What is Lactose Intolerance in Babies?

What is Lactose Intolerance in Babies?

Every baby gets gassy and cries and fusses sometimes. But when babies seem to endure this discomfort more often than not, parents naturally explore what the cause might be. One of the issues that may come up is lactose intolerance (sensitivity). But what exactly does that mean? Today, we’re breaking it down to the basics.

What is Lactose?

Lactose is the primary carbohydrate found in breast milk – and all mammalian milks for that matter. That means that cow’s milk, goat’s milk and sheep’s (you get the point) all have lactose!  If you want to get really geeky – in more scientific terms, it’s a disaccharide (a molecular compound of two sugars) comprised of the monosaccharides (single sugar molecules) glucose and galactose (1).

What Function Does Lactose Serve in the Body?

Carbohydrates are the body’s primary energy source. Children and adults consume carbohydrates from a variety of sources, but for babies, this macronutrient comes from lactose in breast milk or formula. Beyond simply providing energy, a growing infant’s brain relies on carbohydrates (such as lactose) as its primary source of fuel for proper growth and development (1).      

What is Lactose Intolerance?

Many of us may know a friend, colleague, or family member who doesn’t drink milk because they are "lactose intolerant" – a medical condition that occurs when the body no longer produces sufficient amounts of the enzyme lactase, which is needed to digest lactose (1).  

The American Academy of Pediatrics (AAP) defines lactose intolerance as, “a clinical syndrome of one or more of the following: abdominal pain, diarrhea, nausea, flatulence, and/or bloating after the ingestion of lactose or lactose-containing food substances” (1). Symptoms vary from person to person and depend on how much lactose is consumed, the form of the food that they consume, and their degree of lactase deficiency.

Lactose intolerance actually varies by ethnicity, gender, and age and is broken down into four different types (1,3):

According to AAP (1):

  • Primary lactase deficiency is the most common cause of lactose intolerance. In this situation, there is a relative or absolute absence of lactase, which develops in childhood and persists into adulthood.  
  • Secondary lactase deficiency is more common in infancy but can occur at any age. It is the result of a small bowel injury (such as acute gastroenteritis or chemotherapy).  
  • Congenital lactase deficiency is the most rare type of lactose intolerance. Infants with congenital lactase deficiency require specialized lactose free breast milk substitutes.  
  • Developmental lactase deficiency is lactase deficiency that is found in preterm infants younger than 34 weeks gestation.

Lactose intolerance in babies is generally not a problem because they are born with the ability to produce lots of lactase. In fact, they begin developing lactase at 34 weeks gestation (1).  Since they depend on their mother's milk for nutrition in the first year of life and the lactose in mother's milk is needed for brain development, their bodies are born ready! Isn’t nature amazing?

However, even though babies may not be truly lactose intolerant, a high volume of lactose can be overwhelming to a young infant’s immature digestive system. When there is not enough lactase to break down all the lactose, the undigested lactose may cause gassiness and discomfort, and even green, watery or foamy stools. This doesn’t happen in all babies but can be attributed to oversupply or foremilk/hindmilk imbalance in a breastfeeding mother. Since lactase is produced in the small intestine, a baby might temporarily lose the ability to produce lactase after an acute gastroenteritis (1). Lactase production generally decreases as children get older, because in the world of mammals – milk was made for babies – not adults.

Are You Concerned Your Baby Might Be Sensitive to Lactose?    

These days it seems “lactose intolerant” has become the catchall diagnosis for any baby suffering from regular upset tummies and loose stools. While it might be an accurate assessment at the time, as you can see from the evidence above, it’s rarely a permanent intolerance. Babies are built to consume lactose.

You should always consult your healthcare provider with concerns like this, but the general recommendations in these situations are as follows:

  • For breastfed infants who may become sensitive to lactose due to a viral illness, the recommendation is to continue breastfeeding as the immune factors found in breastmilk actually help heal the gut. In situations where lactose sensitivity arises as the result of excessive foremilk consumption (known as ‘foremilk/hindmilk imbalance,’ ‘oversupply,’ or ‘lactose overload’), it’s typically resolved by adapting the feeding practice to fully nursing on one breast (in order for the baby to get enough hindmilk). This is often explained as letting the baby “finish the first breast first.”
  • For infants who are exclusively formula fed and having a similar issue, pediatricians may recommend a “sensitive” formula that is reduced in lactose.

That said, we always recommend that parents reach out to a board certified lactation consultant for breastfeeding support or go to their child’s pediatrician with questions that they might have about formula.


  1. Lactose Intolerance in Infants, Children, and Adolescents, Melvin B. Heyman, Pediatrics Sep 2006, 118 (3) 1279-1286
  2. Antonowicz I, Lebenthal E. Developmental patterns of small intestinal enterokinase and disaccharidase activities in the human fetus. Gastroenterology.1977;72 :1299– 1303
  3. http://nutrigenomics.ucdavis.edu/?page=information/Concepts_in_Nutrigenomics/Lactose_Intolerance accessed July 1, 2016


We aim to provide you with the most honest and credible information possible. This article was reviewed for accuracy by The Honest Team and was written based on trusted sources that are linked at the bottom of the article.