fat “browns” into beige fat as a potential strategy for combating obesity.

Where are the major fat depots in humans and does fat distribution matter?

Fat distribution can be defined as subcutaneous or visceral. Subcutaneous fat is located just below the skin and subcutaneous fat makes up most of the fat in our bodies. The major subcutaneous fat depots are the abdominal, femoral, subscapular, and gluteal fat stores. In contrast, visceral fat is located near or around internal organs in the abdominal cavity including the liver and intestines. Importantly, fat distribution is a major risk factor for metabolic disease, including obesity-related insulin resistance. Body fat distribution affects disease risk independent of total body fat levels or body fat percentage.

Subcutaneous fat is located just below the skin. Visceral fat is located near or around internal organs in the abdominal cavity. High visceral fat increases the risk for type-2diabetes and high blood pressure.

Obesity has been described as an epidemic, with over 30% of Americans and one-fifth of the population of Europe affected. Mechanistically, obesity resulting from overfeeding causes adipocytes to expand in volume (to accommodate increased triglyceride volume); once a threshold is reached, adipocytes will then increase their number and eventually, fat depots will increase in size. 

Visceral fat has been identified as the “bad” fat type when it comes to greater health risk. In particular, obesity associated with the accumulation of visceral fat is correlated with higher risk of insulin resistance, type 2 diabetes, hypertension, and mortality. Interestingly, emerging evidence indicates that people with obesity and type 2 diabetes have limited amounts of metabolically active brown fat and in rodents, the browning of white fat (as well as the activity of brown fat) is associated with protection against obesity.

In total, fat is more than its reputation would suggest. Beyond the storage of energy, fat tissue has a key role in maintaining energy homeostasis and for brown fat in particular, thermogenesis. When determining the potential pathogenic implications of fat, it is all about location, with visceral fat most implicated in negative health effects. On a positive, note, various clinical studies over the years have demonstrated that exercise, especially high intensity exercise, can reduce visceral fat and its associated risks.