Lactose Intolerance

The continued ability to digest lactose is due to a relatively new mutation in a single gene called LCT. Roughly 10,000 years ago, mutations emerged that allowed for increased LCT gene activity in adulthood,^[13] which is technically called ‘lactase persistence’. People with this mutation, which is most common among people of Northern European descent, can digest milk sugar into adulthood. Mutations persist when they confer benefits. Some of the ancestral humans with this mutation benefited from their mutant superpower ability to digest milk sugar into adulthood. Why would that be? In a word, pastoralism. Raising livestock can provide for a steady supply of meat and milk, and certain geographies were set up well for animal husbandry.^[1]

One of those regions, Sweden, provides some interesting data. Nearly all modern Swedish people can digest lactose, but as little as 4,000-5,000 years ago, there was a stark divergence in that area: DNA from Swedish hunter-gatherer populations of that era show only 5% to have had lactase persistence, while DNA from cattle-raising populations of the same era show 75% with lactase persistence.^[14] If you’re a modern-day Swede, you probably owe your milk-digesting ability to the success of the latter population.

Lactose intolerance is sometimes confused with milk allergy, but the two are quite different. Milk allergy is an immune reaction to a protein in milk, usually casein. Lactose intolerance is not an immune condition. People with lactose intolerance don’t have an immune reaction to lactose; they simply have low or no production of the enzyme which breaks it down, and experience digestive symptoms resulting from the fermentation of lactose by their intestinal bacteria.^[2]

No, lactose intolerance is not contagious.

However, primary lactose intolerance, which results from the normal low-to-absent production of lactase enzyme in adulthood, is heritable. In other words, if neither of your parents has the mutation which lets their bodies produce lactase into adulthood, you aren’t likely to, either. Congenital lactose intolerance (the inability to digest lactose from birth) is also heritable, although extremely rare; it’s an autosomal-recessive disorder resulting from a different mutation in the LCT gene (the gene that allows the body to produce lactase).^[12]

Lactose intolerance in adults can be diagnosed by a careful dietary history. Keeping a written record of both food and digestive symptoms can reveal a relationship between lactose-containing foods and symptoms. The digestive symptoms of lactose intolerance usually develop 20 minutes to two hours after consuming lactose.^[2]

The lactose breath test (LBT) is the main diagnostic test for lactose intolerance. In a LBT, a fasted patient drinks a lactose solution. Breath hydrogen is measured before drinking the lactose, and then again every 20 minutes for the next 2–5 hours. A rise in breath hydrogen of ≥20 ppm is considered positive for lactose intolerance. Frequently, breath methane is measured along with hydrogen, and a rise of ≥10 ppm in breath methane is considered positive.^[2] Both hydrogen and methane are byproducts of bacterial metabolism; a sizeable increase in one or both after fasted lactose ingestion indicates that the lactose the patient drank is available to their intestinal bacteria for fermentation, and is therefore not being broken down in the small intestine. Some patients may also experience digestive symptoms during the LBT, but this isn’t required for diagnosis.

Higher-lactose dairy foods to avoid on a low-lactose diet include mammal milks, yogurt, soft cheeses, cream, and foods prepared with any of the above (such as milk pudding, barfi, ice cream, gulab jamun, milk bread, scalloped potatoes, cream sauces and soups, etc.).

Lower-lactose dairy foods, which may be tolerable without symptoms, include lactose-free milks and creams (mammalian milk/cream which has been pretreated with lactase enzyme), hard cheeses (parmesan, swiss, cheddar, etc.), and all plant-based milks and cheeses, which are naturally lactose-free.^[2]

A special note on whey protein: Whey, a byproduct of cheesemaking, contains plenty of lactose. Whey protein concentrate (WPC) contains varying amounts of lactose, depending on how it’s processed.^[15] Fortunately, whey protein isolate is low-lactose, although not completely lactose-free.^[16] Some brands of whey protein concentrate and isolate list lactose content on the label.

All mammals feed their young with milk, but that milk is rich in lactose rather than sugars we see in other foods (namely sucrose, or table sugar). This isn’t likely to be an accident: lactose is made up of galactose and glucose, and not only is galactose a major nutrient for newborns,^[17] it is important for their development.^[18]

Galactose is also part of complex molecules that are essential to the nervous system and immunity. Given that infants have increased nutritional needs, in order to bolster their developing physiology, their intake of lactose makes sense.

This is a much bigger question than lactose intolerance. Don’t presume that the ability to digest lactose with ease also means that you’re meant to consume tons of dairy. And don’t presume dairy is either “good” or “bad”.

Nobody knows all the effects of dairy, either positive or negative. Dairy is linked to acne, and has mixed evidence^[19] related to heart disease, but effects seem to vary widely.

We do know that humans are unique in drinking milk past childhood, and also in drinking the milk of other animals routinely. This arose due to physiological benefits, such as the ability of dairy products to provide water, protein, vitamins, and calcium in times of low food availability. While dairy products remain an excellent source of all of the above, they’re currently far from being the only dependable source of these nutrients. Thus, it might be prudent to test whether you feel better with or without dairy, if you suspect that one or more dairy products may not agree with you.