So, this is my first request post, exciting no? I’ve been asked to explain the biochemistry behind sorbitol intolerance, which I have to say, is quite a good first request because it’s something I’ve never looked at before, and I do love new biochemistry. So, the best place to start is by explaining what sorbitol actually is, and there’s a picture there, for those of you that love a good chemical structure (and I know you’re out there). It’s what is known as a sugar alcohol, so it isn’t a traditional carbohydrate sugar (like glucose for example) although it can be obtained from glucose in the presence of the right enzyme. One of the features of sorbitol is that in comparison to glucose it has fewer calories per gram, and therefore it is frequently used as a sweetener in so-called ‘diet’ or low-sugar foods (like Diet Coke, sugar-free chewing gum and any number of ranges of low calorie cakes and sweets). That said, it isn’t an artificial additive or anything, in fact it’s quite natural, being commonly found in certain fruits, including peaches and prunes.
Now, that last fruit may provide a hint as to one of the negative effects sorbitol can cause! It’s been difficult to track down biochemical reasons for sorbitol intolerance, in terms of why one person might react more negatively than the person sitting next to them. Sorbitol can certainly cause an effect, but why one person reacts differently or more severely to another is a bit of a mystery. The chances are that there is a genetic link, but environmental effects (lifestyle choices, diet etc) may well play a part. However, while I can’t necessarily explain why people are intolerant to sorbitol, I can certainly explain how sorbitol intolerance manifests itself and what’s going on to cause this manifestation!
The most obvious effect is a laxative effect, which can lead to diarrhoea (proof readers watch out for how many times I misspell that throughout this post!) and is caused via osmosis. Osmosis is the process by which water distributes itself evenly (or to use the official term, creates equilibrium), so if you have a membrane (the barrier that surrounds a cell to stop things wandering in and out willy nilly) with lots of water on one side (this is called having a high water potential), and not very much at all on the other side (low water potential), the water will want to get itself to a point where it has the same water potential on each side, because… well that’s just chemistry for you.
Anyway, what has this to do with sorbitol? Well, a lot of sorbitol in the large intestine creates a low water potential, because water potential isn’t dependant simply on how many water molecules there are, but also how many molecules of solute there are in amongst the water, and sorbitol can dissolve in water (as can most sugars). The existence of low water potential in the large intestine leads to water in the cells of the large intestine getting a bit anxious, things are not at equilibrium the way that they would like them to be. And since the sorbitol can’t get through the membrane to achieve equilibrium that way, what can they do but launch themselves through the cell membrane and into the large intestine where the sorbitol is? As I said, it’s just the way they roll. And it doesn’t take a genius to work out that a lot of water in the small intestine is going to result in watery bowel movements.
The other effects of sorbitol are a little trickier to pin down, biochemically. It seems to be reasonably well-established that sorbitol can aggravate irritable bowel syndrome and on top of causing diarrhoea, may also cause, or contribute towards, cramping, bloating and flatulence. It has been suggested that the reason for this is that sorbitol can act as a substrate for enzymes that are involved in the fermentation of bacteria, a process that produces hydrogen gas in the digestive system. Therefore more sorbitol in the digestive system may lead to more fermentation and more gas being produced and, well, that gas has to get out somehow (although often not before causing bloating and painful cramps, which isn’t terribly friendly)
So, that’s sorbitol intolerance for you. As with all my posts, please please let me know if anything is unclear to you or appears inaccurate, confusing or downright wrong. I’m extremely receptive to feedback, and I’d love to hear your thoughts or know whether this post has left you with further questions. I’d also love to know if there are any particular drugs you’d be interested in understanding, or indeed anything else you’d like to see me blogging about. I’m always up for a bit of research.