Quite a few people, including most recently my own lovely Mummylase and the ever-wonderful Aisling, have asked me to write about lactose intolerance and milk allergies, the differences between them, and how they both work. It’s actually a really interesting area, personally I absolutely did not know until recently that there were two different reasons that people might need to eat dairy-free, so once again I got the excitement of learning new things, which always makes me quite stupidly happy.
The crucial difference between the two problems is that one is an allergy, an inappropriate immune response to a usually harmless stimuli, and the other is an inability to properly digest something.
Lactose is a complex sugar that is found in milk, whether it be from cows, goats or people. It’s actually the lovely little molecule that gives milk it’s slightly sweet taste. Because lactose is produced in human breast milk, humans possess a gene containing the instructions to build an enzyme called lactase. Lactase exists purely to chop lactose into little bits that can be absorbed by the body; stripping lactose for parts is its vocation in life.
Normally, in human babies lactase is produced so the baby can digest milk from its mother and absorb the sugars without any digestive issues. As the child gets older, and is weaned off a milk diet, the enzyme then stops being produced. The really interesting thing about lactose intolerance is that this seems to be the original way that things worked… we were supposed to stop producing lactase after we were weaned, because way way back in our evolutionary history, we didn’t really eat or drink any milk once we’d stopped breastfeeding. We’re programmed, or at least we were once programmed, to be lactose intolerant as adults, because frankly the body doesn’t bother producing enzymes that aren’t going to be useful. No-one wants cells full of bored unemployed lactase, hanging around with nothing to do.
If you eat lactose and your body has no lactase to break it down, your body can’t absorb it. The lactose heads for your colon, undigested, and there is metabolised by your friendly gut bacteria. In this case, you might not consider them to be quite so friendly, though, since what they do as a result of metabolising lactose is produce excess gas, causing you considerable discomfort. The sugar and gas can also lead to increased water heading to the colon… which means diarrhea as well.
Clearly, though, a lot of us aren’t lactose intolerant. We can eat milk and ice-cream and yoghurt and lots of things that are packed with lovely sweet lactose, without the ill effects. This is because we’ve evolved to continue to make the lactase enzyme, even after we’ve stopped breastfeeding, mainly in order to be able to digest cow’s milk. This is actually a really good example of where a genetic mutation can be beneficial. A single very straightforward change in the stretch of DNA that controls the production of the lactase gene can keep the gene switched on into adult life. This is basically a mutation that has allowed us to eat ice-cream. I think it’s my new favourite genetic mutation. And, obviously, natural selection has helped us to hang onto this mutation, because the ability to digest milk is a bit ace, so that it has become more and more common in certain parts of the world to be able to digest lactose.
Coping with casein
The other reason someone might need to be dairy free is a milk allergy. This means that your body considers one of the proteins in milk to be a dangerous invader, despite the fact that it’s actually harmless. Once the immune system thinks something is an invader, though, it’s going to respond. It’s suspicious and efficient like that.
The most common milk protein to be allergic to is alpha-S1-casein. This protein is structurally different in different species, so a body might happily accept the form in human milk, but react nastily to the one in cow or goat’s milk. The alpha-S1-casein is recognised by an IgE antibody as an invader, and an immune response is launched, complete with sirens and wildly flashing lights, and all sorts of dramatic goings on. IgE binds to receptors on mast immune cells, which in response release immune chemicals, including cytokines, histamine, and other members of the Inflammation Squad.
The Inflammation Squad can cause a range of symptoms, from digestive discomfort and issues like flatulence, vomiting and diarrhea, through skin rashes and headaches, all the way to severe anaphylaxis. They’re a bundle of fun, honestly. This is your allergic reaction.
So, that’s the key differences between the two conditions, although both mean the same thing to some extent… you need to cut out dairy products. Next week there’ll be a kitchen science post on lactose and alpha-S1-casein in food, and why people can eat certain things and not others. My Mummylase, for example, cannot drink cow’s milk but can still eat cheese, a fact for which she is endlessly grateful.