I have not been diagnosed but they elimination have found them to be the culprits of by bouts of extreme stomach cramping and light headedness. The thing is the symptoms aren't always severe. Is anyone else experiencing this? Have you found anything that helps other than trying to avoid them?
I often have reactions (mouth) to raw onions, but I can tolerate them cooked, usually, although they will upset my stomach sometimes.
I get horrible gas and sometimes cramps and diarrhea with onions. I find spring onions work better. Often I cut onion chucks up bigger for family that want it in a dish.
If you don't give up onions and garlic, it is possible, that your reaction could turn anaphylaxic without warning. Onions causes such severe bloating in me, I had to give them up. Fennel bulb is a great substitute though. Also, anything on the label that says "spice" means onion and/or garlic.
Some people are actually reacting to Fructans (which onions, garlic and other foods have) that are not being absorbed properly in the small intesting. FODMAPs diet has helped some people in this position. I was told that I should go on the FODMAPs diet. I've noticed that I do better low sugar, no legumes. I'm on a modified FODMAP diet in that I allow some sugar, but am working towards being more :P with my foods I eat. My diet is too restrictive already, and my budget too tight.