Endurance training can result in significant increases in daily energy requirements. If we take the example of an endurance runner who completes an average of 40 miles per week of training. They will consume approximately 4000 calories per week whilst training (you burn approximately 100 calories per mile), on top of their normal daily energy expenditure. Therefore they will need to consume at least an extra 4000 calories/week, in addition to their normal recommended daily energy requirements (typically 2-2,500 calories day dependent on body mass, %body fat, gender etc), in order to try to maintain muscle glycogen stores, body size, muscle mass and aid recovery between training sessions. Therefore they would need to consume approximately 2,600-3100 calories/day to match their energy expenditure.
Energy requirements of elite endurance athletes
Among top level elite endurance athletes the calorie requirements can be considerably higher, and greatly exceed the normal recommended dietary intake for healthy adults. An elite marathon runner completing around 100miles/week training would need an average daily calorie intake of approximately 3,500-4,000 calories day. Whereas it’s estimated that tour de France cyclists consume around 6500 calories a day without any increase in bodyweight – more than 2.5 x the normal recommended dietary intake of an equivalent sized adult. Under these conditions of high energy expenditure a the large calorie intake is required to replace the calories consumed during exercise, aid the repair and recovery of muscles and maintain bodyweight.
Nutritional requirements during competition and training
During endurance competition it is vital that enough calories are provided before during and after exercise. If too few calories are provided before, or during, exercise then performance will suffer as the reduced energy levels will lead to early fatigue. If too few calories are consumed immediately after exercise then the rate of recovery will be compromised and the athlete will suffer with reduced training and susbsequent racing performance, and will put themselves at risk of overtraining and illness.
During endurance training it is not always a requirement to consume calories prior to and during exercise especially if the goal of the training session is to enhance fat metabolism. In fact it’sknown that pre-exercise carbohydrate loading can reduce rates of fat metabolism and could compromise training aimed at enhancing fat metabolism. As such completing some training sessions in a semi-fasted state – normally achieved by training early in the morning without breakfast – has become an increasing popular training method for enhancing rates of fat metabolism. However it is important that athletes who use this type of training still maintain the same level of calorie intake and therefore calories not consumed prior to and during exercise training must be made up for after exercise. This training method is still controversial particularly as it can lead to increased rates of muscle breakdown during exercise and places greater levels of stress on the immune system.
Primary energy sources during endurance training
The primary energy sources during long distance events and training is the aerobic metabolism of fats and carbohydrates. At moderate exercise intensities (60 – 70% of VO2max, 65 – 75% HR max), the major energy source is through the metabolism of fats (in the form of free fatty-acids) which provide over 50% of the energy requirements. However, as exercise intensity increases the proportion of energy supplied by fat decreases and the proportion of energy supplied by carbohydrate increases. As exercise intensity increases to around 85% VO2max (around the intensity of the lactate threshold) carbohydrates become an increasingly important energy source, supplying more than half the energy requirements. It is important to note that the total amount of energy supplied by fats doesn’t change (the proportion decreases but the amount remains relatively constant), rather it is the amount of energy provided by carbohydrate that actually increases as exercise intensity increases. In addition to the metabolism of carbohydrates and fats there is a small amount of aerobic metabolism of proteins.
Fat stores, metabolism and requirements for endurance exercise
Under most exercise intensities but particularly during low-moderate intensity exercise, the metabolism of fats provides a significant amount of the total energy requirements. However, there is an almost inexhaustible amount of fat stores for use in metabolism, in even the leanest athlete. This is easy to understand when you consider that just 1kg of fat will provide enough energy to run around 3 marathons. The maintenance of fat staores is generally considered to be easilty met through most diets and therefore significantly increased dietary intakes of fats are not normally recommended for improving recovery from exercise or improving exercise performance. However, there is some recent evidence the consumption of a high fat meal prior to exercise (when muscle glycogen stores are already full) may be beneficial to prolonged endurance exercise due to increased levels of free fatty acids in the blood. For health reasons fat requirements are best met through the consumption of healthy fats including unprocessed vegetable oils (olive oil, sunflower oil), seeds rich in essential omega oils (pumpkin, sunflower, sesame etc) flax seed oil, oily fish (makeral, herring, sardines etc) and nuts (almonds, walnuts, brazil nuts).
