Fat Loss 101 – Understanding the Different Types of Body Fat
Few things in this world garner as much collective agreement as the recognition that excess body fat is detrimental to health. In fact, body fat is the target of a $254.9 billion weight loss industry worldwide.
Despite being villainized, fat is not universally bad. Fat serves some vital functions and even works to regulate the body against accumulating more of itself.
Essential Body Fat
The human body must maintain an amount of body fat within a certain range in order to function properly – not too little and not too much. The bare minimum amount of fat must be present for functions such as hormone regulation and vitamin absorption, without which one cannot thrive.
This essential minimum fat exists in various areas throughout the body: brain, bone marrow, nerves, and membranes around organs. Women need at least 10-13% body fat, whereas men need a minimum of only 2-5%. This sex difference reflects the importance of body fat for female fertility.
An example of the consequences of inadequate body fat is the Female Athlete Triad, a pattern noted among adolescent and young adult female athletes. It is characterized by:
- Low energy availability +/- eating disorder
- Menstrual dysfunction
- Low bone mineral density
Types of Body Fat
White fat. White fat is the predominant fat in our bodies and is what we typically refer to as “fat”. This tissue is specialized to store energy in the form of triglycerides, or fatty acids. The storage and release of fatty acids are controlled by insulin and catecholamines (stress hormones). Though white fat may be found throughout the body, there is an important differentiation between some locations:
- Subcutaneous fat is the fat present just beneath the skin. It is the “pinchable” fat and the most visible collection of fat stores.
- Visceral fat is located within the abdominal cavity and is present around the abdominal organs. It is within the abdominal musculature and is what causes a protuberant abdomen. Compared to white fat, visceral fat is much more inflammatory and is associated with metabolic disease and increased mortality.
Brown fat. The minority fat in our body is brown fat, characterized by much smaller cells and a brownish color. The coloration occurs because of the high density of mitochondria, the energy-producing organelles present in our cells. Brown fat produces heat and tends to disappear as we age. It is primarily located in the ribs, upper chest, and neck.
Beige fat. Beige fat appears to be a hybrid of white fat and brown fat. There is evidence that a particular hormone produced in response to exercise, Irisin, can convert white fat to beige fat.
Fat cell population
Fat cells have a lifespan of approximately 10 years, with about 10% of fat cells being replaced by new fat cells every year. In humans, there is an increase in the number of fat cells in childhood through puberty before leveling off and remaining stable in adulthood. It appears that, once an adult, the total number of fat cells remains the same, but the fat cells may then increase or decrease in size. Thus, your pool of fat cells is set in your adolescence and is stable for life, whether you gain or lose weight as an adult.
As a whole, fat mass creates a state of low-grade inflammation, which has negative consequences such as susceptibility to infection and increased risk of chronic diseases including cancer.
Passive Roles of Fat
Fat is primarily a method of storing excess energy in the body. For example, when excess glucose is present in the blood, it behooves the body to remove it from the circulation by storing that excess glucose as fat. Fat can be the storage vessel for that glucose, effectively removing the glucose from the bloodstream. Thus, in a sense, fat can actually be a buffer against disease – diabetes, etc.
Due to its bulky constitution, fat serves as a cushion against injury, protecting lean body mass and internal organs against direct trauma. It can also function as passive insulation to conserve body heat.
Complex Roles of Fat
That’s not all fat is good for; it does far more than these passive functions. Fat actually signals the rest of the body what is going on in terms of energy supply and modulates food intake accordingly. It does so by hormonal signaling.
There are more than 50 substances produced and released by fat that regulate bodily functions. The most well known hormone produced by white fat is leptin, which is known as the satiety hormone. As the mass of white fat increases, more leptin is produced with the intent of decreasing our appetite and thus the amount of energy consumed. Other common hormones associated with fat mass include adiponectin and adipsin.
Too much or too little fat can adversely affect female fertility, through complex hormonal means. It appears that an optimal bodyfat percentage for fertility is approximately 22-28%.
Fat Cells and Metabolism
White fat has a very low metabolic rate, mostly serving as a passive form of energy storage.
Brown fat, however, with its high concentration of mitochondria can reach a metabolic rate 10 times higher, putting it at a level similar to muscle tissue. Maximally stimulated brown fat can produce up to 300 Watts per kilogram of heat compared to 1 Watt/kg in all other tissues.
In other tissues throughout the body, mitochondria generate heat on-demand only – as the cells need it. This coupling of energy to the cell’s demand is not seen in brown fat, however, where there are uncoupling proteins that allow the mitochondria to simply produce heat. There is a much larger blood supply to the brown fat tissue that allows for distribution of this heat.
In contrast to the inflammatory white fat, brown fat actually protects against metabolic disease such as obesity, diabetes, fatty liver, and more. The more brown fat that is present, the less visceral fat and the better the overall fat distribution. Brown fat does more than just generate heat – it is also involved in hormonal signaling.
Brown fat is also unique in that it is activated by cold exposure – at 64.4 deg F, brown fat becomes active and starts burning glucose to warm the body. There won’t be any muscle shivering at that point because the brown fat can compensate with heat production, but may occur at lower temperatures.
Infant humans have relatively large collections of brown fat which help to regulate their body temperature. This mechanism of heat production is vital for infants who would otherwise be prone to hypothermia due to their high surface-area-to-mass ratio and low muscle mass. Thanks to brown fat, infants are able to better regulate their body temperature and typically don’t shiver.
Activation of brown fat is also one of many mechanisms action identified with the class of medications known as GLP-1 agonists.
Body fat can take many forms and serve many different roles. Beyond just being a depot for excess energy, it is responsible for complex hormonal signals that impact our eating behavior and metabolism.