Lactose Intolerance occurs due to the inability of the body to digest lactose. Lactose is a sugar present in milk and is the main source of calories in milk. Cow’s milk contains approximately 5 g lactose per 100 mL. Lactose is also present in milk products such as yogurt and cheese.

What Is  Lactose Intolerance

Lactase is the enzyme present in the gut that digests lactose in milk. Lactose intolerance occurs due to reduced or absent activity of lactase. Lactose intolerance symptoms include abdominal pain, bloating, flatulence, diarrhea (in some cases constipation), nausea, and vomiting. Symptoms usually occur about one hour after consuming foods containing lactose.

Some people also get other symptoms like memory deterioration, headache, body pains, depression, anxiety, oral ulcers, acne, disturbances of the menstrual cycle, etc.

Most individuals with lactose malabsorption can tolerate at least 12 g of lactose (equivalent to 250 mL of milk) without any issues. They may tolerate larger doses if they consume them with food or spread them over the course of a day.

Tests For Lactose Intolerance

Several diagnostic methods are available to identify individuals with lactose intolerance.

Genetic test

Several studies suggest that lactose intolerance might be a genetic disease. Hence, a genetic test can be used to diagnose it. In this test, the gene responsible for lactose intolerance (LCT gene) is detected in our DNA.

However, this test cannot be used in people who do not have a genetic predisposition for the disease.

Hydrogen breath test

This is the most frequently used method to diagnose this intolerance. It is inexpensive, non-invasive, highly sensitive and specific, and easy to perform and interpret.

The fermentation of lactose by the gut microbial flora occurs when it is not digested. Lactose then accumulates in the gut and leads to the production of gas, including hydrogen. 

The test involves the measurement of exhaled hydrogen by a person in fasting and then after subsequent consumption of 25-50 g of lactose. 

Then the hydrogen measurement is performed every 15 minutes for 3-6 hours. An increase in exhaled hydrogen concentration greater than 20 ppm (parts per million) from baseline is suggestive of reduced activity of lactase enzyme.

Quick lactase test

This involves a biopsy of the gut (duodenal mucosa) and the detection of the activity of lactase enzyme in it.

Lactose tolerance test

A blood glucose test of an individual is done. 50g of Lactose is then consumed by the individual. Then the measurement of the blood glucose levels is again done after 30 min, 60 min, and 120 min. 

The digestion of lactose causes an increase in blood glucose levels. The absence of this increase indicates a lack of lactose digestion absorption. 

This test is rarely performed due to low sensitivity and specificity.

Summary

Lactose intolerance causes symptoms like abdominal pain, bloating, flatulence, and diarrhea. Tests for it include a genetic test, hydrogen breath test, Quick lactase test, and Lactose tolerance test.

References

1. Catanzaro, R., Sciuto, M., & Marotta, F. (2021). Lactose intolerance: An update on its pathogenesis, diagnosis, and treatment. Nutrition Research, 89, 23–34. https://doi.org/10.1016/j.nutres.2021.02.003 
2. Misselwitz, B., Butter, M., Verbeke, K., & Fox, M. R. (2019, November). Update on lactose malabsorption and intolerance: Pathogenesis, diagnosis and Clinical Management. Gut. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6839734/