There are 2 different questions being asked here and they are:
- Does hyperglycemia result in a person being thin?
- Why some people can't gain weight?
To answer your first question, yes, it is directly connected. With low insulin levels the body isn't able to process/utilize the glucose for fuel, so it starts getting the required energy from fats. When these are burned, the person starts losing weight. Be aware that if the body is extremely low on insulin and starts to burn the fats down at a fast rate, this may result in ketoacidosis, which is a life-threatening condition.
To answer your second question, there are various reasons:
- High energy output - this is kind of obvious. A person who moves all the time, doesn't use cars or public transport, tends to be active, etc, has a high energy output, therefore can eat as much as he wants and he won't ever gain weight.
- High methabolism - methabolism is the rate at which a person burns down energy. People with high methabolism burn the energy (the food) very fast, therefore they are unable to gain weight. In a healthy person, a high methabolism is "a part of his DNA", but in some cases, it can also be caused by hyper-thyroid disorder, which can be treated. If high methabolism is a part of person's DNA, it shouldn't be fought with. If such people really want to gain weight for some reason, they should eat 5 - 6 times a day, but certainly not take medication to slow their methabolism down.
- Tape worm - this one is kind of ugly. Ever ate fish? Fish improperly processed? There might be a tape worm living inside of them and after you eat it, it starts living inside of you and 'consuming' the food you consume. Therefore, a lot of the food you eat is actually eaten by the tape worm and you just keep eating and eating and you are constantly hungry.