Carbohydrates, Glycaemic index and glycaemic load

Best vitamin supplements / Category / Emile Du Toit / May 5th 2014


Carbohydrates together with fats are the body’s primary source of energy. They provide energy to your organs, tissues, muscles and brain.

A small amount of carbohydrates is also utilised by your liver to build lipoproteins. A carbohydrate is literally a carbon atom (hence ‘carbo’) attached to a water molecule (hence ‘hydrate’). So the basic unit of a carbohydrate is CH2O. Now there are many different types of carbohydrate molecules but they all have a CH2O molecule in common. For example glucose – which we will be talking about below – is composed of six CH2O molecules.

Difference between simple carbohydrates and complex carbohydrates

Let’s quickly understand the difference between simple carbs and complex carbs:

1. Simple carbohydrates

Simple carbohydrates have either one or two sugar molecules. Saccharide means sugar, so these are called either monosaccharide’s (mono = 1) or disaccharides (di = 2). Examples of monosaccharide’s are glucose (‘blood sugar’ molecule) and fructose (fruit sugar). Examples of disaccharides are sucrose (1 glucose and 1 sucrose molecule) and maltose (2 glucose molecules).

2. Complex carbohydrates

Complex carbohydrates are carbohydrates consisting of more than 2 sugars. Oligosaccharides have 3-10 sugar molecules and polysaccharides have greater than 10 sugar molecules.

What happens to carbohydrates that are consumed?

Carbohydrates are broken down into monosaccharide’s (largely glucose – generally referred to as blood sugar), to be utilised by the body. The brain gets first dibs on glucose, as it requires a huge amount of energy and is only able to use glucose to power it. After this other tissues in your body will get to utilise what they require.

Insulin controls blood sugar levels by enabling glucose from the blood to be absorbed into the cells that need it. The pancreas produces insulin, and when blood glucose levels rise more insulin is released to lower blood sugar concentrations. When the glucose levels drop, the pancreas reduces production of insulin.

Your body’s cells absorb glucose and convert it into energy to drive the cell. Various chemical reactions combine to convert glucose to ATP (adenosine triphosphate), and a phosphate bond in ATP powers most of the machinery in any human cell.

Calories per gram of carbohydrate

Most carbohydrates, as well as proteins have approximately 17 kilojoules of energy for each gram, which amounts to 4.1calories ( kcal) per gram. Fats have the greatest amount of food energy per gram, amounting to 37kj/g or 8.8 kcal/g. And for all of you trying to loose or maintain weight, the next time you go out drinking you need to realise that each gram of ethanol (alcohol) contains 29kj (6.9kcal)!

Carbohydrates are utilised in different ways

1. Fibre cannot be broken down from the body and is mostly excreted. However, a small amount is fermented by bacteria in your colon and broken down into short-chain fatty acids that can then be absorbed.

2. Carbohydrates are largely stored as glycogen (a polysaccharide), to later be converted into energy for the body.

3. After your glycogen stores are filled up, the rest of the carbohydrate is stored as fat in your adipose tissue.

Glycaemic index

Glycaemic index (GI) is a standardised measurement to examine how extremely a particular carbohydrate affects blood glucose levels (blood sugar).

For obvious reasons glucose itself tends to have the most extreme effect on blood glucose. Therefore the glycaemic value for glucose is set arbitrarily at 100 so that all other glycaemic index values are a percentage of the blood sugar increase which is brought about by consuming pure glucose. So a food with a GI of 70 increases blood sugar by 70% of the amount that glucose would. A GI up to 55 is considered low, from 56 – 69 is considered medium and above 70 is considered high. This is actually one of the most important bits of nutritional information that you need to look for whilst shopping. Despite this, in many parts of the world GI (and GL) is still poorly reported.

The next paragraph on how glycaemic load is actually calculated is just for those ‘but why’ kids like myself. If you are not one of those feel free to skip to the paragraph on glycaemic load.

How is glycaemic index actually calculated?

For those of you who are interested in the details of how things work, this is how GI for a particular food is actually calculated. As mentioned it is a relative value as it is a comparison to the impact of glucose on blood sugar. So when testing a particular subject both the glucose value and value for the particular food would be calculated. The glucose value can then be converted to 100 ad the food value would be manipulated proportionally to achieve a GI value. Generally speaking 50g of whichever food tends to be used, after any fibre has been subtracted. This carbohydrate without the fibre weight added in is known as ‘available carbohydrate’. Initially a fasting blood sugar value is done. After the food is consumed blood glucose is measured frequently over the next 2 hours. These points are plotted on a graph, and what is then considered is the incremental area under the curve (iAUC). What this means is that the whole area above the fasting blood sugar is mapped, and its area is calculated. This is a measure of how much the particular food increased the blood sugar of the particular subject (whether it be glucose or carrots).

Food Graph

In this case you can see that the iAUC is that area above the fasting glucose (around 4 for this subject), and fenced in by the 120 minute line and the particular curve of the particular food being used. You will also see that the red line displays a food that raises blood glucose more than the other foods. It would have a higher GI value than the foods measured by the blue and green curves, as its iAUC is larger.

Glycaemic load

Glycaemic load (L) is a relatively new way to examine how carbohydrate impacts on blood sugar level. It considers both how rapidly a particular carbohydrate breaks down into sugar (glycaemic index) as well as the amount of that particular carbohydrate in a particular food.

Essential if you take the GI of a food (e.g. watermelon) and divide it by the percentage of carbohydrate in that food you will end up with the glycaemic load. To express this:

GL = GI/100 x net carbs (carbs minus fibre)

So we would discover that although watermelon has a pretty high glycaemic index there is actually very little carbohydrate in it so the (overall) glycaemic load for a portion is fairly low. A GL up to 10 is considered low, from 11-19 is medium, and from 20 and up is considered high.

Using Glycaemic Load

You can see that the GL value is partially determined by weight of available carbohydrate, and therefore portion size. This is in fact particularly helpful. What it means is that you can manipulate your portion size of a food, or plate of different foods, in order to make sure that the overall glycaemic load remains in the low GL or on occasion medium GL bracket!

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