Lactic acid is synthesized as a normal part of metabolism, then used for energy or eliminated from the body. But the lactic acid associated with lactose intolerance comes from a different source. It's produced when bacteria in the colon ferment lactose that wasn't digested in the small intestine. This lack of digestion is the reason people with lactose intolerance can often consume fermented milk products without experiencing symptoms.
Lactose is a sugar that naturally comes from just one source -- milk. Your body needs a specific enzyme, lactase, to properly digest lactose. Without a sufficient supply of lactase to break down the sugar, it can't be absorbed in the small intestine. This condition is called lactose intolerance.
Lactose intolerance is sometimes caused by conditions such as celiac disease and Crohn's disease, which interfere with the small intestine's ability to produce lactase. In some people, lactase production declines as they get older, which is why lactose intolerance appears in adulthood.
Lactase and Lactic Acid
When lactose isn't absorbed in the small intestine, it passes into the large intestine. Some types of bacteria that live in the colon, called lactic acid bacteria, produce lactase. When they encounter lactose, these bacteria use their lactase to break down, then ferment, the sugars.
During fermentation, lactose is turned into lactic acid. Fermentation of lactose also produces short-chain fatty acids and gases, including methane, hydrogen and carbon dioxide. The gases are responsible for the symptoms associated with lactose intolerance, such as abdominal pain, bloating, gas and diarrhea.
Many people with lactose intolerance can eat yogurt and other fermented foods because they contain lactic acid bacteria. Lactobacillus is the lactic acid bacteria primarily used to ferment foods, but you may see other types of bacteria, such as Leuconostoc, Pediococcus or Streptococcus.
Lactose in Milk Products
The severity of your lactose intolerance determines whether you can eat a small amount of dairy products or you have to avoid all lactose. Condensed milk is one of the top sources, with more than double the lactose of regular milk. Processed cheese, cheese spreads, ice cream and milk, from skim to whole, all have about the same amount of lactose.
If you don't need to exclude all lactose, try eating small portions of cheddar cheese, Gouda, mozzarella cheese, blue cheese, low-fat cream cheese, goat or Stilton cheeses. Their lactose content, which is present in a trace amount, is five times less than milk, according to a January 2008 report in Alimentary Pharmacology and Therapeutics.
Other Sources of Lactose
If you avoid all lactose, you may need to stay away from supplements and other products made from whey protein because it contains a small amount of the sugar. Casein doesn't contain lactose, but consult your doctor before consuming any milk products if your lactose intolerance is severe.
Also be aware that lactose is used as an additive to add bulk, improve texture, bind ingredients and help foods brown. It's in a variety of medications and foods, such as baked goods, processed meats, soft drinks, beer and prepared hamburgers and chicken.
- National Institute of Diabetes and Digestive and Kidney Diseases: Lactose Intolerance
- University of Maryland Medical Center: Lactose Intolerance
- Alimentary Pharmacology and Therapeutics: Review Article: Lactose Intolerance in Clinical Practice: Myths and Realities
- Fermented Fruits and Vegetables -- a Global Perspective: Bacterial Fermentations
- Anaesthesia Education Website: Acid-Base Physiology: Lactic Acidosis