Eli5: Why and how does fat form on people/animals?
In: 1
The simplest answer, without getting into the chemical process is —
Your body (and other animals) turn extra “fuel” into fat. This fat can can be used as fuel when external fuel (aka food) is not readily available. So basically, food is in abundance now so you chow down. Your body “decides” it only needs part of what you’ve consumed, so it stashes some of the excess as fat and tucks it into areas like your belly and butt where it is “out of the way”. A few days/weeks later, and suddenly food is scarce. What you can find is not enough to support your activity levels, so your body grabs and burns fat to sustain your activities.
That’s why some animals, and people, are more prone to weight gain than others. They a genetically descended from creatures that survived in “feast or famine” environments. Their ancestors had the ability to store enough fat during feast periods to survive the famine periods
Fat is a type of cell, to be more precise. When you are born, in addition to al your other kinds of tissues, such as skin cells, organs made of cells, etc., you have fat cells. It’s important to have some of these, because people born without any fat cells [will have a variety of health complications,](https://rarediseases.org/rare-diseases/congenital-generalized-lipodystrophy/) such as diabetes.
As the other commenter describes, when you eat food and only part of the energy contained in the food is used, the rest gets stored as fat. Specifically, the food molecules are converted into fat molecules and stored inside fat cells. [Microscope images of fat cells](https://www.researchgate.net/profile/Saverio-Cinti/publication/226524850/figure/fig2/AS:302182676090880@1449057372169/Light-microscopy-of-human-white-adipose-tissue-A-subcutaneous-adipose-tissue.png) look like bubbles, because fat cells are only used to store the fat molecules, so there isn’t much else inside them.
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