How do some people get fat as fuck so easily and other people struggle to gain weight?

I have a blood sugar disorder. If I don't eat regularly, ny blood sugar drops. The longer I go without food the more intense the effects become. First I feel hungry, then queasy, then a headache kicks in, if I don't take in some calories I'll throw up and potentially pass out. Over a lifetime it's trained me to eat as soon as I feel my blood sugar drop. I live on a low calorie/ low carb diet in an attempt to not gain weight. I eventually end up having to eat carbs or my body will take all available energy away. As soon as I eat, I feel normal. Doctors always think they're going to catch me in a prediabetic high blood sugar so they test me and discover I have low blood sugar. Then, the conversation ends. I've never been offered any medication to help me regulate my low blood sugar. All they ever say is "You really need to lose some weight". BTW I'm about 50 pounds overweight.

/r/NoStupidQuestions Thread