Nutrition: Carbs and the glycaemic index

The smart way to choose cycling fuel

The GI diet allows energy starved muscles to be be re-filled with glycogen by eating plenty of carbohydrate foods, an ideal approach for cyclists.


This makes much more sense than something like the Atkins diet (which relied on protein), especially if you ride a hard race on Saturday and need to replenish muscle glycogen stores overnight for another race or training session on Sunday.

When you need to refuel quickly, eating fatty foods, which usually also contain a lot of protein, is an inefficient way of replacing muscle glycogen. It is also an expensive way – carbohydrate rich foods such as potatoes, baked beans, bananas, rice pudding and dried fruits are much cheaper than good quality cheddar cheese, bacon, ham, fresh cod, mackerel or salmon, grass fed beef and corn fed chicken.

Simple or complex?

The GI concept is not new, and was originally used to educate individuals with diabetes about the effects of various foods on their blood glucose levels. For many years, carbohydrates were classified as being either ‘simple’ or ‘complex.’

It was assumed that simple carbohydrates (sugars) were absorbed quickly, causing a rapid rise in blood sugar, whilst complex carbohydrates (starches in bread, pasta, potatoes) were absorbed slowly and resulted in a gradual rise in blood sugar.

However, researchers at the University of Toronto were the first, in 1981, to make accurate measurements of the effect of different carbohydrate foods on blood glucose. A stable blood glucose, one which does not rise too high or fall too low, is important for physical and mental health.

Your brain can only use glucose as a source of energy. During a long ride, your muscles can burn fat and use this as a fuel to keep the pedals turning. However, a low blood glucose will affect your brain, leaving you feeling tired, dizzy and weak – even though your muscles have plenty of other fuel to draw upon.

Knowing which foods will result in a rapid rise in blood glucose, or help sustain it during a time trial or endurance ride is very helpful. The Toronto group invented the phrase ‘Glycaemic Index’ or GI. to describe the effect that a food has on blood sugar.

They measured this by asking volunteers to swallow a drink containing exactly 50grams of glucose. Blood samples were taken regularly over the next couple of hours to measure the rise and fall in glucose over this time.

Then, 50grams of carbohydrate in the form of other foods (raisins, white bread, rice cakes, dried apricots) were fed to the volunteers. The response from the first glucose drink is taken as 100 and all other foods are individually compared with this. Hence foods are rated between 1 and 100 in the GI scale.

These studies showed some surprising results in that starchy foods, such as white bread and mashed potato, which were traditionally thought to be digested slowly were not and had a high GI value similar to sugary foods. Also some sugary foods, such as chocolate, had a low GI score.

We now know that the form in which the carbohydrate is held within the food (for example the sugar in raisins is bound with fibre whilst the sugar in chocolate is mixed with cocoa butter) is important. This affects the speed at which it can be digested and the rate the glucose is absorbed into the bloodstream.

Wholemeal bread – a starchy food – has a high GI value, while the sugar fructose, found in honey and most fruits, has a low value. This is because, when the bread is cooked the starch granules are broken open and the starch is easily digested and absorbed.  

Fructose sugar needs very little digesting, but must be carried across the gut into the bloodstream, while glucose simply rushes through by a different mechanism. Not surprisingly, high GI foods are the ones to eat during an endurance ride, or at the end of a time trial when you want to replenish your muscle and liver glycogen stores quickly.  

High or low?

However, if you are finding it difficult to lose weight, perhaps your everyday eating patterns include too many high GI foods. This is because your body responds differently to carbohydrates when sitting for hours at a desk compared with being out on your bike thrashing up hills.

Low GI foods are the best ones to eat when you are inactive, such as sitting in front of the television. The carbohydrate that low GI foods (such as oatcakes, fresh apples, plain yoghurt, oranges or porridge) contain is absorbed slowly through the gut, and blood glucose levels rise gently.

This stimulates the production of a small amount of the hormone insulin, which makes sure that the levels of glucose in your blood remain constant.

The situation is very different if you eat a high GI food such as fruit, yoghurt, a bowl of cornflakes, biscuits or pretzels. There is a rapid rise in blood glucose and large amounts of insulin are needed to regulate the system. It is as if you are working in a shop assembling bicycles with a steady flow of frames, wheels and brake systems delivered to the door. Suddenly there is an excess of wheels, and they have to be put somewhere. You stash them in the back of the shop and then carry on making up the bikes.

The excess glucose in your body is the same as the extra wheels – it has to be dealt with, otherwise it will clog up the system. Your body responds by converting the excess glucose into fat and dumping it into your ever-increasing fat stores. In men, the fat is deposited most easily around the belly, while in women it tends to get stored in breasts, hips and thighs.

Not surprisingly, your pancreas will start to tire of this continued over-stimulation as it is forced to produce large quantities of insulin to control the effects of a regular onslaught of high GI foods throughout the day. If this happens over a long enough period of time, your pancreas may still produce insulin, but not enough to remove the glucose.  

This means that you have a higher than normal blood glucose, which leads to other health complications, and is a symptom of type two diabetes. At this point, if you switch to lower GI foods, it’s possible to control the problem.

However, if you continue to snack on high GI foods throughout the day, thinking that the long commute home justifies it, your pancreas is under persistent strain and may pack up altogether. The best time to eat high GI foods is just before a ride. The surge in blood glucose which results is then used to fuel your muscles, rather than overworking your pancreas.

Knowing a food’s glycaemic index score is a very useful, but it is not the whole story. Next week we look at GL – glycaemic load.


Dr Chris Fenn Accredited Nutritionist