++ to what Equus stated about getting enough carbs. Protein also!

I have to be very careful even when I'm not doing a lot of training about eating the right foods, consistantly, and enough of them or I experience massive headaches, vision problems, random stabbing pains, and feel very rundown and quickly fatigued when I fall off the wagon.

Thus, I've learned to be thrice as careful when I'm working towards a new fitness goal and pushing myself to extremes so I don't experience yucky-ness or burn out.