Wednesday, January 23, 2013

Lactose Intolerance

Ironically, for my first illness post I’m covering something that isn’t actually an illness but can definitely make people sick. Lactose intolerance is the inability to digest the sugar (lactose) in milk products.
Figure 1: Milk hates some people. 
Image from Rakka on Flickr.

Let’s look at a list of the recognized symptoms from the Mayo Clinic:

·         Diarrhea
·         Nausea, and sometimes, vomiting
·         Abdominal cramps
·         Bloating
·         Gas

How nice of them to put the worst symptoms first! I say this because in the popular media lactose intolerance is often joked about, laughed off, and generally considered a simple nuisance. On the popular science nerd sitcom Big Bang Theory, Leonard is often made fun of because his lactose intolerance makes him gassy. The truth is that the symptoms can range from just mild gas and bloating to full-on diarrhea and vomiting that feels like a velociraptor is tearing apart your insides while you sweat and cry in the bathroom in the middle of the night and wonder if you’re dying of some weird nocturnal intestinal disease. Why nocturnal? Well, because the symptoms are caused by lactose making it to the large intestine. The average digestion time for food to pass through your stomach and small intestine is around 6-8 hours but many people begin feeling gas and bloating symptoms only a few hours after eating dairy. Everyone is different. Assuming 6-8 hrs, if you eat dairy for supper it’s going to hit your large intestine in the night. When lactose gets to the large intestine, that’s where things get interesting… if by interesting, I mean painful bloating, gas, and diarrhea. Which I do. So where does the gas come from and why does lactose cause such terrible cramping and diarrhea?
Figure 2: Lactose is made up of the simple sugars glucose and galactose.
To start with, lactose is a sugar (aka a carbohydrate, aka a saccharide). Sugars can be made of one carbon ring (monosaccharides), two rings (disaccharides), or more than two (polymers), which means they’re made up of monosaccarides (mono=1) linked together.  Lactose is made up of two linked monosaccharides, glucose and galactose, which each look like a hexagon (Figure 2). The monosaccarides are connected by an oxygen atom. It’s at this oxygen that the enzyme lactAse breaks lactOse down into glucose and galactose. Glucose, as you may know, is the sugar the body uses for energy. The galactose molecule is less stable than glucose and is quickly converted to glucose also for energy. This reaction takes place in the small intestine… and as long as there is enough lactAse around that’s the end of it; no problems. When there’s a shortage of lactAse, the lactOse travels on down to the large intestine where there is a whole host of hungry little guys ready to eat it up.

The human body has taken on lots of little buddies during its evolution including bacteria, with some even becoming an integral part of cell function (holla to the mitochondria!). In the large intestine, bacteria can breakdown the leftover undigested lactose into usable products like short chain fatty acids, which are further broken down for energy. Great, right? It is except the byproducts of lactose breakdown by bacteria include CO2, H2, and methane gas. And where do those gases need to go? Out. But the way out is generally blocked by previous, um, material so the pressure builds up until, well, you know. The first bowel movement in an attack is usually normal in all respects except urgency, about 3-4 on the Bristol Stool Scale. Once that’s out of the way comes the diarrhea, which can range from a 5 to a watery 7 on the scale.

Figure 3: Lactose in the large intestine leads to excess water loss 
due to osmosis.
But why does lactose in the large intestine lead to diarrhea? Essentially, because lactose is a bigger molecule that’s not normally there and the body likes to keep the gradients between concentrations the same between boundaries in the body. If the concentration gets too high in the large intestine (Figure 3), water will get sucked into the large intestine to balance the concentration to that of the body. This water then gets flushed out of the body with the poop. Lots of water in your poop makes diarrhea.

So with all of these uncomfortable symptoms, why isn’t lactose intolerance considered an illness? Historically, lactose intolerance has been the default setting in adult mammals. Mammals only needed to survive on milk until they could eat solid food so after infancy lactAse production decreased. In Europe, people realized they could drink the milk of other mammals and make delicious products like cheese and ice cream (okay, not really ice cream way back then). As they ate more dairy at later ages the production of lactAse stayed higher. This is why it’s rare for people of European descent to be lactose intolerant. Surprisingly, up to 75 percent of adult humans are lactose intolerant to some degree and the highest rates are in people from Southeast Asia, Africa, and in Native Americans, cultures with traditionally low rates of dairy consumption after infancy.

In another post, I’ll tell you about the tests that can be used to diagnose lactose intolerance and how it’s treated.

I am not a medical doctor, none of this is intended as medical advice, and I cannot diagnose you in the comments. If you are concerned about your health, please see your physician. 


  1. Awesome post! I enjoyed it thoroughly. Are you open to subject suggestions?? :)

  2. Thanks! Yes, I'm definitely open to suggestions! You can email them to me (sicknessisfascinating at gmail dot com) or tweet them at me. I'd like to put a Submit an illness form on the blog but haven't figured out how to yet.