Published June 21, 2021

If you are one of the many people who experience unpleasant symptoms after eating dairy foods like milk and cheese, you may have asked yourself “How do I know if I’m lactose intolerant?”

But let’s start at the beginning: just what is lactose? It is a ‘disaccharide’ – that is to say, two simple sugars which have bonded together. A major constituent of the milk produced by mammals to nurse their young, lactose is a rich source of energy. A digestive enzyme called lactase breaks lactose down into other sugars – glucose and galactose. The latter plays an important role in the development of the immune and nervous systems and it can only be obtained from lactose.

When mammals are weaned, there is, of course, no further need to digest lactose since it does not occur outside milk, so the ability to produce lactase is gradually switched off. However complications arose when some early populations began keeping cows and decided to supplement their diet with cow’s milk and make cheese. As a result the ability to produce lactose lingered on in some populations – in particular, Europe. Dairy foods have never been as popular in other parts of the world and as a result, and lactose intolerance has remained the norm in those regions. On a global scale around 70% of people are lactose intolerant to varying degrees.

Even those who are currently lactose tolerant can develop intolerance – if example, following a bacterial or viral infection.


Symptoms of lactose intolerance

It follows then that the symptoms of lactose intolerance are caused by an inability to digest lactose – or at least difficulty doing so – not an allergic reaction to this natural sugar as is sometimes supposed. 


Typical symptoms of lactose intolerance include:

  • Bloating
  • Intestinal gas
  • Stomach pain
  • Diarrhea or constipation
  • Vomiting

By contrast, eczema is more likely to be a symptom of an allergy to milk. Allergic reactions involve the immune system, and the skin plays an important role in this.

On a fundamental, digestive level, lactose is a carbohydrate. Low levels – or a complete absence of – lactase means that undigested lactose may accumulate in the colon, where it will then undergo fermentation by the bacteria in our stomachs, producing high levels of gas – specifically methane, hydrogen and carbon dioxide – along with acid and water. Only some of this will be absorbed by the body, and this excess can cause bloating, cramps, flatulence, diarrhea and even nausea in sensitive individuals. 

Methane, one of the primary gases produced by lactose fermentation, also contributes to constipation, a rarer symptom of lactose intolerance, by slowing down the amount of time it takes for food to pass through the digestive system.

Over time, people with lactose intolerance may find their symptoms increasing. This is because gut flora become increasingly efficient at digesting lactose the more they are exposed to, and as a result produce even more uncomfortable gas and water.

Other, less common symptoms linked to lactose intolerance include:

  • Muscle pain
  • Fatigue 
  • Difficulty concentrating 
  • Headaches


How to find out

Of course, none of the symptoms above are unique to lactose intolerance: they could indicate other conditions. So how do you find out for sure? 

There are a number of possibilities. You could talk to your GP and ask to be referred for specialist testing. A hydrogen breath test, for example, can detect the abnormal levels of hydrogen in your breath produced by lactose malabsorption. 

But why wait months for an NHS referral when you could take a simple-to-use food sensitivity test at home? With the detailed results you can quickly begin taking positive steps to alleviate your symptoms and improve your health.

You will need to experiment to identify your own level of sensitivity. Symptoms vary from person to person – some people can eat or drink small amounts of dairy with no problems while others must avoid it altogether. We’re all different so a personalised plan ensures optimal results.


Written by Bev Walton, BSc Nutritional Science

I achieved a First-Class Honours degree in BSc Nutritional Science, Nutrition Sciences from the University of Reading and now have over 35 years experience in all types of cuisine, dietary plans, recipe development, health and nutrition. I have been writing for over 10 years for magazines and websites as well as ghostwriting for ebooks, Kindle and fully published books. I’m also a proud member of the Guild of Food writers.

Bev Walton | University of Reading BSc Nutritional Science, Nutrition Sciences


Share This