Why do you need carbohydrate?
Carbohydrate is your main source of energy. Your brain, nervous system and heart need a constant supply of carbohydrate (in the form of blood glucose) in order to function properly. You also need carbohydrate to fuel your muscles. The carbohydrates in your food are converted into glycogen and stored in your muscles. Rather like filling your car up with petrol before a journey, you need to ensure your muscles are well fuelled before working out.
The more active you are, the more carbohydrate you need to fuel your muscles. Regular exercisers training up to 2 hours daily need around 4–7 g of carbohydrate for each kg of their body weight, or approximately 50–60 per cent of their total calorie intake. Serious athletes who train 4 hours or more a
day may need as much as 10 g. The box below provides a guide to your carbohydrate needs, based on body
weight and activity level. For example, if you weigh 70 kg and work out 3–5 hours a week, you’ll need 4–5 g per kg a day, or between (70 x 4) and (70 x 5) = 280–350 g carbohydrate daily.
Simple or complex?
Carbohydrates are traditionally classified as simple (mono- or disaccharides) or complex (polysaccharides) according to the number of sugar units in the molecules. But this tells you very little about their effect on your body and your blood glucose level. Today, carbohydrates are more commonly categorised
according to their glycaemic index (GI).
What’s the GI?
The GI is a measure of how the body reacts to foods containing carbohydrate. To make a fair comparison, all foods are compared with a reference food, normally glucose, and are tested in equivalent carbohydrate amounts. Glucose has a GI score of 100.
How does GI affect the body?
High GI foods cause a rapid rise in blood glucose levels and have a GI number above 70 (glucose has the highest score at 100). They include refined starchy foods such as potatoes, cornflakes, white bread and white rice as well as sugary foods such as soft drinks, biscuits and sweets.
Foods classed as low GI fall below 55 and produce a slower and smaller rise in blood glucose levels. They include beans, lentils, coarse grain breads, muesli, fruit and dairy products. Moderate GI foods such as porridge, rice and sweet potatoes have a GI between 55 and 70. Protein-rich foods such as meat, fish, chicken and eggs and pure fats such as oils, butter and margarine contain no carbohydrate so these foods have no GI
value. But adding these foods – as well as fats and low GI carbohydrate foods – to meals will reduce the GI of the entire meal. Cooking and ripening (of fruits) tends to increase the GI value.
Low GI eating at a glance
Essentially, a low GI diet comprises carbohydrate foods with a low GI as well as lean protein foods and healthy fats. Low GI foods include:
The more acidic the fruit, the lower the GI. Apples, pears, oranges, grapefruit, peaches, nectarines, plums and apricots have the lowest GI values while tropical fruits such as pineapple, papaya and watermelon have higher values. However, as average portion size is small, the GL (see page 7) would be low.
Most vegetables have a very low carbohydrate content and don’t have a GI value (you would need to eat enormous amounts to get a significant rise in blood glucose). The exception is potatoes, which have a high GI. Eat them with protein/or healthy fat or replace with low GI starchy vegetables.
Low GI starchy vegetables
These include sweetcorn (GI 46–48), sweet potato (GI 46), and yam (GI 37).