Lactose Intolerance FAQs


Written by: CDHF

Updated: February 6th, 2023

People who are lactose intolerant have unpleasant symptoms after eating or drinking milk or milk products. But why? Normally, when someone eats something that contains lactose, an enzyme that’s produced in your small intestine called lactase, breaks down lactose into its simpler components (which are simple sugars called glucose and galactose). These simple sugars are then able to be absorbed in the bloodstream and give you the energy you need.

If you have lactose intolerance, the body doesn’t make enough lactase to break down lactose, and as a result allows undigested lactose molecules to pass to the lower parts of the intestine. At this point, the undigested lactose encounter bacteria which begin digestion through a process of fermentation. The outcome of this fermentation (hydrogen, carbon dioxide, methane gases, and short-chain fatty acids) leads to many of the telltale symptoms for lactose intolerance – like gassiness and diarrhea. (1)

man with lactose intolerance holding milk

We’ve compiled a list of frequently asked questions to help you further understand this digestive condition!

What are the types of lactose intolerance?

Lactase deficiency is a spectrum- some people are more deficient than others. Lactase deficiency can be primary, secondary, or congenital.

1. Primary lactase deficiency

This is the most common cause of lactose intolerance, and is most prevalent in adults. Most people have sufficient amounts of lactase at birth and in early childhood, when breast milk is the primary source of nutrition. In some people, the amount of lactase declines with aging; in others lactase production persists. This is a genetically inherited condition where the parent passes down a genetic mutation to child.

2. Secondary lactase deficiency

Injury to the lining of the small intestine may result in a deficiency of lactase and lactose intolerance, which is usually associated with other indications of bowel damage such as continuous diarrhea, intestinal bleeding, and weight loss. Further, intestinal infections such as Salmonella and parasites such as Giardia can result in lactose intolerance. Lastly, digestive conditions such as celiac disease and Crohn’s disease may include lactose intolerance as part of their symptoms. While most people with lactose intolerance have primary lactase deficiency, the possibility of an underlying intestinal disease should be looked at, especially if there is accompanying symptoms such as weight loss or low blood count (anemia).

3. Congenital lactase deficiency

This is a very rare inherited condition where lactase production is absent from birth. In this case, both parents have passed on the gene for lactose intolerance to their child, preventing the small intestine from producing enough lactase. Affected infants cannot tolerate milk products and must be nourished with non-milk formulas.

Is lactose intolerant genetically inherited?

Primary lactase deficiency and congenital lactase deficiency are both genetically inherited.

Primary lactase deficiency is a genetically inherited condition that involves passing down a genetic mutation from parent to child. It may seem like primary lactase deficiency is developed because symptoms may not appear until adulthood, but it is in fact hereditary.

Congenital lactase is also inherited. The genetic mutation responsible for congenital lactase deficiency is passed on in an autosomal recessive inheritance pattern. This means both parents must have a copy of the mutated gene to pass on the condition. (4)

Can lactose intolerance come and go?

Lactose intolerance will never fully go away for someone genetically predisposed to it.

That being said, it is possible to manage symptoms and many people find that their symptoms go away within a couple of days after decreasing the amount of dairy in their diet. (1)

Is lactose intolerance an allergy?

Lactose intolerance is NOT an allergy. A milk allergy is an immune response to protein in cows’ milk and results in skin rash, symptoms of inflammation of the esophagus or intestine or, occasionally with life-threatening anaphylaxis or shock. A food allergy happens when your immune system overreacts to a specific food protein, causing an allergic reaction. Unlike food allergies, food intolerances do not involve the immune system, but happen when an individual is missing the enzyme lactase. Milk allergies are generally diagnosed in the first year of life, while lactose intolerance occurs more often in adulthood. (2)

Can you suddenly become lactose intolerant?

It often manifests early in life, but it is not uncommon to develop later in life, seemingly out of the blue. As you get older (starting from the age of 2), the body generally produces less and less lactase. By adulthood, up to 70% of people no longer produce enough lactase to properly digest the lactose in milk, leading to symptoms when they consume dairy. This is particularly common for people of non-European descent. (3)

Is it possible that I have celiac disease as well as lactose intolerance?

It is possible. 25% of patients who have been clinically identified as lactose intolerant, have celiac disease. In Canada, that means about 73,500 people have undiagnosed celiac disease which is the causal agent for their lactose intolerance. If you think you have celiac disease, you should speak with your doctor.

Can I diagnose myself with lactose intolerance?

No. It’s important to listen to your body and know when to get help, but only medical professionals can assess if one is lactose intolerant or not. Medical professionals can also point you in the right direction, so that you do not end up with any deficiencies or misinformation.


1- NHS. 2016. Lactose Intolerance.

2- Mayo Clinic Staff. 2018. Lactose

3- Torborg L. 2016. Mayo Clinic Q & A: Lactose intolerance can develop at any 

4- Malik, T. (2021) Lactose Intolerance.

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