The Genetics of Dietary Intolerance

If the mere thought of milk makes your stomach hurt, you probably know a thing or two about dietary intolerance. Intolerances are surprisingly common, affecting the majority of adults in one way or another — and advances in personal genomics are telling us more than ever about how genetic conditions like lactose and gluten intolerance work.
People who are lactose intolerant have difficulty digesting lactose, a sugar found in milk and other dairy products, because their bodies don’t produce a particular enzyme that’s needed to break it down. It’s not uncommon for individuals who experience symptoms of lactose intolerance — bloating, upset stomach, and all that fun stuff — to blame how much milk they drank growing up or just a run of bad luck. In reality, genetics are a primary factor.

Adults who can drink milk without side effects have a trait called lactase persistence. Historically, humans drank milk only as infants and thus had no need to digest dairy products after adolescence, so their bodies stopped lactase production over time. These regulatory mechanisms are coded in the DNA at a gene known as MCM6 which act as a switch, controlling whether lactase production is turned on or off. Multiple biological factors control this switch, so some individuals whose DNA codes for lactose intolerance can actually drink milk without any discomfort; however, the majority of people that can drink milk have a variation in the MCM6 gene. In these individuals, the body continues to produce an enzyme (lactase) even in adulthood. In addition to genetic factors, the ability to digest lactose can be influenced by the type and quantity of bacteria naturally present in a person’s stomach and intestines.

It turns out that these little hitchhikers can actually play a big role in how you process your food, and some evidence indicates that your genetics can influence what kinds of bacteria exist in your stomach, which can have an indirect effect on nutrient and vitamin absorption.
More directly, variations in genes can also affect how well you metabolize nutrients, including a gene known as FUT2. This gene is specifically associated with vitamin B12 metabolism, and growing evidence suggests FUT2 variations may be associated with several conditions relating to digestion and gut bacterial infections (although more research is needed to conclusively link these risks with genetics).

And then there’s gluten intolerance, which has gotten a lot of visibility in recent years as restaurants and grocery stores have started offering a wide array of gluten-free products. Gluten is a natural component of many grains that helps dough rise and keep its shape. Gluten intolerance is topic of very active research; so far, studies suggest that some forms of gluten intolerance are associated with changes in DNA, which has prompted the use of genetic testing to differentiate between various forms of the condition.

Many times food intolerances are subtle — so subtle, in fact, that you may not even realize that a particular ingredient or nutrient is giving you trouble. By assessing your DNA for variations associated with various intolerances, you may be better equipped to manage your diet and reduce discomfort.

And remember: your genes aren’t your destiny. Some of your dietary preferences — and the various ways that your body responds to food — are influenced by your environment and personal history. Science has shown that genetics can combine with these factors to affect your body’s ability to process some foods, which means that genetic testing has the potential to help you determine what foods to avoid and how simple dietary changes might improve your wellbeing.

Citation: “Lactose digestion and the evolutionary genetics of lactase persistence.” Ingram, Mulcare, Itan, Thomas, Swallow. Copyright Springer-Verlag 2008