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Carbohydrates


Dr Samantha Stear
http://www.bbc.co.uk/health/healthy_living/fitness/energy_carbs.shtml

No matter what type of exercise you do, your body will always use some glucose for energy. The main source of glucose are the carbohydrates - sugars and starches - in your diet.

The best way to keep your stores of glucose stocked up is to eat a diet rich in carbohydrates. How much carbohydrate you need depends on the amount of training you do - the more glucose you use, the more you need to eat to replenish your stores.

Sugar and starches
All sugars and starches are carbohydrates, which can be divided into three groups:

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

  • Glucose - found in most carbohydrate foods. Most carbohydrates are eventually digested or converted into glucose for energy fuel.
  • Fructose - found in fruits, vegetables and honey. Converted into glucose by the liver.
  • Galactose - found as part of lactose, the sugar found in milk.

    Disaccharides
    These are two molecules of sugar joined together. They are broken down into the monosaccharide sugars by digestion. The disaccharides are:

  • Sucrose (glucose + fructose) - normally comes from sugar beet and cane, but can be found naturally in all fruits and vegetables.
  • Lactose (glucose + galactose) - found in milk and milk products.
  • Maltose (glucose + glucose) - formed when starch is broken down.
    Starches
    Starch is hundreds of molecules of glucose sugar joined together. When starch is digested, it's first broken down into maltose and then into glucose.

    How much do I need?
    This depends on the amount of exercise you do. If you're physically active, the optimal diet is one that contains 60 to 70 per cent of energy from carbohydrates.

    The simplest way to calculate your daily carbohydrate needs is to first work out how much you require depending on the number of hours of exercise you do each week, then multiply that by your weight in kilograms. Use the following list to work out your training programme needs (expressed in grams per day for every kilogram you weigh - g/d/kg):
    Physical Activity Carbohydrates
    3-5 hrs/week 4-5 g/d/kg
    5-7 hrs/week 5-6 g/d/kg
    1-2 hrs/day 6-7 g/d/kg
    2-4 hrs/day 7-8 g/d/kg
    4 + hrs/day 8-10 g/d/kg


    If you weighed 70kg and exercised about an hour each day, for example, your daily carbohydrate requirement would be 420g (70 x 6).
    The majority of packaged foods will tell you how many grams of carbohydrate per 100g a food contains. Also use this list to discover roughly the amount of carbohydrate you are getting from everyday foods and snacks:

    Medium portion of food Carbohydrate (in grams)
    2 tsp honey or jam 10
    Banana, apple, pear 20
    10 jelly beans 30
    500ml sports drink, milk, squash 30
    Breakfast bar, 3 digestive biscuits 30
    Bran cereal, muesli, 2 pieces wheat cereal 30
    Baked beans, sweetcorn (1 can) 30
    2 slices of bread, 1 bread roll 30
    Bagel, flapjack, slice of fruitcake 40
    Baked potato, pasta, rice 50
    Crisps 60

    Which type should I eat?
    As most carbohydrates are broken down into glucose, one type is not necessarily any healthier than the next. When we're exercising, what's important is how quickly the carbohydrate is converted to glucose - that's where the glycaemic index (GI) comes in.

    The GI is a measure of a food's effect on blood glucose levels. It's worked out by comparing the rise in blood glucose after eating a food containing 50g of carbohydrate with the rise after eating 50g of a reference food (glucose or white bread). The faster the rise, the higher the GI.

    Generally, foods are divided into three categories:

    High GI above 70 Medium GI of 50-70 Low GI below 50
    Glucose Sucrose Fractose
    Honey Museli bar Chocolate
    Jelly beans Crisps Sponge cake
    Sports drink Squash Milk
    Bagel Bread Fruit cake
    Wheat cereals Museli Bran cereals
    White rice Brown rice Pasta
    Baked potato Boiled potato Baked beans
    Watermelon Banana Apple


    There's no easy way to tell the GI of a particular food. 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). Use the above list to guide you.

    When should I eat?
    Allow about two hours after a meal before exercising, then have a 50g carbohydrate snack five to 30 minutes before your workout. This will help to maintain your glucose levels so you can train more efficiently.

    If you exercise continuously for more than an hour, you'll need to consume carbohydrates during your workout too. One of the best ways to achieve this is by drinking sports drinks. Not only do these provide carbohydrates, they also help to keep you hydrated. See fluids.

    The best time to refuel and restock those vital glucose stores ready for the next workout is immediately after exercise. Try to eat a minimum of 50g carbohydrates and preferably 1g carbohydrate per kg of body weight - ie, 70g if you weigh 70kg - within the first two hours. Again, sports drinks are useful as they replace fluids at the same time.

    Between exercise sessions, include a mixture of low- to medium-GI foods in your high-carbohydrate diet. Watch out, though, and don't overload your bread, potatoes and pasta with lots of butter and cream - that would be a high-fat diet!

    Also go easy on more fatty carbohydrate snacks such as cakes and biscuits. After all, aside from the health and fat issue, gram for gram fat has twice as many calories as carbohydrate. This is something to consider if you need to watch your energy intake.

    This article was last medically reviewed by Dr Rob Hicks in September 2005.
    First published in May 2001.