As outlined in our last Nutrition piece, the Glycaemic Index is a useful tool to help select the right type of carbohydrate foods. Foods with a high GI value (70 or more) result in a rapid rise in blood glucose. These foods are useful immediately before a time trial, during an endurance ride or immediately after a training session when muscle and liver glycogen stores need to be quickly and effectively replenished.
The majority of the rest of your diet, unless you spend most of your day cycling, should be based on foods with a low GI value (55 or less). This ensures a steady supply of glucose to your brain, without the sudden onslaught of sugar – which is quickly made into fat; and you wonder why you are no longer a lean, mean cycling machine!
However, GI is not the whole story. Anyone who is switched on about the links between nutrition and performance, as most cyclists generally are, will also need to consider the GL (glycaemic load) of a food.
There are plenty of books on GI, several of which have enjoyed a regular place in the top ten best sellers list. Foods are rated between 0-100 and the aim is to avoid those rated over 70 where the carbohydrates are absorbed rapidly. This means that mangoes and grapes should be restricted whilst ice-cream (with a lower GI value) can be eaten more freely.
Whilst the GI value is useful, it only tells part of the story and I believe that the concept of GL may soon take its place.
There are a few books on the market about GL, but I am sure many more are on their way. The GI value only tells you how quickly your blood glucose will rise after eating a particular food; it doesn’t take into account the portion sizes that we normally eat. For example, watermelon has a high GI value (72), but you have to eat a huge chunk to get a significant amount of carbohydrate. A normal portion of watermelon will supply a mere 6grams of carbohydrate, and you would need to chew through the whole melon to get 50 grams.
In contrast to get 50 grams of carbohydrate from wholemeal bread, you would only need to eat two large slices. Although carrots and chocolate share the same GI value (48) you need to munch through two large carrots to get the same amount of carbohydrate from two tiny squares of chocolate.
Just to confuse things further, the GL system uses a different index – with foods being scored from zero to 20. Foods with a low score are considered healthier, in that they impact less of a carbohydrate load on the body.
If carrots and chocolate are recalculated in terms of the GL then carrots rate as 3.9 whilst chocolate has a value of 14. So, if you want a good slug of carbs, just before heading out for a ride, choose foods with a high GL. If you aren’t training for a couple of days but want to maintain a steady input of carbs, base your eating pattern on foods in the low GL range.
This is because the glycaemic index (GI) can be misleading. Some foods have a low GI value, but a high GL. For example, most types of pasta have a GI of less than 55 – so it is classed as a low GI food. However, a normal portion size of pasta means that you are getting a lot of carbohydrate – in other words, a high GL. This is fine if you want to load up on carbs. If not, then mix butter beans, couscous or chickpeas into a sauce or soup; they all have a low GL.
Dr Chris Fenn www.chrisfenn.com