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  • Annie Zhou

Lactose Intolerance: Everything You Need to Know

Annie Zhou, HK BioTek Intern

What is it?

Lactose intolerance occurs when the body is unable to digest lactose, a sugar found in milk and other dairy products. The body usually digests lactose by an enzyme called lactase. Lactase breaks down lactose into glucose and galactose so that it can be absorbed into the bloodstream. When people don’t produce enough lactase, lactose remains in the digestive system where it is fermented by bacteria, creating various gases that cause unwanted symptoms.

It is easy to confuse lactose intolerance with milk allergy or sensitivity since they may show similar symptoms. However, they are caused by two different reactions: Reactions from allergies and sensitivities involve the immune system, while reactions from lactose intolerance do not. As mentioned above, lactose intolerance is due to a deficiency of lactase, and its reactions only involve the digestive system.

Symptoms usually develop within a couple hours of consuming lactose. They depend on the amount of lactose consumed and also on the person’s level of intolerance. Symptoms may include:

  • Bloated stomach

  • Gas

  • Stomach cramps and pains

  • Diarrhea

  • Nausea, sometimes vomiting

Who is affected?

The most common form of lactose intolerance, called primary lactose intolerance, occurs in adults and is genetically determined. A decrease in lactase production is expected with age, but it is more common for people with Asian, African, or Hispanic ancestry. Below is a map showing the prevalence of primary lactose intolerance in 2013.

Source: Food Intolerance Network

Another form of lactose intolerance is secondary lactose intolerance, where the small intestine decreases lactase production after an illness, injury, or surgery involving the small intestine. Most cases in children fall under this condition, caused by inflammation from an infection of the digestive system. These are the only cases of lactose intolerance that are temporary.

There is also a congenital or developmental lactose intolerance, where babies are born with a complete absence of lactase activity due to a rare recessive gene. Premature infants may also have lactose intolerance because of an insufficient lactase level.

How to test for it?

See your family doctor or a healthcare practitioner if you have symptoms that suggest lactase deficiency. There are a few ways for your doctor to confirm a diagnosis of lactose intolerance:

1. A lactose tolerance test gauges your body’s reaction to a liquid with lactose by measuring glucose levels in your blood 2 hours after consumption. If your glucose level doesn't rise, it means your body isn't properly digesting and absorbing the lactose-filled drink.

2. A hydrogen breath is very similar, but it measures the amount of hydrogen in your breath at intervals after consumption. If your body doesn't digest the lactose, it will ferment in the colon and release hydrogen and other gases to be exhaled. Larger than normal amounts of exhaled hydrogen measured during a breath test indicate that you aren't fully digesting and absorbing lactose.

3. A stool acidity test is used for infants and children who can’t undergo the other two tests. The fermentation of undigested lactose creates acids that can be detected in a stool sample.


While there is no cure for lactose intolerance, you can lessen the symptoms by cutting down on dairy intake, consuming lactose free alternatives, and using lactase enzyme tablets before consuming dairy products. However, lactase enzymes are not effective for everyone. If that is the case, you may try taking probiotics before consumption of dairy products. Probiotics are live cultures that help maintain a healthy digestive system, potentially assisting the digestion of lactose.

Most people with lactose intolerance can enjoy some milk products without any severe symptoms. It may be possible to increase your tolerance to dairy products by gradually introducing them into your diet.

Since dairy is a main source of calcium and vitamin D, it is recommended to take supplements or find lactose-free alternatives with calcium and vitamin D. You should talk with your doctor or a healthcare practitioner to properly diagnose and treat lactose intolerance.


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