A lot of things can cause fainting. Low blood pressure or "orthostatic hypotension" (you can look it up), iron deficiency or anemia, or more serious things such as diabetes. It may be something small like the lack of nutrition in the morning as you suspected, but it is a very serious situation if you could pass out at any time. You could fall and crack your skull, and also shouldn't ever drive if you are prone to fainting attacks. I'd go in for some routine labs and just make sure everything checks out. They'd check for anemia, glucose, thyroid panel, and so on. Hopefully though, it was a nutritional deficiency that you are already taking care of.