Use diet to keep your energy levels up while exercising, writes GILES WARRINGTON.
WHETHER YOU are physically active or a serious athlete, the primary role of nutrition is to optimise health and physiological function during training. Although the importance of good nutrition and its relationship with athletic performance is widely accepted in the sporting community, many aspects are still misunderstood.
For those involved in sport and physical activity, the primary functions of a balanced diet include: maintaining health and reducing illness; delaying fatigue; accelerating recovery; regulating weight and body composition; and ultimately enhancing work capacity and athletic performance.
During exercise, carbohydrate and fat are the key sources of energy fuelling the muscles. Carbohydrate is the body’s preferential fuel for high-intensity work such as running and repeated sprints. An increase in exercise intensity will elevate the contribution of carbohydrate to total energy supply.
Fat contributes to energy supply over a range of activities and intensities, but is the predominant fuel used during low-intensity exercise such as walking or jogging.
While the absolute rate of energy provided by fat remains about the same, the overall proportion of energy derived from fat decreases as the exercise intensity increases due to the elevated energy demand being provided by carbohydrate.
Protein contributes to energy supply at rest and during exercise but under normal circumstances this contribution is small and probably no more than about 5 per cent of total energy demand.
Carbohydrate is stored as glycogen in the muscles and liver and as glucose in the blood.
While it is the fuel of choice during exercise, its supply is only limited and must be replenished on a daily basis.
Replenishment can be achieved by consuming a high- carbohydrate diet. In contrast, fat stores in the body are virtually unlimited – most people have enough fat stores to run about 50 marathons only at a low intensity!
Daily energy requirement of athletic populations will depend on a number of factors including individual size, activity levels and exercise type. Recommended carbohydrate levels typically range between 6-10g per kg body weight per day.
Although protein requirements are slightly elevated in active people, contrary to common belief, they are not excessive. For endurance athletes, protein recommendations are 1.2-1.4g per kg per day and for strength-trained athletes 1.6-1.7g per kg per day, which can be generally met through a balanced diet.
Diet can play an important role in sports performance and recovery from training and competition. Therefore, the timing of nutritional intake is critical.
A pre-exercise meal or snack should be relatively high in carbohydrate, low in fat and moderate in protein and based on foods which are familiar to the athlete and easily digested. This meal should be usually consumed two-three hours before training or competition. Foods which cause gastroinstestinal distress should be avoided.
The main function of nutrient consumption during exercise is to maintain energy levels. However, so long as energy stores are topped up before, this is probably necessary during only prolonged continuous activities lasting more than 90 minutes or high-intensity intermittent events of over 60 minutes duration.
This may be achieved by taking an isotonic sports drink (containing 4-8 per cent carbohydrate), which is easily consumed, at appropriate intervals or during breaks in play.
Depletion of carbohydrate (glycogen) stores is one of the key factors which leads to muscle fatigue during exercise. It is therefore essential to start to replenish depleted energy stores within the first 30-60 minutes after exercise.
This is referred to as the “window of opportunity” and is the time when the muscles most rapidly absorb nutrients. This will help guarantee adequate energy stores are available for your next training session.
During this critical window it is recommended to consume about 1g per kg of body weight of carbohydrate focusing on high glycemic index (GI) food which are most rapidly absorbed.
The addition of a small amount of protein (0.2g per kg body weight) combined with the carbohydrates will enhance post-exercise recovery. This will typically equate to about 50-70g of carbohydrate and 10-20g of protein taken immediately after exercise. This can be in either liquid and/or solid form.
Suitable carbohydrate snacks are two medium bananas, or three slices of bread with jam/honey, or one large banana and 500ml of isotonic sports drink.
Good sources of protein include low-fat yogurt, a tuna sandwich, smoothie or low-fat milkshake. Ideally, this post-exercise snack should be followed within two hours by a high-carbohydrate meal.
Next week we will look at the role of fluid intake for sport and exercise and the importance of maintaining good hydration levels.
- Dr Giles Warrington is a sport and exercise physiologist and lecturer in the School of Health and Human Performance at Dublin City University (DCU)