Bodies have an awareness of how much fat they have, because fat plays crucial roles, and those roles are not, as many diet explainers only grudgingly admit, just insulation and energy storage. Body fat stores vitamins (A, D, E, and K); coats nerves and connective tissues; and regulates hormones, including estrogen, insulin, and cortisol. Certain immune cells found in fat are even anti-inflammatory. While body fat is more flexible in quantity than most other kinds of tissue in the body—that is, we can store it and burn it more easily than, say, bone or organ mass—its presence is still critical for all of those essential bodily functions. This is one reason why bodies work so hard to manipulate energy levels and metabolism to prevent from losing too much body fat too quickly and, in a lot of cases, seem to work uphill against any kind of change in body-composition status quo: complex systems threaten to shut down when bodies don’t have all the fat they need.
Casey Johnston, A Physical Education: How I Escaped Diet Culture and Gained the Power of Lifting
