Nutrition and hydration during exercise

Did you know?

Male Tour de France cyclists need up to 12,600kJ a day. However, an average man weighing 70kg only needs about 1,470kJ when jogging for 30 minutes or 1,680kJ swimming for 30 minutes.

Understanding your body’s need for food and drink can make a big difference to how you feel during and after physical activity. Knowing when it is the best time to eat, and how much, can lead to a big boost in energy levels and results.

When to eat

Eating a big meal before exercise can slow you down and leave you feeling sluggish. It’s also not a great idea for your digestive system and can lead to stomach cramps, and even diarrhoea, as your muscles and digestive system compete for energy resources. However, skipping food before physical activity can be just as bad for you. The resulting low blood sugar levels can leave you weak, faint or tired, and can slow down your reactions.

Try these tips to get the most out of your workout.

Eat a full breakfast

The majority of energy you got from dinner has been used up by the morning. If you plan to exercise within an hour of breakfast, eat a smaller amount or drink something, like fruit juice, to raise your blood sugar.

Time your meals

Large meals should be eaten at least 3-4 hours before exercising; smaller meals 2-3 hours.

Don't skip meals

Low blood sugar can make you feel weak and lightheaded. If you don’t have a lot of time to eat beforehand, try snacking on a piece of fruit.

Eat after your workout

To help your muscles recover, eat a meal that contains both protein and carbohydrates within 2 hours of exercise.

What to eat

If you're taking part in competitive sport, your nutrition needs will probably be quite specialised. For those of us who are ‘exercising’, rather than training, we can find the nutrients we need in a balanced diet. That should include carbohydrate for energy and protein for muscles, as well as vitamins and minerals.

These all play an important role in helping our bodies function properly and shortages can have an adverse effect, eg, low calcium and magnesium levels may lead to muscle cramps, while good levels of iron will help prevent exercise-induced anaemia.

MRC Human Nutrition Research, an independent UK body carrying out scientific research into relationships between nutrition and health, published an article outlining the role of nutrition in exercise.

Muscles use two main fuels:

Carbohydrates. These provide easily available energy and are the most important energy source for short, intense physical activity like sprinting. The body stores limited amounts of carbohydrates - enough for approximately 20 to 30 minutes of moderate exercise. After this, if fat can't be converted to energy, the body becomes tired. Cereals, breads, vegetables, pasta, rice, fruit, sports drinks and fruit juices are sources of carbohydrate.

Fat. This can be used for supplying energy during longer periods of exercise at a more moderate pace. For those people exercising to help lose weight, exercise for longer periods at a moderate level to burn fat, e.g. walking or jogging for longer than 30 minutes.

Whatever the intensity of the exercise, some carbohydrates are always used. Experts recommend you start replacing them as soon as possible after physical activity as the muscles' capacity to refuel is greatest during that first hour.

Maintaining fluid levels

The water in our blood is used to carry nutrients such as sugar (glucose) to cells and to remove waste products. During exercise our bodies get hot, and our main way of cooling down is to sweat, which means we lose fluid containing electrolytes — elements like potassium, calcium, sodium and chlorine. If we don't replace that fluid, our heart rate increases and our temperature rises, putting us at risk of dehydration.

Every day, men need 3.4L and women 2.8L of water from all sources: bodily breakdown of food, food we eat, beverages we drink. [D] More is needed on days when the temperature and humidity are high. When exercising, drink at least 1 glass of water before and afterwards, and then every 10-15 minutes during exercise.

Signs that tell us our bodies need fluid

• thirst
• fatigue
• loss of co-ordination
• irritability
• dry skin
• high body temperature
• fewer visits to the toilet.

Energy drinks can help maintain energy levels if you are exercising for 60 minutes or more, but the energy boost is only temporary. And, remember, many of these drinks contain large amounts of sugar and caffeine, which can be dehydrating.