When your baby drinks breast milk or formula, an enzyme in their body breaks down a natural sugar in the milk called lactose. If there is an insufficient amount of this enzyme (called lactate), the intestines cannot digest it and your baby may act fussy, have diarrhea, bloating, nausea, cramps, or gas. Collectively, these symptoms indicate a problem called lactose intolerance.

Usually, a person who is lactose intolerant will show symptoms later in life (often around age five or older) and it becomes quite common in adults. Roughly 30 million American adults will have some lactose intolerant symptoms by age 20. In infants, lactose intolerance is relatively uncommon. Only about 2-3 percent of newborns will show signs of lactose intolerance.

Lactose intolerance tends to be more common in people with Asian or African heritage and less common in Caucasians. Intestinal infections can also trigger lactose intolerance, especially in children, as can bowel procedures and intestinal diseases such as celiac sprue. Lactose intolerance due to common diarrheal illnesses is usually self-limited, meaning that it improves after a few weeks.

Lactose intolerance is often confused with a milk allergy, but the two are very different conditions. A milk allergy occurs when the immune system mistakenly identifies the milk protein to be harmful and attacks it. Both share gastrointestinal upset (stomach upset and diarrhea, for instance), but a milk allergy may also cause skin rash and, in more severe forms, hives or wheezing may occur.

Your doctor may test for lactose intolerance by sampling your baby’s stool for acidity. Undigested lactose creates lactic acid that is detectable in a stool sample. Because lactose intolerance is uncommon in babies, special attention should be made during diagnosis to determine the cause of the digestive symptoms.

If diagnosed with lactose intolerance, your baby’s diet may need to be adjusted. Breast milk or cow’s milk-based formula can be replaced with an infant formula that is more easily digested.

Your pediatrician will be able to recommend a nutritional plan that will ensure your baby gets the vital nutrients he or she needs to grow healthy and strong.

Reviewed by Dr. Sara Connolly, April 2019


  • Lactose intolerance is a condition in which a person lacks the enzyme to digest lactose.
  • Lactose intolerance is common in teens and adults but rare in infants and young children.
  • Gas, diarrhea and abdominal discomfort are common signs of lactose intolerance.
  • A temporary lactose intolerance is common after a diarrheal illness and will improve over time.


  1. American Academy of Pediatrics. Milk Allergy.
  2. National Institutes of Diabetes and Digestive and Kidney Diseases. Lactose Intolerance.
  3. National Institutes of Health. Lactose Intolerance.


  1. What’s the next step after formula if your baby is lactose intolerant? Are soy milks safe?

    1. It depends so much on the degree of symptoms, age of the baby, and overall growth. Soy formula is sometimes very well tolerated in people with lactose intolerance. With true allergy, this is less so. I’d discuss this with your pediatrician, so that you know for sure you are making the correct choice. If your child is under the age of 1 they need some version of formula as their digestive systems are not ready for the other “milks” (almond, coconut, goat, etc).

  2. I had no idea that “lactose intolerance” and “milk allergy” were not one and the same. Interesting info.


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