Lactose intolerance: Those uncomfortable symptoms of cramping, gas, bloating, and diarrhea shortly after eating dairy. A significant proportion of the population has it. How have we evolved that some people can tolerate milk and some can’t? 

Lactose is the type of sugar found in dairy.  All mammals can digest lactose as newborns and for the first few months/years of life. This is because they produce the enzyme lactase, which can break-down lactose into glucose and galactose in the gut so that it can be absorbed.  

Lactose intolerance

Image of cat drinking milk indicating lactose intolerance in mammals from

However, after the first few years of life, all mammals, apart from humans, stop making lactase or produce substantially less and thus can no longer break-down lactose. This means that the lactose enters the intestine undigested and becomes fermented by the gut bacteria,  producing gas. The gas can subsequently lead to the classic symptoms of lactose intolerance of bloating, gas, cramping, and diarrhea. So yes, those cute photos of cats lapping up milk, are the “before”  shots to digestive issues!  Adult cats shouldn’t be given milk as they can’t digest it! 

Lactase Persistence

In humans, some of the population have what is called “lactase persistence” which means they keep producing the enzyme lactase into adulthood. They can, therefore, digest lactose and so can tolerate dairy (unless they have allergies to the protein in dairy). 

Genetic polymorphisms for lactase persistence are found in a region of the gene MCM6.  What is interesting is that it is those without a polymorphism, the C-C genotype who are lactose intolerant.  This genotype is often shown as -/-, indicating no polymorphisms in either the allele from the mother or father. 

Those with polymorphisms in both alleles, +/+  T-T have the lactase persistence. 

There is also a percentage of the population who are +/- C-T and so have a polymorphism in just one allele from one parent. This genotype generally also confers lactase persistence. 

Evolution of lactase persistence

It’s interesting to see how lactase persistence has evolved in different areas of the world. 

Take a look at this video for the background and to see those areas where persistence originates from and why we evolved with these differences. 

Does age affect lactose intolerance?

Yes. As humans,  we can all digest lactose for the first couple of years of life. After that time, lactase production will start to drop in those who are non-lactase persistent.

By the age of 10, the number of people intolerant to lactose in different populations is around:

10-15% of Americans and Northern Europeans

25% of Chinese and Japanese

47% of Mexican Americans

60% of black South Africans

90+% of Peruvians. 

In some populations, the loss of enzyme activity gradually decreases over 20 years or more. In others, it is lost rapidly in childhood. 

What can you do if you are lactose intolerant?

Many people with lactose intolerance find the symptoms too uncomfortable, so stop consuming any dairy products.  Others find that they can consume a small amount and remain symptom-free, but have to restrict their intake. 

Options with lower lactose levels include the following:

  • Fermented dairy products are often easier to consume as the microorganisms involved in the fermentation have broken down some of the lactose. Thus we don’t need as much lactase enzyme to digest it. Many people can, therefore, tolerate yogurt, particularly if you choose one that has been fermented for 12 hours or more. Check the labels. Those with long fermentation times will tell you. The other option is homemade yogurt where you can control the fermentation time.
  • Some cheese may be well tolerated.  This is because lactose is mainly found in the whey part of dairy. The whey is the liquid part of cheese that is separated from the cheese curds. As cheese ages, it continues to lose even more whey so the older the cheese, the less lactose will remain. Types of cheese with low lactose levels include aged gouda, aged cheddar, Parmigiano-Reggiano, and romano.  So these may all be OK for lactose intolerant people to enjoy.  
  • Reduced lactose or lactose-free dairy products are also available, along with products which help digest the lactose.  

Is there less lactose in goat and sheep dairy? 

Goat and sheep dairy do have slightly less lactose than cow dairy, but it’s not a big difference. This means that if you are lactose intolerant, switching from cow to goat or sheep dairy typically won’t help.  

If you do find a significant difference between cow and other dairies, it could be because you are reacting to not the lactose but other proteins in the cow’s milk.  

Take home message

If you are lactose intolerance, avoiding dairy may be beneficial for you.  Continuing to eat dairy could lead to chronic inflammation in the gut as you are not able to digest it properly. 

Reacting to dairy doesn’t always mean it is the lactose that is the problem. It could be an allergy or sensitivity to the proteins, not the sugars. Talk to a health-care practitioner to help you figure it out. If you are avoiding dairy, make sure you are getting good levels of calcium, magnesium, vitamins D and K2 in your diet. 

If you are interested in other posts about dairy, take a look at these:

How do the different milks compare? Cow vs. goat vs. sheep milk

Cow and goat milk estrogen levels. Does it matter?

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