low blood sugar..

Hi, my middle brother is getting irritated so easily. Sometimes with small-time things often in his chatting with someone like my cousins. He is opinionative, and hard to convince. I know that this is ok, and many people out there like bro, but this is affecting his health. Two days ago, he was sick, low sugar and heart little down. He said that the doctor told him he has a lack of sugar in blood ( not sure about this because I wasn?t there with him, and I wonder when went to the doctor without me !). This will cause trouble in the heart for him.

Are there such things; lacking of sugar in blood ( he isn?t diabetic ) that affect the heart, or cause the person to be down, despair, frustrated.

Thanks