system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If body fat is stored energy, how come a morbidly obese person would die of starvation before all their fat is used up?

Let's assume here they're still drinking water and taking in basic minerals and vitamins that they need, but are consuming zeroish calories.
gpt
It's not starvation that morbidly obese people will die of when they literally stop eating. Its malnutrition. The body stores any energie it does not use in the form of fat. Energie intake can be done via different sources: carbohydrates, proteins, and fat. Note, we need all of the aforementioned in various degrees, but in western societies, they are heavily over consumed. Vitamins and minerals don't supply us with energy, but are necessary to perform functions, such as potassium is needed to flex a muscle. Vitamins and minerals can't be stored by the body, and an overconsumption of these will leave the body via excretion or fecal matter, or can even poison the body.   

Malnutrition is observed in lots of poor third world counties, but also in Western societies. It is because of people are not conscious of what their body needs.  

Here is an analogy for your morbidly obese person: an African child gets to eat nothing but a cup of rice each day. The cup of rice gives the cild the energy it needs. Energy alone however, is not enough. It also needs its vitamins, minerals, fatty acids, proteins and others to have all its cells perform its functions. The child does not have a healthy intake of vitamins and minerals, and slowly, its body functions are shutting down.   

The morbidly obese person has plenty of fat storage to provide the body with energie, however, the body does not need all this energy, it needs other nutrients, the obese person is not taking in, so it dies.