Assortment of dairy products

What is Lactose Intolerance? Symptoms and Treatment


Written by: CDHF

Updated: December 1st, 2022

Have you ever had an upset stomach after a dairy-heavy meal, or felt bloated after having too much cheese pizza? Lactose intolerance is quite common but is often ignored by those with symptoms. 

What exactly is lactose intolerance?

Lactose intolerance is an impaired ability to digest lactose, a sugar found in milk and other dairy products. 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 (more on that below!). Milk allergies are generally diagnosed in the first year of life, while lactose intolerance occurs more often in adulthood.

What happens in my body if I’m lactose intolerant? 

Normally, when someone eats something that contains lactose, an enzyme that’s produced in your small intestine called lactase breaks down lactose into it’s 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. (2) 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.

Lactose intolerance is fairly common! You might be surprised to know that as you get older, you generally produce 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. (1)

What are the symptoms of lactose intolerance?

Lactase deficiency varies from person to person – with some people being more affected than others. The likelihood and severity of symptoms depends on how little of the enzyme lactase you have, and the amount of lactose ingested. If the amount of lactose consumed overwhelms the available lactase, symptoms will develop.

Symptoms of lactose intolerance follow the ingestion of milk or milk products and include:

Stomach pain and bloating

Stomach pain and bloating are common symptoms of lactose intolerance in both children and adults. When the body is unable to break down lactose, it passes through the gut until it reaches the colon. Carbohydrates such as lactose cannot be absorbed by the cells lining the colon, but they can be fermented and broken down by the naturally occurring bacteria that live there, known as the microflora.

This fermentation causes the release of short-chain fatty acids, as well as the gases hydrogen, methane and carbon dioxide. The resulting increase in acids and gases can lead to stomach pain and cramps. The pain is usually located around the navel (your belly button) and in the lower half of the tummy.

The sensation of bloating is caused by an increase of water and gas in the colon, which causes the gut wall to stretch, also known as distention. In severe cases, this can even lead to nausea and vomiting. (3)


We all experience this symptom from time to time. Lactose intolerance causes diarrhea by increasing the volume of water in the colon, which increases the volume and liquid content of the stool.

In the colon, microflora ferment lactose to short-chain fatty acids and gases. Most, but not all, of these acids are absorbed back into the colon. The leftover acids and lactose increase the amount of water that the body releases into the colon.

That being said, it is worth noting not all carbohydrates that cause diarrhea come from lactose. In fact, 2–20% of any carbohydrates consumed will reach the colon undigested in healthy people. (4)

Increased gas

In people with lactose intolerance, the colon microflora become very good at fermenting lactose into acids and gases. This results in more lactose being fermented in the colon, which further increases gassiness.

The amount of gas produced can differ enormously from person to person due to differences in the efficiency of the microflora, as well as the rate of gas reabsorption by the colon. (5)

So how do you treat lactose intolerance?

While there is no actual cure for lactose intolerance, most people manage their symptoms by decreasing the amount of lactose in their diet or using commercially available lactase enzymes.

Decreasing lactose intake

A lactose-free diet excludes milk and milk products containing significant amounts of lactose. Lactose intolerance is not an all or none phenomenon, so reducing the amount of lactose in your diet rather than strict avoidance may be sufficient.

Lactose products avoid (or consume in moderation) include:

Tips: Things like butter and hard, aged cheeses such as parmesan, swiss and cheddar have much lower lactose content and may be easier to digest because most of the lactose is actually eliminated while the cheese is being made.  Items such as dark chocolate, and probiotic yogurt are considered lower in lactose as well. Any of the products above labelled ‘lactose-free’ are also an excellent option!

It’s important to note that people who are extremely sensitive to lactose must carefully read labels to detect unexpected sources. You might be surprised to know that lactose is often used as a filler in prescription and non-prescription tablets and may be a source of continuing symptoms in spite of a presumed lactose-free diet!

Lactose is also present in small amounts in a variety of processed foods. So be sure to check the ingredients and speak to a registered dietitian if you are unsure about food choices and wish to implement a lactose-free diet.

Luckily, there are a number of alternative foods and drinks available in grocery stores to replace the milk and dairy products you may need to decrease consumption of. Food and drinks that do not usually contain lactose include products such as:

There are also a large number of lactose-free dairy products available such as lactose-free milk, yogurt, cheeses, and ice cream that are great tasting and suitable for those with lactose-intolerance. These options allow someone living with lactose-intolerance to enjoy typical dairy products without sacrificing taste and quality!

Lactase enzymes

Taking a lactase enzyme before you eat foods that contain dairy, may prevent the symptoms of lactose intolerance. Options include low-lactose milk with lactase, which is available in most grocery stores, and lactase tablets that are available in pharmacies. It’s important to consult a registered dietician or your physician before beginning use of lactase tablets as a way to combat lactose intolerance. (7)


The simplest, and probably most reliable way of diagnosing whether an individual is lactose intolerant or not is to remove all lactose products from the diet for 1 to 2 weeks and see if the symptoms resolve. Keep in mind dairy products are an important source of calcium and vitamin D so those who avoid dairy need to ensure adequate sources, either through calcium/vitamin D supplements or calcium-rich non-dairy foods, including soya and other plant-based milks, tofu, green vegetables, fortified citrus juices, and lactose-free products with added calcium. (7)

If you are showing signs of lactose intolerance, it is important to consume products with lactose in moderation. Additionally, another option is introducing lactose-free variants of standard dairy products such as cheese, milk, and yogurt. Lactose-free alternatives can help you get back to enjoying the foods you love – without the side effects!  


(1) CDHF (2018). What is lactose intolerance?.

(2) Medline Plus. (2020). Lactose Intolerance.

(3) Sanderson, J. (2007). Review article: lactose intolerance in clinical practice – myths and realities.

(4) Hammer, J. (2012). Diarrhea caused by carbohydrate malabsorption.

(5) Deng, Y. (2015). Lactose Intolerance in Adults: Biological Mechanism and Dietary Management.

(6) Chaudhuri, A. (2000). Lactose intolerance and neuromuscular symptoms.

(7) Malik, T. (2021) Lactose Intolerance.

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