What is a Carbohydrate?

What is a carbohydrate? Carbohydrates,

as the name suggests, are compounds consisting of carbon, hydrogen and oxygen. The common abbreviation for carbohydrates is therefore CHO. CHOs contribute a substantial proportion of energy to the daily diet – the brain relies almost exclusively on CHO to function, and the other tissues, nerves, and muscles use CHOs for a significant proportion of their energy.

CHOs are broken down to monosaccharides in the mouth to a small degree, but digestion is mainly in the small intestine. Monosaccharides are the basic units, or building blocks of CHOs; glucose being the most well known of the three. The other two monosaccharides are fructose (commonly found in fruit) and galactose (occurring as part of the disaccharide lactose).

Disaccharides are made when two monosaccharides join together:

Maltose = glucose + glucose
Sucrose = glucose + fructose
Lactose = glucose + galactose

Polysaccharides are linked branched chains of monosaccharides (e.g. starch).

It is from the above chemical explanations that the well-known definitions of simple and complex CHOs came about. Monosaccharides and disaccharides were classified as simple sugars and were thought to be non-nutritious, very sweet and could cause a fast rise and fall in blood glucose and insulin levels, and be a large contributor to dental issues. Complex carbohydrates (starches) were thought to be far more nutritious, and would create a more sustained rise and fall in blood glucose and insulin levels.

Foods defined as Complex Carbohydrate-rich:

Bread (e.g. multigrain, wholemeal)
Breakfast cereals
Rice, pasta, noodles
Potato, sweet corn
Dry biscuits
Legumes (e.g. red kidney beans, soy beans, baked beans)
Grains including cous cous, barley and oats

Foods defined as Simple Carbohydrate-rich:

All types of sugar
Honey, golden syrup
Jam, marmalade
Fruit (e.g. canned, fresh, dried)
Jelly
Soft drinks and cordials
Sugar-based confectionery
Dairy products

GI Index – The Glycaemic Index

The GI index (glycaemic index) came about because nutritionists needed a different method to classify carbohydrates (CHOs). This is because blood glucose and insulin responses are not predicted accurately with only the classifications of simple and complex carbohydrates.

Two classic examples of those inaccuracies are dairy products and white bread. Dairy products would be classed as a simple CHO as they contain lactose. But dairy products only produce a small gradual rise in blood glucose and insulin. White bread causes a large blood glucose response but is classed as a complex CHO due to its starch component.

The Glycaemic index was discovered because of these discrepencies. The index ranks CHO-rich foods based on immediate effects on blood glucose levels, compared to a reference food (glucose or white bread).

Most of the research to date in this area has been undertaken in New South Wales (Australia) by Associate Professor Jennie Brand Miller, Kaye Foster-Powell, and Dr Stephen Colagiuri. An excellent publication on this topic is “The GI factor: The Glycaemic Index Solutions”, written by the above researchers.

The theory of the GI index states that “CHO foods that break down quickly during digestion have the highest GI factors. Their blood sugar response is fast and high. CHOs which break down slowly, releasing glucose gradually into the blood stream, have low GI factors” (Brand Miller, Foster-Powell, and Colagiuri).

The GI index of a food can be influenced by several factors:

1. How much cooking or processing of the food has been done. Both can disrupt the cell structure within the food and can increase the GI index. For example, a hot potato has a higher GI than a cold potato, due to cell breakdown when hot, and subsequent reformation of the cell structure when cold. Also, in the case of grains, if their fibrous coat is intact, this coat acts as a physical barrier and slows down digestion (the enzymes which break down the starch cannot get into the starch as easily).
2. Content and interactions of fructose, lactose, protein or fat. All slow down the blood glucose response.

3. Food absorption blocked by “antinutrients”: e.g. phytates in wholegrain foods slow down the blood glucose response.

CHO-rich foods are given a GI index value up to 100:

High GI: >70
Moderate GI: 55 – 70
Low GI: <55

Low Glycaemic Index Foods Slow release CHOs (GI <55):

All Bran, rice bran, rolled oats, barley bran
Pumpernickel, rye, fruit loaf, mixed grain bread and crispbreads, oat based breads
Spaghetti, pasta, Basmati rice
Apples, pears, firm bananas, grapefruit, peaches, oranges, plums, cherries
Sweet potato, sweet corn
Lentils, kidney beans, chick peas, butter beans, haricot beans, black-eye peas, baked beans
Milk, yoghurt, custard, low fat ice cream, chocolate

Moderate Glycaemic Index Foods Medium release CHOs (GI 55-70):

Crumpets, pita bread, Nutrigrain, arrowroot biscuits, Ryvita, Mini-Wheats, Vita-Brits, one minute oats
Ripe bananas, mangoes, pineapple, sultanas, new potatoes, orange juice, beetroot, broad beans
Pastry, cous cous, taco shells
Regular ice cream

High Glycaemic Index Foods Fast release CHOs (GI >70):

White bread, wholemeal bread, bagels, scones, pre-split muffins, rice cakes
Coco Pops, Cornflakes, Rice Bubbles, Weet-Bix
White rice, brown rice
Parsnip, potatoes, watermelon
Pretzels, potato chips, morning coffee biscuits, jelly beans
Glucose, sports drinks

The longer it takes to be digested and absorbed, the lower the GI of a food. This information is particularly relevant for both weight management and sporting activity.

The GI and Sports Performance

Sports performance can be assisted with knowledge of the GI. It is advisable to consume low GI foods as part of the pre-event meal, as CHO are released slowly into the blood stream.

Unfortunately not all athletes can tolerate low GI foods right before exercise. This is because the fibre, fructose or lactose can slow digestion too much. Combine this with a little nerves and it can cause abdominal issues including diarrhoea and cramps.

An athlete would need to consume some foods and fluids with a high GI during exercise to help boost energy levels. This would also help delay fatigue. After exercise, consuming high GI foods help to quickly refuel glycogen stores. There is no point consuming a low GI food with half an hour of competition remaining!!

Rebound Hypoglycaemia If blood glucose drops below normal baseline levels after consuming CHO, then Rebound Hypoglycaemia occurs. Some athletes who consume glucose 30 minutes before exercise seem to have trouble maintaining a high intensity of exercise. This is because when CHO is consumed, insulin levels increase. Some athletes more than others.

Once insulin increases, fat oxidation decreases, so there is an increased dependance on CHO. The increased use of CHO causes a slight drop in blood glucose levels.

This drop is generally rectified within 30 minutes of exercise and is rarely noticed by the athlete. The benefit of the extra CHO outweighs this blood decrease.

All studies since the discovery of Rebound Hypoglycaemia have found that CHO before exercise helps performance, or has little effect.

Yes, there are some athletes that react negatively to CHO before exercise. In these athletes, blood glucose leves reduces quickly. Symptoms including feeling shaky, clammy and light-headed can be felt. To avoid this, these athletes should try low GI foods before exercise to avoid a large hike in blood glucose levels. They should also do a few high intensity sprints to prompt the liver to release glucose into the blood.

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