Carbohydrates

No matter what type of exercise you do, your body will always use some glucose for energy. Glucose is formed from the breakdown of carbohydrates (sugars and starches) in your diet and is stored as glycogen in the muscles and liver. However, the body can store a limited amount of glycogen - and to keep it topped up you need to refuel with carbohydrates within two hours of exercise as well as eating a carbohydrate-rich diet overall. How much carbohydrate you need really depends on the amount of training you do - the more glucose you use the more you need to eat to replenish your stores.

Sugars and Starches

All sugars and starches are carbohydrates. Basically, they can be divided into three groups:

1. Monosaccharides
These are single molecules of sugar. The monosaccharides are:

Glucose
Fructose
Galactose

Glucose is found in most carbohydrate foods including sugars and starches. Most carbohydrates are eventually digested or converted into glucose for energy fuel. Fructose is also known as fruit sugar and is found in fruits, vegetables and honey. It is converted into glucose by the liver. Galactose is part of lactose, the sugar found in milk.

2. Disaccharides
These are two linked sugar molecules which are broken down into the monosaccharides by digestion. The disaccharides are:

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

Sucrose (table sugar) normally comes from sugar beet and cane, but can be found naturally in all fruits and vegetables, and also in most herbs and spices. Lactose is found in milk and milk products. Maltose is formed when starch is broken down.

3. Starches
Starch is simply hundreds of molecules of glucose sugar joined together. When starch is digested, it is first broken down into maltose and then into glucose.

So, as you can see, the only difference between sugars and starch is the size of the molecule. Ultimately, most carbohydrates will end up as glucose to provide that vital energy fuel.

Carbohydrate Needs
Just how much carbohydrate you need depends on the amount of exercise you do. Research has shown that a diet high in carbohydrate, obtained either from simple sugars or complex carbohydrate, is equally effective in improving exercise performance. If you're physically active, then the optimal diet is one that contains 60-70 per cent energy from carbohydrates. However, it's difficult enough to guesstimate the proportion of carbohydrate containing foods in one meal, let alone drinks and food for the rest of the day!

The simpler way to calculate your daily carbohydrate need is to first work out how much you require, depending on the number of hours of exercise you do each week, and then multiply that by your weight in kilograms. See Table 1 to work out how much carbohydrate - expressed in grams per day for every kilogram you weigh (g/kg/d) - you need to have for your training program. For example, if you weigh 60kg and exercise about an hour each day your daily carbohydrate requirement would be: 60 x 6 = 360g.

Table 1.
Carbohydrate Needs for Exercisers
Physical Activity / Carbohydrates
3 - 5 hrs/week / 4 - 5 g/kg/d
5 - 7 hrs/week / 5 - 6 g/kg/d
1 - 2 hrs/day / 6 - 7 g/kg/d
2 - 4 hrs/day / 7 - 8 g/kg/d
4+ hrs/day / 8 - 10 g/kg/d

Thanks to food labeling, the majority of packaged foods will tell you how many grams of carbohydrate per 100g - and often per portion - that food contains. Plus, see Table 2 to discover roughly the amount of carbohydrate you are getting from everyday foods and snacks.

If you are training frequently, then your daily carbohydrate requirement will be high and you will need to eat frequent snacks and meals to achieve this. Therefore, you must remember to look after your teeth by brushing twice a day with a fluoride toothpaste and visiting the dentist regularly.

Table 2. Carbohydrate Content of Everyday Foods
Medium portion of food / Carbohydrate (g)
Banana, apple, pear or 2 kiwi fruit / 20
2 slices of bread or 1 bread roll / 30
Bagel, flapjack or slice of fruit cake / 40
Nutri-Grain bar or 3 digestive biscuits / 30
All-Bran, Muesli or 2 Weetabix / 30
Baked potato, pasta or rice / 50
Baked beans (135g) or sweetcorn (1 can) / 20
1 Mars bar or 50g chocolate / 30
10 jelly beans or 60g fruit gums / 30
2 tsp Honey or jam or 150g low-fat yoghurt / 10
200ml orange or apple juice / 20
500ml sports drink, milk or squash / 30

Which Carbohydrate?
The next question we need to consider is: which type of carbohydrate do you need? As most carbohydrate foods, for example, pasta or sugars are eventually broken down into glucose, one type is not intrinsically better than the other. However, what is important is how quickly the carbohydrate is converted to glucose - and that's where the glycemic index (GI) comes in.

The GI of a food is a measure of that food's effect on blood glucose levels. It is worked out by comparing the rise in blood glucose after eating a food containing 50g of carbohydrate with the blood glucose rise after eating 50g of a reference food (usually glucose). The faster the rise in blood glucose, the higher the GI (and the greater the insulin response). Generally, foods are divided into three categories. Table 3 gives the GI category of some everyday foods.
Unfortunately, there is no easy way to tell what the GI of a food is. Some sugars have a high GI (glucose) and others a low GI (fructose). Some complex carbohydrates have a low GI (pasta) whereas others have a higher GI (rice).

Table 3. The glycaemic index (GI)
High GI above 70 / Moderate GI of 50 - 70 / Low GI below 50
Glucose / Sucrose / Fructose
Honey / Mars bar / Chocolate
Jelly beans / Crisps / Sponge cake
Sports drink / Squash / Milk
Bagel / Bread / Fruit cake
Weetabix / Muesli / All-Bran
White rice / Brown rice / Pasta
Baked potato / Boiled potato / Baked beans
Watermelon / Banana / Apple
Parsnip / Sweetcorn / Lentils

Before, During and After Exercise
Just before, during and immediately after exercise, try to eat high- and moderate-GI foods to help stimulate glycogen synthesis.

It has been argued that low-GI foods, such as fructose, are a good thing to eat before exercise. This is because they provide a readily available energy source with minimal insulin response and therefore encourage your body to greater fat burning. However, this theory is not well founded, and it is also irrelevant because insulin secretion is suppressed during exercise. In addition, the rate of glucose supply to the bloodstream from the digestion of low-GI foods is generally not fast enough while exercising.

Allow about two to three hours after a meal before exercising, to minimize gastric upset. Then, five to 30 minutes prior to your workout have a 50g moderate- to high-GI carbohydrate snack (see Table 4 for suggestions). This will help you maintain your glucose levels so that you can train more efficiently. It is probably advisable to avoid the more bulky (fiber-rich) carbohydrates here, as these can cause abdominal discomfort. Try different snacks to find which ones suit you best.

If you exercise continuously for more than an hour, it is likely that you will need to consume carbohydrates during your workout to avoid fatigue. One of the best ways to achieve this is by drinking sports drinks, which provide water as well as sugar and therefore help hydration - see the section on fluids.

The best time to refuel and restock those vital glucose stores ready for the next workout is immediately after exercise, because this is when muscle glycogen is replaced fastest. Try to eat a minimum of 50g of carbohydrates and preferably 1g carbohydrate per kg of body weight every two hours for up to six hours after a hard workout.

Most people don't feel hungry immediately following exercise, so this is where sports drinks can be useful, as they are better tolerated and provide both carbohydrates and fluid - all helping speed up the recovery process. Furthermore, when the appetite is suppressed a more concentrated, moderate-high GI, carbohydrate source such as glucose, sucrose, or maltodextrin is ideal.

In between exercise sessions, try to include a mixture of low- to moderate-GI foods in your high-carbohydrate diet. However, be careful not to overload your bread, potatoes and pasta with lots of butter and cream as that would be a high-fat diet! Go easy on more fatty carbohydrate snacks, such as cakes and biscuits, too. After all, gram for gram fat has twice as many calories as carbohydrate and could lead to weight gain.