I am wondering if it is possible that people with very fast metabolism have some genetic mutation. There are people who can eat a lot while others would definitely get fat with such a diet. In other words, they seem to have very fast digestion and never feel so much full after eating but what's surprising is that they still stay very skinny?
Can it be a result of some disorder/mutation?
The expression "fast metabolism" is very general and makes the question hard to answer. I will hence answer in the most general sense possible and then just give one example.
Yes, it is most definitely the case that there is genetic variation underlying the variation in metabolic abilities in the human population (btw, this post summarizing the partitioning of phenotypic variation might be helpful to you). For example, Greenlandic Inuits have a series of mutations that affect their ability to digest polyunsaturated fatty acids allowing them to cope with the impressive amount of animal fat that is/was found in their traditional diet (Fumagalli et al., 2015).
