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Why is Lactose Intolerance so Common?

Lactose intolerance describes the symptoms an individual experiences after consuming dairy products. Lactose intolerance normally causes people to suffer digestive issues such as diarrhea, bloating, gas, and/or vomiting. This is due to an inability to tolerate lactose, the sugar found in milk and other dairy products. Our body produces an enzyme, lactase, to breakdown this sugar for digestion. Some individuals produce lower levels of lactase or may stop producing lactase entirely over time. Without the enzyme, lactase, we are unable to break down the lactose and thus experience an upset stomach. According to Lomer, Parkes, & Anderson (2007), we produce the highest amount of the lactase enzyme at 34 weeks gestation. Our body produces less lactase following the first few months of life (Lomer, Parkes, & Anderson, 2007). This occurs at different rates for different people as a normal part of development (Lomer, Parkes, & Anderson, 2007). Only about 30% of the population continues to produce lactase into adulthood and is more commonly found in people of North European descent (Lomer, Parkes, & Anderson, 2007). This is believed to occur due to the increased prevalence of dairy farming and frequency of intake over the last 10,000 years (Lomer, Parkes, & Anderson, 2007). Lomer, Parkes, & Anderson (2007) found that DNA indicates those of North European descent did not have prolonged lactase production until the rise of dairy farming. Lomer et al. (2007) describes this as the culture-historical hypothesis and that it attributes the difference in lactase production to the heavy consumption of milk in their diet compared to others.

In general, there are three terms that describe lactose intolerance which is also known as lactase deficiency, including: congenital, primary, and secondary lactase deficiency (Lomer et al., 2007). See Table 1. for complete definitions of the three terms describing lactase deficiency. Congenital lactase deficiency describes individuals that produce the lowest amount of lactase. This is evident soon after birth by poor formula tolerance and digestion issues when offered breastmilk. Congenital lactose intolerance is very rare (Lomer et. al, 2007). Most of the population, around 70%, suffers from primary lactase deficiency (Lomer et. al, 2007). This describes those that produce less lactase over time and due to a natural decline in lactase production to <10 u/g of tissue (Lomer et. al, 2007). Secondary lactase deficiency describes individuals who are unable to digest lactose due to a secondary reason such as disease like Celiac or Chron’s Disease (Lomer et. al, 2007). There are other factors that also influence one’s tolerance level of dairy.

Nutrition and genetic factors play a role in our tolerance level (Lomer et. al, 2007). Ethnicity can increase your risk of lactose intolerance. Northern Europeans have the lowest rate of lactose intolerance, whereas about 50% of the population in South America, Africa, and Asia are lactose intolerant (Lomer et. al, 2007). According to Lomer et al. (2007), Asians experience an 80-90% decrease in lactase production after 4 years of age, whereas Northern Europeans may not experience this until after reaching 18 years of age. Similarly, in the US, about 15% of adult Caucasians and 85% of African Americans are lactose intolerant (Fawley, Kay, Porto, & Tolia, 2021). They also found that individuals of Asian, Hispanic, Native American, and Jewish decent had high rates of lactose intolerance (Fawley, Kay, Porto, & Tolia, 2021).

You may have heard of some individuals who are lactose intolerant still being able to tolerate small amounts of dairy products like cheese and yogurt. This is because all dairy products have varying amounts of lactose and is also dependent on how much lactase your body is able to produce. Milk alternatives like lactose-free, soy, almond, and oat milk are great ways to include calcium in your diet without simultaneously suffering an upset stomach. Lactose-free milk specifically is the same as cow’s milk but does not cause the same discomfort that regular cow’s milk does. Lactose is a complex sugar composed of galactose and glucose. Lactose-free milk breaks down lactose into these simpler sugars so that it is easier for our body to digest. See Table 2. below for common dairy products and their lactose content. If you feel that you may be lactose intolerant or experience discomfort when drinking cow’s milk, talk to your doctor about changing your diet.



Lomer, C. E., Parkes, G. C., & Anderson, J. D. (2007). Lactose intolerance in clinical practice myths and realities. Alimentary Pharmacology & Therapeutic, 27 (2). 93-103.

Fawley, R. K., Kay, M. H., Porto, A. F., & Tolia, V. (2021). Lactose intolerance. American College of Gastroenterology.



Author: Mercedes Martin

Editor: Kaitlyn Longstaff

Public Health Scientist: Mercedes Martin


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