Carbs, Sucrose, Fructose and High Fructose Corn Syrup

Given how important sugars and carbohydrates are in the diet, I thought people might find a very quick and easy guide to their chemistry helpful and useful.



Most people have a pretty good idea of what carbohydrates, or carbs, are when they come across them.  Things like bread, potatoes and pasta have high carbohydrate content and are most people’s main source of energy.  What doesn’t seem to be so well known is how close the relationship is between carbohydrates and sugars.  Carbohydrates are basically strings of sugar molecules.   The technical name for them of polysaccharides is much more informative.  The chain lengths vary from a few tens of sugar units to several thousands.

When you eat carbs the body breaks them down into sugars pretty quickly.  In fact the process starts almost instantly with the enzyme amylase in the saliva getting straight to work on them.  You can experience this for yourself if you simply chew a bit of bread without swallowing it for a few minutes.  You’ll notice it develops a sweet taste.  The most common sugar in the carbs you eat is glucose, and from the point of view of how it ends up eating carbs is pretty similar to eating glucose.



Glucose is a sugar, one of a large family of similar molecules.  The sugar we use in cooking and in coffee is another member of the same family, which I’ll come to in a moment.  Glucose is key ingredient in the diet.  It is the main source of energy for the body and also the way the body transports energy.  Your body picks up glucose from the blood stream to power whatever it is doing.  The level of glucose in the blood is important.  Industrial and road accidents are more frequent early in the morning when people’s blood glucose levels are low.  If you are short of glucose in your blood you are less able to solve problems and even to exert willpower.

And if this weren’t enough, the level of glucose in the blood is used by the brain to monitor how hungry you are.  All in all, an important molecule.  And it does all this with only 6 carbon atoms.




The sugar we are most familiar with is known by chemists as sucrose.  It is a disaccharide because it is composed of two sugar units, one of glucose and the other of fructose.  The same enzymes that break down carbs are just as effective at breaking down sucrose into its two building blocks.  So eating sucrose is not too different to eating carbs.  Both supply energy and both lead to an increase in blood glucose levels. It isn’t surprising that sucrose gets glucose into the blood stream more quickly.

I like the structure of sucrose because it always looks to me like a man and a woman holding hands, the fructose being the female who is slightly smaller than her companion.




Fructose is a five carbon rather than a six carbon sugar but aside from that it has a lot of similarities to glucose.  It is a naturally occurring sugar that our bodies are well equipped to handle.   But it is less common than glucose and whereas any cell can handle glucose metabolism, fructose is mainly metabolised in the liver.  On the face of it, fructose is an unlikely candidate to give rise to any health problems.  But recently some work has suggested that fructose may interfere with the hunger mechanism.  This is far from a proven fact, but it does sound plausible and if it turns out to be true would explain why we find it so hard to resist eating sweet things like cakes once we have started.

High Fructose Corn Starch (HFCS) is a relatively new sweetener – it only started being used in any quantity in the seventies – which is a processed form of corn starch where the carbohydrates have been broken down to their constituent sugars, glucose and fructose.   There are different grades with different proportions, but very roughly 50:50 in most cases.

HFCS therefore is basically not very different to sucrose.  Both are sources of glucose in a form that can get into the blood stream very quickly, and both are much richer sources of fructose than typical carbohydrates.  If it turns out that fructose does interfere with our ability to know when we are full, then I would imagine that both sucrose and HFCS would have much the same effect.  If HFCS is slightly worse than sucrose would seem to be a bit academic.

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