Foods That Are Difficult to Digest

Some foods can be good for you, but there are foods your body cannot digest. Many hard to digest foods contain carbohydrates that are not properly broken down during digestion, often due to a common deficiency of an enzyme. High-fiber foods can also be difficult to digest.

Digestion starts in the mouth, where food is mechanically broken down by chewing, and enzymes in saliva begin to break down starches. Stomach acid and enzymes in the stomach continue to break down the food particles in preparation for passing into the small intestine. Enzymes, bacteria, bile and water in the small intestine complete the process of breaking down food into its nutrient components of carbohydrates, proteins and fats.

The National Institute of Diabetes and Digestive and Kidney Diseases states that the walls of the small intestine absorb these nutrients and pass along any non-nutritive substances into the large intestine. The large intestine wrings out excess water that was taken in by the small intestine to aid in digestion and gleans any remaining nutrients for absorption. The remaining fecal material passes through the rectum and anus as waste.

Some foods are more difficult to digest and can contain substances that the body cannot break down. Some individuals lack enzymes necessary to properly digest certain foods and may experience gas, bloating and abdominal pain when they eat these foods. A food intolerance is the inability to digest a particular type of food.

Read more: 10 Foods That Give You the Worst Gas

Fiber is a plant-based substance that is made of carbohydrates. According to Harvard's T.H. Chan School of Public Health, most carbohydrates break down into simpler sugar molecules, but the carbohydrates in fiber cannot be broken down or digested. Too much fiber, or a sudden increase in dietary fiber, can result in gas, bloating and sometimes diarrhea.

Soluble fiber forms a gel-like substance when combined with water, which helps to maintain blood glucose and cholesterol levels. Soluble fiber also keeps you feeling fuller, which can promote weight loss. Foods high in soluble fiber include oats and nuts.

Insoluble fiber doesn't absorb water but is beneficial in moving materials through the digestive tract. Foods high in insoluble fiber include whole grains and vegetables such as carrots and tomatoes.

Lactose is a sugar in milk and other dairy products that requires the enzyme lactase to break down. According to the Mayo Clinic, the absence of lactase — or an insufficient amount of the enzyme — is common in adults, especially people of African, Asian, Hispanic, Mediterranean or Native American descent.

Without lactase, lactose isn't broken down in the small intestine and passes undigested into the large intestine. Within two hours of consuming a dairy product, a person with lactose intolerance may experience gas, bloating, cramping, diarrhea and nausea.

A group of carbohydrates are known to be poorly absorbed in the small intestine in people with irritable bowel syndrome, resulting in fermentation of undigested material in the gut if the appropriate enzymes are lacking. These carbohydrates are collectively called FODMAPs, which stands for fermentable oligosaccharides, disaccharides and monosaccharides and polyols.

Fermentation produces gas, which leads to bloating and abdominal pain in the small intestine. Fermentation in the colon causes diarrhea because undigested FODMAPs attract water molecules which makes fecal material retain liquid.

Read more: Why Do Sweet Potatoes Hurt Your Stomach?

Fermentable oligosaccharides include beans, legumes, grains and hard to digest vegetables such as onions, artichokes and leeks. Difficult-to-digest disaccharides include maltose in molasses and cooked sweet potatoes and lactose in milk.

Monosaccharides such as glucose are generally easy to digest, but fructose found in fruits and table sugar are harder for some people to absorb. Some vegetables also contain fructose, such as broccoli, asparagus and okra. Sugar alcohols in sweeteners like xylitol and sorbitol are polyols and can cause intestinal upset because they are not absorbed in to the bloodstream like glucose.