Carbohydrate stores, metabolism and requirements for endurance exercise
Unlike the large amounts of energy stored in our body fat, we have relatively small stores of carbohydrate within our body. Following 60 minutes of intense aerobic exercise, muscle and liver glycogen (the body’s store of carbohydrate) can decrease by around 50%, and following 2 hours of exercise they may be almost fully depleted. Since, carbohydrates provide a large proportion of energy during moderate/intense physical activity, when carbohydrate stores are depleted the athlete will experience a significant increase in fatigue and performance will decrease significantly.
Another, key point to remember is that, carbohydrates are involved in the complete metabolism of fats, and therefore when carbohydrate stores are drained the level of fat metabolism is also decreased. It is this point at which carbohydrate stores drop significantly, and metabolism is compromised, that is described as hitting the wall, by marathon runners. Therefore, it is important to keep your stores of carbohydrate topped up before, during, and after exercise, so as to optimise endurance performance.
Because carbohydrate stores are relatively small, compared with fat stores, endurance athletes must constantly consume large quantities of carbohydrates (4-5g of carbohydrate per pound of body weight per day) to maintain adequate stores. This means that a 150lb endurance athlete must consume around 600g of carbohydrate per day in order to maintain adequate carbohydrate stores. This is better taken as a number of small meals throughout the day (4-6 small meals), rather than 3 large meals. By consuming smaller meals you will ensure that more of the carbohydrate is used to replaced muscle and liver glycogen stores, and less is converted to, and stored as body fat. It is particularly important that athletes consume adequate amounts of carbohydrate over the few days building up to a race. Many athletes also look to consume a high carbohydrate meal 3-4hours before competition to ensure glycogen levels remain full before competition.
Protein stores, metabolism and requirements for endurance exercise
The contribution of energy from protein metabolism is low during exercise of less than 1 hour (typically < 2%) but can rise significantly to ~ 5-10% as exercise duration increases and as glycogen stores become depleted. In order to metabolise protein the muscle must either utilise free amino acids within the blood or breakdown protein into amino acids. It’s known that the muscle can metabolize branched chain amino acids and alanine. This has led to the popularity of energy drinks containing small amounts of branched chain amino acids in the belief this may benefit aerobic metabolism, spare muscle glycogen levels, and help to reduce the rates of muscle breakdown when exercising.
Unlike fats, and carbohydrate, our bodies do not store protein for use specifically during aerobic metabolism – small amounts of protein are present as free amino acids in the blood and larger amounts are present within muscles. The human body consists of just over 40% muscle and approximately 20% of muscle mass is made up of protein – the human body typically contains around 5-6kg of muscle protein. During prolonged endurance exercise (e.g. > 1hour) or when muscle glycogen levels are depleted our bodies start to breakdown muscle protein for use during aerobic metabolism. The rate of muscle breakdown can be reduced by one of two ways: 1) maintenance of muscle glycogen levels through adequate carbohydrate intake before, during, and after exercise; 2) Consumption of protein (especially BCAAs or proteins high in BCAAs like whey protein) before, during and after exercise.
Endurance Nutrition Summary:
- Endurance training significantly increases the daily energy expenditure, which may be more than double normal values following the training volumes employed by elite athletes.
- Endurance athletes must ensure they consume adequate calories to match daily energy requirements so as replenish/maintain muscle and liver glycogen stores, minimize muscle protein breakdown, ensure adequate rates of recovery and reduce the risk of illness
- Ensuring adequate carbohydrate intake before and during competition will help to maximize endurance exercise performance.
- Our bodies contain large fat stores which can be metabolized during aerobic exercise. There is some evidence that consuming high fat meals prior to exercise may aid fat metabolism, providing that glycogen levels are maximal
- Protein metabolism contributes a small amount of aerobic energy. This can increase significantly during prolonged exercise or exercise in a glycogen depleted state.
- The consumption of BCAAs or protein with a high BCAA content can reduce the rates of muscle breakdown, enhance recovery and may aid muscle glycogen resynthesis rates.