The “T3” thyroid hormone triggers calorie burning in brown fat.
Everyone knows about FAT: we all care about it and everyone worries about it.
Fat is among the most popular topics in the urbanized world, perhaps slightly less of a concern to the rural population and in some societies, an asset to a few (sumo wrestlers, for example).
Our conversations about FAT are limited, most of the time, to “White” Adipose Tissue, referred to in the medical literature as WAT. Most of us know nothing about brown fat, our Brown Adipose Tissue, jocularly nicknamed BAT. [1]
We have two types of Fat:
WAT, the infinitely expandable energy storage layer, fills out the areas between our bony protuberances, providing the “curvaceous” outline in which healthy young women take pride and without which we don’t look “well”.
It is found under the skin, but also in many other parts – for example:
– In the “mesentery”, a double-layered sheet that looks like cellophane, attached to the midline, from which the small bowel hangs.
– In a similar sheet, hanging off the stomach, supporting the middle of the large bowel.
– Around the heart arteries: this small deposit looks like white fat, but it originally evolved from brown adipose tissue and it has brown, or beige-like, features: especially, it has a nerve and blood supply, and immune cells, just like BAT – and it works like BAT, supplying the heart muscles with energy and temperature regulation.
– Around the outside of the peritoneum (the lining of the abdominal cavity): white fat is spread out around the kidneys, bladder, and the large blood vessels in the pelvis.
– Around the eyes: this is special: there are two separate fat pads in the upper eyelid, three in the lower lid and a bigger fat pad behind each eye. – The ocular fat pad layout is really complicated and too much to describe in detail in this blog post: if you’re interested, click here.
– There is even white fat in the Pinnae (the ears), although in tiny quantity.
Some lucky body parts don’t carry any WAT
About the only places where there isn’t any white fat are inside the skull, the lining of the chest cavity, the penis and testicles in the male, the clitoris and ovaries in the female, inside the joint cavities and in the nail beds of the fingers and toes.
BAT, on the other hand, is a completely different, fixed-volume, energy-producing, glucose and cholesterol-burning organ. It is situated discreetly beside the spine, in the shoulders, the base of the neck and the “mediastinum”, where the heart, the esophagus and our “breathing pipes” lie hidden, deep within the chest.
The difference between WAT and BAT:
WAT volume is variable in the newborn, depending on the maternal diet and metabolism (diabetic mothers make fat babies). It has a sparse blood and nerve supply and is composed of large, unilocular cells, each containing a single large lipid droplet, which contains fatty acids, and only a few small mitochondria. Its cells are mostly nondescript, but the tissue contains some progenitor (“stem”) cells, which have a few extra mitochondria and can be converted to BAT-type cells.
WAT increases its volume by storing the products of excess food intake, as additional fatty acids and oils, in the intracellular globules, without increasing the number of cells.
BAT adipocytes are small, multilocular cells, containing multiple small lipid droplets and numerous mitochondria with high iron and cytochrome content. They are densely innervated by the sympathetic nervous system and have a generous blood supply.
Brown fat is abundant in the newborn, although reduced in the presence of maternal/fetal malnutrition. It possesses many progenitor (“stem”) cells, so the volume of the energy- burning organ can be increased, under special circumstances, by increasing the number of cells. Generally, the total volume of BAT is constant, but there is a tendency to gradual attrition over time and some obese individuals have little, or no BAT. It remains to be seen whether those with little BAT become obese, or whether obese people’s BAT shrinks, because it’s not being used (a much more likely scenario).
Anyway, everyone knows that white fat is just an unexciting, boring, energy storing embarrassment, so let’s talk about brown fat, first!
Graphic representation of BAT, from “ice bath science”, by Thomas P. Seager, PhD
See Dr. Seager’s article, at https://www.morozkoforge.com/post/ice-bath-science [1]
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