Food allergies or intolerances can be quite common, and they don't always show themselves as skin rashes or a tightening of the throat. Some allergies like the ones to peanut butter, can be quite extreme and obvious, but an increase in irritability or a change in skin texture can also be telling signs that you may want to cut down or eliminate something from your diet.
Sugar is one of those substances that can have delayed effects to the body, since it takes so long to digest. Unlike natural foods like fruits and vegetables, the human body cannot recognize refined granulated sugar as any sort of substance, so it has to attach one of its own nutrients in the body to the sugar molecule in order to disguise it as something it can digest. Because it cannot digest it naturally, you may see reactions to youself which are not natural in themselves. Such reactions can be itchy skin on the hands or legs, blemishes on the face, or sudden irritability.
Other foods that people can have allergic reactions/intolerances to are wheat, eggs, nuts, and dairy products.
But not everybody is allergic to these foods all at one time, and you don't have to cut them all out of your diet. If you suspect you have a food allergy, try monitoring your eating habits and observing what reaction you have afterwards (this could be as little as an hour afterwards or as much as a day afterwards). Then you can slowly start to cut the food category out of your diet, while replacing it with something just as healthy (for example, soy milk instead of dairy milk)
For more information on this topic, you can go to:
http://en.wikipedia.org/wiki/Food_intolerance
http://www.hc-sc.gc.ca/fn-an/securit/allerg/fa-aa/index_e.html
