Lactose: Intolerance, Definition And Symptoms

Lactose is a type of sugar present in milk. When people cannot digest it naturally, they have lactose intolerance.


There are some foods or products of animal origin, specifically those made with milk, that contain lactose. They can cause indigestion or intolerance symptoms.

But why does this happen? In this article, we will discover what it is and why people have intolerance to it, what are its symptoms and the dairy and non-dairy products that we can find at the grocery store. 

What is lactose?

Lactose is a disaccharide, a carbohydrate formed by 2 monosaccharides: β-D-galactose and β-D-glucose. The anomeric carbon of the β-D-galactose molecule reacts with the C-4 hydroxyl group of the β-D-glucose molecule to form the glycosidic bond.

This element is the sugar present in milk obtained from mammals —except for seals and walruses. Milk is the only natural source.

Lactose is not directly absorbed into the intestinal mucosa, but the two monosaccharides that compose it are absorbed when they are released by the lactase enzyme. The activity of this enzyme is maximum when we are children, but as we grow up it only remains in people who have the phenotype persistence of lactase. This enzyme is encoded by a gene that is only expressed in cells of the small intestine.

It is then broken down into substances that are absorbed into the bloodstream by the cells lining the small intestine.

When we eat dairy products such as cheese, yogurt or milk, this component is introduced into the digestive system and is broken down into the simplest sugars: glucose and galactose.

The liver is the organ responsible for transforming galactose into glucose, which then enters the bloodstream and increases blood glucose levels.

The use of lactose as a fermentation substrate is one of its most important functions. What the lactic acid bacteria produce comes from lactose. This is the beginning of the formation of fermented dairy products.

Lactose intolerance

This condition is defined as the  inability of the organism to digest this disaccharide. It occurs when due to a lactase deficiency, lactose is not completely degraded in the body and the blood glucose level does not increase.

While not a very dangerous condition, it causes serious discomfort. It is believed that 75% of the general adult population is not able to produce enough lactase enzyme and is therefore at risk for some or all of the symptoms of lactose intolerance.

Lactose intolerance is caused by conditions and lesions of the digestive system that result in much lower lactase production. Although there are cases of people being born without the ability to produce this enzyme, it is also possible that the body naturally stops producing the enzyme.


Among the main lactose intolerance symptoms are nausea, cramps, diarrhea, bloating, and gas. These usually appear between 30 and 120 minutes after eating dairy food. Vomiting, lower stomach pain, and constipation are other common symptoms.

The types of symptoms associated with this condition vary depending on the amount of lactose consumed by the person and their ability to tolerate it. As a consequence, the severity of lactose intolerance is also different in each case.

Aunque se suele pensar en la leche de vaca, cualquier leche de procedencia animal contiene lactosa.
We usually think of cow's milk when talking about lactose, but all milk of animal original contain it

Dairy products

The main foods containing lactose are whole milk, yogurts, as well as cheeses, cream, chocolates and ice cream. However, there are other types of products that, although less known, also have it.

For example, all lunch meat like turkey, pork, chicken or sausages contain it as a preservative. Jelly beans and snacks such as chips with added flavors, flavored seeds or nuts and even chewing gum are products that also contain it.

Also, foods that are low in calories contain a component known as Lactitol which is a derivative of this substance used as a sweetener. Cookies also contain whey, milk powder or simply as an additive.

Finally, there are other foods such as bread —generally sliced bread— wines and spirits and medicines that also contain it. In the case of alcoholic drinks, there are some companies that use dairy products such as whey to make them.

Also, there are some medications, especially antibiotics, antidepressants, and contraceptives, which present lactose as an excipient to preserve their properties.

En todos los supermercados podemos encontrar ya secciones específicas para los productos sin lactosa.
In all supermarkets, we can already find specific sections for lactose-free products

Dairy-free products

In many grocery stores we can find a special section for dairy-free products, among are:

  • Lactose-free milk
  • Condensed milk
  • Cheese
  • Yogurts
  • Mayonnaise
  • Candies
  • Milkshakes
  • Chocolate bars
  • Cakes
  • Biscuits
  • Sauces
  • Yeast
  • Sliced bread

Natural and fresh foods such as fruit, cereals, seafood, meat, fish, legumes, tofu and vegetable drinks —coconut, oatmeal, rice or soy— are foods that do not contain lactose per se.

Nonetheless, it is important to look at the composition of the foods we eat if we are lactose intolerant.


Dew, S. E. (2004). Disaccharides. In J. J. Lagowski (Ed.), Chemistry: Foundations and Applications (Vol. 2, pp. 14-17). New York: Macmillan Reference USA.