Intestinal gas is a normal by-product of digestion, but excessive gas can cause loud, smelly flatulence, which can be both uncomfortable and downright embarrassing. Certain foods tend to increase the levels of intestinal gas, but not all of them cause foul-smelling flatulence. Foods that produce malodorous gas are usually those high in sulfur, which produce a smell similar to rotten eggs. Dairy products can also cause smelly flatulence if you have problems digesting them, particularly those with a high sulfur content. Avoiding problematic foods can often help eliminate the issue of smelly flatulence, but if the problem continues it could be the sign of a digestive problem.
Intestinal gas is a normal by-product of bacteria breaking down food in your digestive tract, although it can also occur when you suck in too much air, such as when you eat too fast or suck on hard candy. If the buildup of gas is excessive in your large intestine, which is usually due to bacteria breaking down foods, it often results in flatulence -- the passing of gas through the anus. Most flatulence consists of odorless vapors that contain carbon dioxide, oxygen, nitrogen, hydrogen and sometimes methane, according to the Ohio State University Wexner Medical Center. On the other hand, foods that contain sulfur cause bacteria in the large intestine to release gases that produce foul-smelling flatulence.
Foods that contain sulfur get broken down by bacteria in the large intestine and produce hydrogen sulfide, which is responsible for the bad smell. The sulfur-rich foods most commonly associated with foul-smelling flatulence are eggs, meat and cauliflower. Cruciferous vegetables such as broccoli, cabbage, kale, mustard greens, collards and Brussels sprouts are also high in sulfur and can cause smelly gas. Garlic, horseradish and onions are also high in sulfur.
Foods with sulfites, a sulfur-based preservative, are also problematic. Wines and dried fruits are common examples, but many other foods are preserved with sulfites or contain them. Sulfites are usually noted on food labels in the ingredients list, so it's important to read carefully.
Dairy products -- those made from cow's milk -- can sometimes cause foul-smelling flatulence, because many people cannot properly digest lactose, or milk sugar. The condition, known as lactose intolerance, is caused by a lack of the proper enzyme, lactase, to break down the lactose. This can result in a variety of digestive symptoms, including foul-smelling flatulence, particularly with dairy products such as whole cow's milk and some cheeses that contain sulfur. Thirty million to 50 million people in the United States, both adults and children, are lactose intolerant, according to the Ohio State University Wexner Medical Center.
Avoiding foods high in sulfur or sulfites can help prevent smelly flatulence, but consult a qualified health practitioner before doing so to ensure you can find suitable substitutes, as many of the sulfur-containing foods also contain valuable nutrients. Taking prebiotics or probiotics might also be beneficial, according to Frank W. Jackson, M.D. -- they create an acidic environment that makes it difficult for the bacteria that produce hydrogen sulfide to thrive. If you suffer from lactose intolerance, avoiding dairy products can be beneficial. Consuming dairy products labeled lactose-free or taking an enzyme that has lactase in it might also help avoid a foul smell in your flatus, although those products won't really help with the sulfur content. If taking preventive measures doesn't help, consult a qualified health practitioner to rule out a digestive disorder.