Lactic Acid & Lactose Intolerance

Lactose intolerance is an impairment of your body's ability to produce lactase, an enzyme that digests lactose, a sugar found in milk. If your body can't digest that sugar, the lactose may ferment in your intestines and produce lactic acid.

Milk is the only natural source of lactose.
Image Credit: Pakorn Kumruen / EyeEm/EyeEm/GettyImages

Understanding Lactose Intolerance

Although it is possible to be lactose intolerant from birth, it's very rare; the U.S. National Library of Medicine reports that congenital lactose intolerance is most common in Finland, where it's estimated to affect 1 in every 60,000 newborns. Its worldwide incidence is unknown.

But developing lactose intolerance in adulthood is much more common, as your small intestine stops producing enough lactase to break down the lactose in your food. This can happen as the result of an injury, disease or infection, or it can be hereditary. As noted in the University of Rochester Health Encyclopedia, in cases of hereditary lactose intolerance you may start to notice symptoms during your teen or adult years.

The National Library of Medicine also notes that about 65 percent of humans have a reduced ability to digest lactose after infancy, with the condition most common in people of East Asian, West African, Arab, Jewish, Greek and Italian descent.

Exercise vs. Digestion

Lactic acid is a normal byproduct of muscular activity — a very different context than your digestion. As exercise researchers at the University of New Mexico explain, lactic acid is either broken down and used as a source of energy or removed from your body.

But as the Mayo Clinic explains, if you're lactose intolerant, the undigested lactase ferments in your colon and creates lactic acid. As part of the process of diagnosing lactose intolerance, your doctor may ask you to submit a stool sample to be tested for lactic acid from lactose that has fermented in your colon. They may also test your breath for excess amounts of exhaled hydrogen — another sign that lactose is fermenting in your colon.

Read more: If You're Lactose Intolerant, Is Chocolate Off-Limits?

Lactose Intolerance Symptoms

Although your doctor can test for lactic acid in your stool or an excess of exhaled hydrogen to diagnose lactose intolerance, you'll probably notice other symptoms on your own. The University of Rochester notes some of the most common lactose intolerance symptoms, including abdominal pain or cramps, nausea, bloating, gas and diarrhea.

If you are diagnosed with lactose intolerance, your doctor may suggest doing some experimenting to see which dairy foods you can tolerate with fewer symptoms, because these are often an important source of calcium and vitamin D in your diet. Other possible remedies include taking lactase supplements, combining milk or milk products with other foods (which may reduce your symptoms), and eating hard cheeses and yogurt, which naturally have lower levels of lactose.

You can also purchase dairy products that have the lactase enzyme added to them; these may be labeled as "lactose-free" or "lactose-reduced." And finally, some people with lactose intolerance report that they can eat fermented dairy products.

Read more: Do 'Lactose-Free' and 'Dairy-Free' Mean the Same Thing?

Foods That Contain Dairy

If your symptoms are severe, your lactose intolerance treatment may include abstaining from dairy foods entirely. Get used to reading food labels, because lactose can be added to processed foods, including bread, cereal, lunch meats, salad dressings and cake mixes.

Learning to spot dairy-related ingredients such as butter, cheese, cream, milk solids and whey can help you avoid hidden sources of lactose in your food. And of course, lactose can be present in those stand-alone ingredients as well.

You can also use non-dairy substitutes for these products. But if you end up abstaining from dairy entirely, talk to a doctor or registered dietitian about whether you need to take calcium supplements or additional vitamin D.

Read more: List of Dairy-Free Foods

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