One of my favourite blogs is that of Jeff Duntemann; I first came across Jeff as a columnist in the computer magazine Dr Dobbs Journal nearly twenty years ago, and have followed his career ever since. I find his blog very interesting, as it contains a wide variety of subjects. True, I'm not normally interested in his musings on Catholicism, and sometimes his material about publishing becomes tedious, but frequently he brings up interesting issues.
His latest entry (at the time of writing) is about fructose, or rather high-fructose corn syrup. He points the interested reader to this article which contains a fairly detailed explanation of the biochemistry of fructose, which normally enters the body as a component of table sugar.
Many people don't know this, but my degree is in Food Science, which is composed of four subjects: food analysis, biochemistry, microbiology and food manufacture. I've listed the four in the order of my interest, which was not necessarily the ratio in which we were taught. I'm sure there have been vast improvements in the syllabus since I graduated 28 years ago. My dissertation was on methods of analysing xylitol in food; xylitol is a sugar alcohol (not a true sugar) which apparently has the ability to heal dental caries whilst being sweeter than fructose, which in turn is sweeter than table sugar. Not surprisingly, one of the 'foods' which I examined was chewing gum.
One of the more interesting pieces of data about xylitol was why it is so sweet. This is due to the physical layout of its hydroxyl groups and nothing to do with its chemical properties or metabolism. The xylitol molecule happens to 'sit' on the sweetness receptors in the tongue better than other sugars and thus is sweeter.
I was reminded of my xylitol research when reading the article about fructose: why is fructose metabolised only in the liver whereas glucose can be metabolised by every cell in the body? Why can fructose cause raised levels of uric acid (which can cause gout) whereas glucose does not? Why does fructose not stimulate the pancreas to produce insulin? The question which I am trying to ask is what were the carbohydrates available to the early primates and why do our genes code the ability to handle glucose one way and fructose another.
Obviously, a few million years ago, there weren't purified mono- (fructose, glucose) and disaccharides (sucrose aka table sugar, lactose aka milk sugar) available. But there were starches, and these are polysaccharides almost completely composed of glucose units. Other saccharides (like the trisaccharides in cabbage) were probably undigested as our ancestors were lacking the enzymes for their metabolism. So now that I think of it, if starch were the primary carbohydrate source, then it's not surprising that we have enzymes for its metabolism (readily available in one's saliva), it's not surprising that we are geared to deal with its metabolic product, glucose, and it's not surprising that we are less well equipped to deal with the metabolism of other sugars.
I thought that fructose was converted into glucose and then metabolised, but obviously not.
Judging by the articles that Duntemann mentions, it seems that fructose - and especially high-fructose corn syrup - is something to be avoided at all costs. Fortunately because of my dietary problems, I eat very little preprocessed food, so I hope that my fructose intake is minimal. I have enough dietary problems as it is.