Fructose is absorbed in the small intestine without help of digestive enzymes. Even in healthy persons, however, only about 25–50 g of fructose per sitting can be properly absorbed. People with fructose malabsorption absorb less than 25 g per sitting. Simultaneous ingestion of fructose and sorbitol seems to increase malabsorption of fructose. Fructose that has not been adequately absorbed is fermented by intestinal bacteria producing hydrogen, carbon dioxide, methane and short-chain fatty acids. This abnormal increase in hydrogen may be detectable with the hydrogen breath test.