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An important dietary energy need is for adequate carbohydrate (CHO) to meet the fuel requirements of the training programme. The energy requirements for exercise are met largely by oxidation of fat and carbohydrate. Whereas body fat stores are adequate to supply the energy cost of exercise for may days, CHO stores are vulnerable. With prolonged training each day the body may turn over in excess of its total body CHO stores. Furthermore, there is an obligatory need for CHO oxidation when the intensity of exercise is high (e.g. >70% V02 max). Therefore, daily replacement and supplementation of CHO intake may be needed to provide the fuel needs for a strenuous training programme, particularly to maintain the intensity of training sessions. Inadequate CHO intake will gradually lead to a depletion of muscle glycogen stores and may reduce the ability of the athlete to complete their desired training load.

It is worth noting that many athletes do not appear to optimize fluid and carbohydrate intake strategies during training sessions as well as they do in the competition setting. Sometimes this is simply due to practical limitations: during competitions, access to fluid and CHO is improved due to the provision of aid stations, or to the scheduled breaks which allow intake. However there is some evidence that athletes may not rate attention to these needs as highly during training. In some cases, the erroneous (and dangerous) belief that an athlete may become ‘tougher’ by exposing themselves to dehydration during training still persists. It is important that the athlete optimizes their performance during training by attending to fluid and CHO needs as well as possible. Furthermore, the training situation provides an opportunity to perfect competition intake strategies. This might include assessing the extent of fluid losses during exercise and learning to tolerate fluid intake while exercising, or assessing the optimal amount to carbohydrate that may be needed during a prolonged exercise event. 

Preparation for competition should aim to match the body’s carbohydrate stores of liver and muscle glycogen to the anticipated fuel needs of the event. Normalized glycogen stores can be achieved by a high carbohydrate intake, in conjunction with a reduction in exercise volume and intensity for the 24-36 hours pre-event. This is considered sufficient for most sports events, particularly events lasting less than 60 minutes. Athletes who compete in events longer than this (particularly events longer than 90 minutes) may try to maximize their muscle glycogen stores by undertaking a exercise-diet programme known as glycogen (or carbohydrate) loading. The original CHO loading protocol, as described by Scandinavian researchers in the late 1960s, used extremes of diet and exercise to first deplete the supercompensate glycogen stores.

Recent work has demonstrated that trained athletes do not need to undertake the severe depletion phase to subsequently achieve an increase in glycogen stores. Instead they need only to taper their training and ensure a high (8-10g/kg BM/day) carbohydrate intake over the 72 hours prior to and event to achieve similar increases in muscle glycogen to those reported by the more extreme regimens. Some studies have reported that athletes do not have sufficient practical nutrition knowledge to achieve such carbohydrate intakes and may require dietary counseling.  The pre-event meal offers a last chance to fine-tune fluid and fuel levels prior to the event, as well as to ensure gastrointestinal comfort.