Lactose, the natural sugar found in milk and other dairy products, causes a variety of side effects in some people. An enzyme called lactase, naturally occuring in the human intestines, helps digest milk, but many adults have low levels, according to the National Digestive Diseases Information Clearinghouse. Lactase levels decrease with age and are impacted by heritage. These low levels of lactase make it difficult to ingest milk, resulting in lactose intolerance.
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Gas in the digestive tract may occur when bacteria in the large intestine break down undigested foods. Low levels of lactase caused by lactose ingestion increase the amount of undigested sugars in the human body, making flatulence, the expulsion of gas through the rectum, more common, according to the National Digestive Diseases Information Clearinghouse. Changing the diet to avoid dairy and other foods that contain lactose lessens the incidence of flatulence, and over-the-counter medications are available to treat gas, as well.
Drugs.com explains that nausea is a side effect of ingesting lactose when the body has an intolerance for it. Nausea, the sensation of having an urge to vomit, results from irritants in the body, like lactose. According to MedlinePlus, nausea may cause vomiting, which rids the body of the irritating substance. For those sensitive to lactose, avoiding dairy and other other foods with lactose puts a halt to nausea.
Malnutrition may occur as a side effect of lactose ingestion. According to KidsHealth.org, those who are lactose intolerant often avoid milk and other dairy products to avoid side effects. Seventy-five percent of calcium in the U.S. food supply comes from dairy products, which puts sufferers at increased risk of malnutrition. Physicians define malnutrition, a condition impacting both mental and physical health, as the lack of nutrients necessary to maintain health. Malnutrition can lead to weight loss, fatigue, dizziness, swollen and bleeding gums, muscle and bone weakness, and immune deficiency.