A simple question, right? Worth a whole article?

Yes - because the answer, from the metabolism’s point of view, is not as simple as you might think.

Different Forms

If I limit myself to the “sugar” you can buy in the supermarket, this chapter ends right here. But it doesn’t, because sugar is also the name of an entire chemical class of molecules - and, on top of that, a pragmatic way of looking at things that helps us judge our food by its effect rather differently than before.

Tellingly, Wikipedia keeps a “disambiguation” page for sugar - and it isn’t only needed because of the many people who carry the name. Don’t worry, here we mean the (usually) white powder, and not Lord Sugar, Sugar Ray Robinson or Marilyn Monroe’s Sugar Kane. My own definition is a touch more pragmatic, but the disambiguation page and the various different pages about sugar (the substance) do help to map out the territory. And: I’ll be ignoring all kinds of sugar that are not directly relevant for the metabolism.

Beyond that there is the EU Sugars Directive(1), which legally defines roughly a dozen sugar products with more than a dozen names - and that list is nowhere near exhaustive when it comes to how the various ingredients in our food get labelled.

Chemically

Let’s start at the molecular level:

are a class of substances made up of single, double and multiple sugars. Their chemical names typically end in -ose.

Monosaccharides

The smallest building blocks of this class are the single sugars(2):

The ones that matter to us and our metabolism are glucose (the lead actor, also known as blood sugar), fructose (colloquially fruit sugar), and galactose (a component of milk sugar).

Of these single sugars, fructose is the sweetest-tasting substance, followed by glucose and galactose. From these three building blocks, ultimately, all of our common carbohydrates are assembled.

Glucose also turns up as “grape sugar” or “dextrose”.

Disaccharides

Here we already reach the first clash of worlds: table sugar is a so-called double sugar. It is made up of equal parts glucose and fructose. Because the two components are easily split apart early in digestion, the sweetness of the fructose sets the tone here. Its chemical name is sucrose.

Worth knowing: table sugar is only half glucose - so it has a presumably surprising effect on blood sugar, one that doesn’t quite match expectations. But more on that later.

Other common double sugars are maltose (malt sugar, two units of glucose) and lactose (milk sugar: galactose and glucose). As neither contains fructose, they are less sweet than table sugar.

Oligosaccharides

From a metabolic point of view this class of sugars isn’t terribly interesting. Anyone who really wants the full picture will find information on oligosaccharides at Wikipedia, or can simply treat them as relatively short polysaccharides, about which there is rather more to say.

Polysaccharides

From a metabolic perspective we usually know polysaccharides as starch - a long chain of glucose molecules. Those building blocks already hint that starch arrives in digestion as glucose - which is why eating starch raises blood sugar fairly equivalently to its mass.

Another polysaccharide of metabolic relevance is cellulose - a structural molecule in many plants, which humans cannot break open, however. For that reason it is classified metabolically as .

Metabolically

The ones especially relevant for metabolic considerations are glucose, starch and fructose. So I’ll limit myself to those. Sucrose - table sugar - is only a footnote, but still worth a mention, though the heavy lifting is done by its two components.

First, the usual baseline here: all carbohydrates are for humans.

Glucose: since glucose is the blood-sugar molecule, it deserves a closer look

  • The body constantly needs a certain, relatively stable, blood-sugar level
  • The body can maintain that blood-sugar level entirely on its own and without any help from dietary carbohydrate
  • Even a doubling of the blood-sugar level over a longer period is toxic

The absolute scale of it: reckoning on 100 mg/dL of blood sugar, a body with 4 l of blood normally circulates about 1 teaspoon of glucose (4 g). A can of cola with 33 g of table sugar (or a comparable ingredient) tops that up with 9 teaspoons, half of it pure glucose.

Starch:

  • These are long chains of glucose molecules that can be split apart relatively easily - and even with a net energy gain.
  • Which is why starch arrives in digestion as glucose.
  • Because of the splitting required and the sheer size of the molecule, starch doesn’t reach the bloodstream quite as quickly as single or double sugars. That’s why starch also counts as a “slow carbohydrate” - all the more so when it is taken in together with fibre, which stands, purely mechanically, between the starch and the enzymes, or between the glucose and the gut lining.

