The fattiest organ in the human body is the brain, which is typically composed of 60% fat. By weight, water accounts for c. 77% of the brain, with lipids (fats) accounting for 10–12%.

The brain is composed of neurons and glial cells and is the largest part of the central nervous system. "Grey matter", which comprises 40% of the brain's weight, is pinkish-grey in colour (owing to the blood surging through it) and contains 100 billion signal-transmitting neurons. "White matter", accounting for 60% of the weight, connects the grey matter together, carrying the nerve impulses between neurons, as well as providing nutrition and support; the name comes from layers of a white lipid-rich (fatty) substance called myelin that protects and insulates the nerve cell axons.