Adult lactose digestion

Lactase is the enzyme that digests lactose, the sugar in milk. Most mammals lose it after weaning, but a variant near the LCT gene keeps it switched on into adulthood in many people.

What this variant switches on

Lactase, the enzyme that splits the sugar in milk, is made by the LCT gene. In most mammals it switches off after weaning, and the same is true for most humans. This variant sits in a control region inside the neighbouring MCM6 gene and acts as a switch that keeps LCT running into adulthood.

So lactase persistence is the derived state and lactase non-persistence is the ancestral one. What a genotype file calls lactose intolerance is, in evolutionary terms, the default human condition.

How strong the evidence is

This is one of the most statistically overwhelming genotype-to-phenotype links in human biology. In a study of 10,708 people the association between this variant and measured lactase activity had a p-value below 1e-1400, which is far beyond the threshold used for genome-wide significance.

The variant also carries one of the strongest signals of recent natural selection in the European genome, tied to dairying, which is why the trait is often used as the textbook example of gene-culture coevolution.

The gap that matters: this is the European variant

Lactase persistence evolved independently at least four times. The variant read here is the one that spread through Europe. Separate variants in the same control region, including G-13907, G-13915 and C-14010, produce persistence in East African, Middle Eastern and other populations, and none of them are read here.

The practical consequence is specific and important: someone of African or Middle Eastern ancestry can be lactase persistent through a variant this file does not test, and would be reported here as likely lactose intolerant. In 1000 Genomes reference data the European variant is carried by roughly 85% of European-ancestry individuals, about 23% of African-ancestry individuals and essentially nobody in East Asian samples.

Non-persistence is not the same as intolerance

Losing lactase does not mean losing dairy. Symptoms depend on how much lactose arrives at once, whether it comes with other food, and on the gut bacteria that ferment it, and those bacteria adapt with regular exposure. Most people with lactase non-persistence tolerate around 12 grams of lactose, roughly a cup of milk, taken with a meal.

Hard cheeses and yoghurt contain far less lactose, and lactase supplements and lactose-free milk work. If dairy causes symptoms that a low-lactose approach does not fix, that is a reason to look for another cause rather than to assume the genotype explains it.

What each rs4988235 genotype means

rs4988235 has three possible genotypes: CC, CT and TT.

Likely lactose intolerant (rs4988235 CC). You don't carry the variant that keeps the lactase enzyme switched on, so your ability to digest lactose (the sugar in milk) tends to fade after childhood. Dairy can then cause bloating, gas or stomach upset, which is what lactose intolerance is. Lactose-free milk and hard cheeses are usually easier to handle, and this is the most common pattern worldwide.

Lactase persistent (rs4988235 CT). You inherited one copy of the variant that keeps the lactase enzyme working into adulthood. Lactase digests the sugar in milk, so you can most likely enjoy dairy without the bloating or upset that lactose intolerance causes.

Lactase persistent (rs4988235 TT). You have two copies of the variant that keeps the lactase enzyme switched on for life. Your body keeps making plenty of lactase, so you can digest milk and dairy comfortably as an adult.

Evidence & sources

Across global populations, about 23% of people carry two copies of this variant, and about 44% carry none.

This variant appears in 3 published genetic studies: PubMed 29875488 · PubMed 34648354 · PubMed 37277652.

Common questions

What does the MCM6 gene do for lactose digestion? The lactase enzyme itself is made by the LCT gene, but a control switch sitting inside the neighboring MCM6 gene decides whether LCT stays on in adulthood. The variant tested here, rs4988235, is that switch. The T version keeps lactase switched on so you can digest the sugar in milk, while the C version lets it fade after childhood, which is the usual cause of adult lactose intolerance.

Does the CC genotype mean I am definitely lactose intolerant? Not always. A CC result means you lack the common European persistence variant, so lactase most likely faded after childhood and dairy may cause bloating or gas. But some people, especially of African or Middle Eastern descent, stay lactase persistent through different variants this test does not read, and tolerance also depends on gut bacteria and how much dairy you eat at once. Treat it as a strong hint, not a diagnosis.

Which rs4988235 genotype is lactose intolerant? For this European variant, CC is the genotype linked to lactase non-persistence and typical adult lactose intolerance, while CT and TT keep the lactase enzyme switched on so milk is usually digested comfortably. Your own genotype and its meaning are shown in the possible results above.

My result says lactose intolerant but I drink milk fine. Why? Two common reasons. This file reads the European persistence variant only, so persistence caused by an African or Middle Eastern variant reads as non-persistence here. And non-persistence itself is a spectrum: most people without lactase still tolerate about a cup of milk with food, especially with regular exposure.

Can lactose tolerance change over time? The genotype does not change, and lactase activity typically declines through childhood and adolescence rather than in adulthood. What does change is tolerance to a given amount, since gut bacteria adapt to regular lactose intake, which is why cutting dairy out entirely can make a later reintroduction feel worse.

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