Read "fructose malabsorption" in Wikipedia.
http://en.wikipedia.org/wiki/Fructose_malabsorption
Wheat has fructose like chemicals. 35-40% of Central Europe has fructose malabsorption. Other sources say about 1/3 of all humans do.
Wheat and high fructose corn syrup are the worst fructose offenders in the typical diet. I quit eating apples based on the Wikipedia article. I do eat banana, as the article says those fruits which have an equal proportion of glucose to frutose do not cause problems. Oranges, kiwi, most berries, should be ok.
I read another article which said the average American ate 0.5 lbs of fructose per year in 1970 (I assume fruit), and now ate 59 lbs per year (I assume high fructose corn syrup).
Interestingly, plain corn syrup such as Caro is 100% glucose.