Onions can be a particular problem for some individuals, especially those who already experience digestive issues, such as irritable bowel syndrome. This is because onions contain high levels of fructans. According to Food Map, fructans are the natural way that some fruits and vegetables store their carbohydrates. Such as in the case with raffinose, the human intestines do not produce the enzymes needed to break down fructose. As a result, these compounds also end up in the large intestines, where they are broken down by gut bacteria, causing gas.