White adipocytes (white fat cells) have a single lipid droplet, brown adipocytes contain many small lipid droplets, as well as a very high number of iron-containing mitochondria. Brown fat gets its dark red to tan color from its high iron content.
Brown fat has more capillaries than white fat, because of its higher oxygen consumption. Brown fat also has many unmyelinated nerves, providing sympathetic stimulation to the fat cells.
Definition of lipid - put simply, lipid is another word for fat. Specifically, lipids are substances that dissolve in alcohol but not in water, such as fat, oil or wax. They contain oxygen, hydrogen, and carbon. Lipids have a much lower oxygen content than carbohydrates. In this text, the word lipid(s) mainly refers to fats.
The fat that builds up around our waists and thighs is the white type, while brown fat is mainly found around the neck areas (front and back).
Scientists are only now just beginning to understand what the functions of brown fat are. From what we can tell so far, main differences between the two are:
- White fat is the result of storing excess calories. When we consume too many calories, the body converts them into a contingency energy reserve in the form of white fat.
- Brown fat generates heat by burning calories. When it is cold, brown fat's lipid reserves are depleted, and its color gets darker.
PET/CT scan showing brown fat in a woman (in red)
A human newborn/infant has approximately 5% of body weight made up of brown fat. This gradually drops with age. Brown fat can be detected in adults using a PET (positron-emission tomography) scan - detection is clearer when a person has been exposed to cold temperatures. Most of it can be found in the lower neck and supraclavicular regions of an adult.
Ironically, the fatter a person is the less brown fat they have, or the thinner they are the more they have. Endocrinologists and other experts say brown fat probably plays a key role in keeping people lean.
Brown fat generates heat in newbornsIn newborn infants, a large amount of brown fat is located on the upper half of the spine and towards the shoulders; its main function is to protect the baby from hypothermia (drop in core body temperature). Hypothermia is a serious problem for premature newborns, and a major cause of death.
Older humans have a larger surface area, more muscle, a lower proportional surface area of the head, the ability to shiver, and the ability to move from cold areas - newborns do not. High brown fat levels in newborns provide them with an alternative way of regulating heat.
Should we try to increase our brown fat content?Experts say they do not yet know how humans could increase their brown fat content, or even whether this is recommendable or desirable. In a study carried out by scientists from the Universite de Sherbrooke, Canada, they found that volunteers with higher brown fat levels started shivering at lower temperatures compared to those with lower levels. (Link to article)
They also found that when the brown fat cells were active, the volunteers burned an extra 250 calories; a 1.8 times increase in calorie burning rate.
The researchers said it is too early in their research to make any conclusions regarding the weight-loss benefits of brown fat. They did find that lean people have more brown fat than obese individuals.