Lactose intolerance

Is my baby allergic to milk sugar?

Lactose intolerance is the inability to digest lactose, the main sugar in milk. While common in adults, it is extremely rare in babies and uncommon in children younger than five years. Lactose intolerance is not an allergy. It occurs when people are missing the lactase enzyme, a protein that breaks down lactose. If the lactose is not broken down, it becomes food for microbes in the gut and leads to gas, bloating, diarrhea, and cramping. In rare cases, a sick baby may develop lactose intolerance because of damage from infection, malnutrition, or disease of the small intestine.  

A) Describing lactose and lactose intolerance

Lactose, a double sugar, is made in the breast by connecting two single sugars: glucose and galactose.

Lactose intolerance is the insufficient breakdown of lactose sugar in the small intestine part of the gut. It is not an allergy, which is a response by the immune system to a perceived threat.

Many words associated with lactose intolerance sound similar, including:

  • Glucose: a sugar and the main fuel for the body.
  • Galactose: a sugar that provides fuel for the body and forms parts of complex molecules important to human health.
  • Lactose: the main sugar in milk, consisting of paired glucose and galactose molecules.
  • Lactase enzyme: the protein that starts processing the lactose by splitting lactose into glucose and galactose.
  • Lactase persistence: the situation in which people continue to make the lactase enzyme past the age of five years.
  • Lactase deficiency: the lack of lactase enzyme in a person.
  • Lactose intolerance: the inability to process lactose because of lactase deficiency. Lactose intolerance is marked by a group of symptoms such as nausea, bloating, pain, cramping, and diarrhea.

It may help to remember that:

  • Sugar names end with “-ose”.
  • Enzyme names end in “-ase.”

B) Cause of lactose intolerance

Lactase enzymes in the small intestine break down the lactose, the main sugar in milk, into simpler forms of sugar called glucose and galactose. These are absorbed through the gut and into the bloodstream and then processed and used to fuel a baby’s growth.

If there is little or no lactase enzyme in the gut, the lactose is not broken down and absorbed. Instead, it stays in the bowel and becomes food for the gut microbes (microbiome). One by-product of this is gas (carbon dioxide and hydrogen), which causes bloating. Other by-products draw water into the bowel, resulting in diarrhea and cramping. Together, they make a rather nasty combination!

C) Lactose intolerance in adults

The symptoms of lactose intolerance rarely develop before the age of five years (Heine 2017).  

1) Genetic causes of lactose intolerance

Lactase persistence is a recent adaptation of humans and is estimated to have developed in the last 10,000 years (Ségurel 2017). It is highest in groups of people who traditionally kept dairy animals and not surprisingly, that also began about 10,000 years ago.

About 30% of all people are lactase persistent and can digest lactose without symptoms (Heine 2017). Lactase persistence is found in about 95% of people with origins in Northern Europe but in only 10% of those with East Asian, African, and Native-American origins.  

2) Other causes of lactose intolerance

Celiac disease and gut viral or parasitic infections may cause lactose intolerance.

The gut microbiome can affect the severity of lactose intolerance.

D) Lactose intolerance in babies

There is no evidence that giving lactase drops to unhappy babies decreases crying (Hall 2012). There are tests for lactose intolerance (Heyman 2006). Please see your health-care providers if you think your baby is lactose intolerant. 

1) Congenital lactase deficiency

A very small number of babies are born unable to make the enzyme lactase. This is called congenital lactase deficiency. This is due to a genetic condition. The highest rates are found in Finland where 1 in 60,000 babies is affected. These babies become very sick soon after starting to breastfeed or take in breast milk.

2) Acquired lactase intolerance

In rare cases, a sick baby may develop lactose intolerance because the small intestine, a part of the gut, is damaged and can’t produce the lactase enzyme. This includes babies who:

  • Are under three months old and have severe diarrhea.
  • Are ill with parasites.
  • Have celiac disease, Crohn’s disease, or other gut abnormalities.
  • Are severely malnourished.

Lactose-free infant formula can be bought but is more expensive. It is generally not recommended. Infant formula-fed babies with persistent diarrhea or who have conditions that can cause acquired lactose intolerance may benefit from a two-week period of lactose-free infant formula. 

In the past, babies with infectious diarrhea were thought to be unable to digest lactose and breast milk was replaced by juice, broth, tea, rice water, or even flat soda (fizzy) drinks. None of these provide the necessary nutrients to help the bowel heal or the right balance of salt and water to replace the fluids lost.

Otherwise healthy breastfed babies who develop infectious diarrhea should continue to breastfeed (Heine 2017). In addition, babies should be seen promptly by their health-care providers if they: 

  • Are less than four months of age.
  • Have a fever.
  • Are also vomiting.
  • Are sleepy or weak.
  • Have a dry mouth.
  • Have peed less than usual.


Hall B, Chesters J, Robinson A. Infantile colic: a systematic review of medical and conventional therapies. J Paediatr Child Health. 2012 Feb;48(2):128-37
Heine RG, AlRefaee F, Bachina P, et al. Lactose intolerance and gastrointestinal cow's milk allergy in infants and children - common misconceptions revisited. The World Allergy Organization Journal. 2017;10(1):41
Heyman M. Lactose intolerance in infants, children, and adolescents. Pediatrics 2006; 118(3)
Kuokkanen M, Kokkonen J, Enattah NS, et al. Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency. Am J Hum Genet. 2006;78(2):339–344
Ségurel L, Bon C. On the Evolution of Lactase Persistence in Humans. Annu Rev Genomics Hum Genet. 2017 Aug 31;18:297-319