Good nutrition and hydration are essential to help you perform at your best when exercising. For anyone who will be exercising for an extended period, it is important to plan what you eat and drink before, during and after exercise. This is especially important for anyone involved in sport where optimal nutrition and hydration can make a significant difference to your performance. For anyone participating in exercise at a more moderate level, a healthy diet that includes plenty of fruit, vegetables and water will usually suffice.
Fuel for exercise
The body’s main sources of fuel during exercise are fat and carbohydrate, and the one you need to focus on is carbohydrate. This is because we all have ample stores of fat to undertake even the longest bouts of exercise (unless in a state of starvation). However, our carbohydrate stores (in the form of muscle glycogen) are more limited and can become significantly depleted during vigorous exercise in excess of 90 minutes or moderate exercise of a longer duration (several hours).
Carbohydrate stores can also become depleted over the course of several exercise sessions, if not replenished through appropriate nutrition between times. Depleted muscle glycogen will impair your capacity to exercise, limiting how fast you can run, cycle or swim. This is clearly evident in some people at the end of a marathon, in which their muscles have become depleted of glycogen and they struggle to maintain a speed faster than a slow jog.
Fluid for exercise
Maintaining your body in a fully hydrated state is essential for both your health and performance when exercising. If you are dehydrated you will have a reduced blood volume and less fluid available to form sweat. Dehydration will reduce your capacity to deliver oxygen to your muscles and your ability to prevent your body from overheating, both of which will adversely affect your exercise performance. So it’s important to be fully hydrated when you start exercising, and to maintain a regular intake of fluids while exercising to prevent dehydration.
Food and fluid before exercise
In most circumstances, most of us will have enough stored fat and carbohydrate to fuel our next exercise session without needing to make special arrangements (as exemplified by those who walk or jog before breakfast). However, if you are undertaking a more prolonged or vigorous bout of exercise, you may wish to maximise your glycogen stores before you exercise, and you can do this by eating a meal between one and 4 hours beforehand. This allows enough time for the meal to be digested. Ideally, you should eat a small amount of food that is low in fibre for easy digestion.
To increase your body’s fuel supply, the meal should be predominantly carbohydrate — approximately 2 to 5 grams of carbohydrate per kilogram of bodyweight. Meals based around rice, pasta, bread or potatoes are often advocated by sports dietitians, and you’ll need to try different ones to see what suits you best. The meal should also be low in fat and protein, to minimise any gastrointestinal discomfort.
For most sports and types of exercise, it is recommended that you drink 400-600 mL of fluid one to 2 hours before the activity, and another 200-400 mL 15 minutes before exercising. Water is usually a suitable choice of fluid to drink before exercising.
Food and fluid during exercise
Fluid loss can impair performance and can affect your body’s ability to control its own temperature. If you are exercising for less than 60 minutes, you should drink approximately 200 mL of fluid every 15-20 minutes. Water is appropriate in this situation. In longer duration activities where there is a risk of glycogen depletion, such as more than 60 minutes of vigorous exercise, a sports drink containing glucose and electrolytes can be most effective. And for activities lasting several hours these sports drinks can be supplemented with energy bars.
Post-exercise food and fluid
After exercise it is important to restore your body’s fuel and fluid stores to normal levels. For most people this is easily achieved by following a healthy diet that includes plenty of fruit and vegetables, and plenty of non-alcoholic, non-caffeinated fluids.
Replacing fluid stores largely depends on how much fluid was lost during exercise. This can be calculated by comparing your pre- and post-exercise bodyweight. A simpler method is to check your urine. You need to drink sufficient fluid until you are passing clear, dilute urine.
The amount of fluid that you'll need to drink will depend upon how much you’ve sweated and the temperature of the environment. So on hot days after a vigorous bout of exercise, you may need to drink several litres. In doing so, be wary of consuming it in the form of sugary drinks, as you may take in more calories than you’ve burned off during the exercise. This may not be a problem if you are a highly trained sports person with a good body composition, but will be contrary to your goals if you are trying to lose weight.
To replenish your glycogen stores after exercising vigorously, you need to eat 1-1.5 grams of carbohydrate per kilogram of bodyweight within the first couple of hours after exercise. Ideally, this should be in the form of high GI foods, such as sports drinks, muffins or white bread. Over the 24 hours after exercise, a total of 7 to 10 grams of carbohydrate per kilogram of bodyweight should be ingested to maximise the glycogen stores again, thereby preparing you for your next bout of exercise.
Note of caution
People with diabetes, people with metabolic disorders and those on special diets should consult their specialist and dietician for advice on how to modify food and fluid intake in accordance with exercise.