Coming back to the normal blood-sugar content of 1 teaspoon of glucose: a typical portion of pasta brings roughly 90 g of starch with it - so more than 20 teaspoons of glucose, which have to be taken up into the blood in the course of digestion and distributed around the body. Sounds rather as though a can of cola would be preferable to a plate of pasta? Your liver, being one of the main distribution hubs, might even agree.

Fructose: fructose occupies a small special position - it is a carbohydrate and is counted as one, yet it has a fundamentally different molecular structure from glucose, and is processed exclusively in the liver.

The body’s easiest route for breaking fructose down heads in the direction of fat and is called . The associated pathway is almost identical to the route by which alcohol is broken down in the liver(3). This pathway is the reason I couldn’t say for certain above whether the liver prefers 20 teaspoons of glucose from pasta or 9 teaspoons of fructose and glucose from cola: glucose can be processed throughout the whole body, fructose almost exclusively in the liver.

Measuring the Effects

To describe the various properties of the various kinds of sugar, we have several options. Two common ones are the glycaemic index (GI) and the glycaemic load (GL).

The GI is a measure of how quickly blood sugar rises - and also of how high it rises. The shorter the glucose chains, the faster the glucose shows up in the blood: the GI is high.

If, on the other hand, the body first has to free the glucose from its molecular chains, and possibly navigate around obstacles on the way (fibre), then the GI is rather low. But since ultimately all of the glucose from the food ends up in the blood, the GL is comparable to that of the short-chain sugar: the curve rises less steeply and doesn’t reach the same height, but it lasts longer in return. The area under the curve can therefore be comparable.

Let’s recall the intriguing point about table sugar teased above, the one that may violate expectations. It’s about the difference between table sugar and starch: you’d think that sugar, with its small molecule, makes itself felt much faster than starch. But because sugar is half fructose, which doesn’t affect blood sugar, its GI is considerably smaller than that of starch, which consists almost entirely of glucose(4).

Ingredients: Creative Accounting

On the ingredients list it’s popular to hide sugar. That’s fairly easily done, by using different names for different ingredients.

From the EU’s list of legally defined sugars(1) alone you can compose impressive ingredient lists: sugar, invert sugar syrup, glucose syrup. And that’s only the legally protected names - English-language catalogues of “different names for sugar” run to 50-plus entries, well worth a search if you like collecting them: dextrose, maltodextrin, fruit juice concentrate, barley malt syrup, and so on.

Why do it? On an ingredients list, ingredients have to be given in order of their mass - that is, the dominant ingredient has to go at the top. If, say, sugar at 50% of the product would unambiguously sit in first place, another ingredient can move up front if instead there is only 18% sugar. And 17% invert sugar syrup. And 16% glucose syrup.

And other sweetening ingredients sound so much more natural: “apple concentrate”, for instance. It may not be 100% as sweet as sugar, but it gives a far nicer ring to the ingredients list.

In the US, high-fructose corn syrup (HFCS) is a popular industrial ingredient. Comparable to table sugar, but with 55% fructose and 45% glucose. Sugar isn’t expensive, but HFCS - especially at scale - is cheaper still.

Pragmatically

Sugar (in all its forms) generally represents an “empty” calorie bomb: carbohydrates don’t inherently contain micronutrients all that often, unless these are separately added or brought along from the surroundings. (This is in contrast to fats and oils, which can contain fat-soluble vitamins - animal fats typically do.)

My pragmatic definition is simpler than the one above: for me, sugar is everything that arrives in the digestive tract as sugar. What baffles most people, but at least agrees with the chemical definition: starch is sugar - and I don’t necessarily have to draw a particularly sharp line between it and the sweet-tasting stuff.

Looking back over the article now: “what is sugar?” turns out not to have been such a simple question after all. The most important part of the answer, though, is this: sugar isn’t necessarily sweet. Most of what we eat isn’t.