Carbohydrate is a critical fuel source for the muscle and central nervous system. The availability of carbohydrate plays a key role in the performance of exercise lasting an hour or more. Therefore, sports nutrition guidelines promote carbohydrate intake before, during and after exercise to meet the fuel requirements of the activity.

Carbohydrate intakes - should they be described as percentages or grams?

Nutrition guidelines for the community express goals for carbohydrate intake in terms of the percentage of total energy they should provide - for example 55% of energy intake should come from carbohydrate. This general guideline is appropriate when muscle fuel needs are moderate - for example, for people who are undertaking less than an hour of exercise each day, or doing exercise that is of low intensity.

However, in situations where maximal glycogen storage is desirable or the athlete must meet the fuel bill of prolonged exercise sessions, carbohydrate needs are higher and more specific. Studies have determined the absolute requirement of the muscle for carbohydrate in these situations (see Table 1). However, some sports nutritionists have continued to describe the increased needs for carbohydrate in terms of energy ratios - for example, an endurance athlete should eat 70% of energy from carbohydrate.

In these specific situations it is best to set definite carbohydrate intake goals for athletes, scaled to their body size and therefore, their muscle mass. A guideline to consume 7-10 g of carbohydrate per kilogram body mass (BM) is not only tailored to the muscle's needs, but is more "user-friendly" for the athlete. An athlete can calculate a definite carbohydrate target in grams, and use food tables or information on food labels to plan meals to reach this goal. Even better, an athlete can see a sports dietitian for advice to further narrow this target range according to his/her specific situation, and have an individualised meal plan fitted to their needs.

Table 1

Why is it confusing to continue to use % energy targets for athletes with high-energy needs?

The amount of carbohydrate needed to fuel the gruelling training or competition schedules of many athletes is higher than the typical carbohydrate intakes of most people. And it may require an athlete to devote more of their total energy intake to 'fuel foods' to do so. Typically, it might require the athlete to earmark 60-70% of their energy intake for carbohydrate needs. However, in practice the total energy needs and the muscle fuel needs of an athlete are not always synchronised, so this energy ratio is not fixed. Some large athletes have very high energy requirements to cover the cost of growth as well as their training. For these athletes, total intakes of 800-1000 g of carbohydrate representing 8-10 grams per kilogram BM may be consumed from only 45% of their energy budget. On the other hand, athletes who are restricting their energy intake to stay lean may need to devote 70% of their energy budget to achieve a carbohydrate intake of even 6-7 grams per kilogram. Therefore, it is difficult and confusing to give carbohydrate intake guidelines on the basis of an energy ratio alone. It is not certain that this will guarantee the absolute amount of carbohydrate that the muscle needs.

Can you tell if an athlete is fuelling up well enough if they are eating more than 50% of their energy from carbohydrate?

Some people judge the fuel intake of an athlete to be 'deficient' or 'inadequate' based on the percentage of energy derived from carbohydrate. However, as in the situation described above, an athlete might be consuming a large intake of carbohydrate in grams, adequate in terms of their fuel requirements, but be judged to be following an inadequate carbohydrate intake from the perspective of energy ratio. On the other hand an athlete might be eating a very high proportion of his/her limited energy intake from carbohydrate foods, and still only eat a small number of grams of carbohydrate. So, percentage figures are not a good guide.

And although total amounts of carbohydrate in grams may be a better guide, they still must be regarded with some flexibility. In all areas of nutrition, judgments of adequacy or deficiency cannot be made from a single piece of evidence. This is particularly the case when the evidence comes from a food record or another type of self-reported dietary assessment. Athletes, just like sedentary people, are not necessarily accurate in describing their real food intake - and most often they underestimate or under-reported their true intake of food and nutrients.

Dietary assessments should be done by an expert such as a sports dietitian, who will work with an individual athlete to collect different sources of information related to their nutritional needs and eating patterns. Specific information about the athlete's training load and their ability to recover between sessions may help to fine-tune carbohydrate intake targets. It is important, particularly in terms of judging everyday carbohydrate intake, to regard guidelines as an approximation rather than a fixed rule. Generally, though, for athletes who have an important or increased need for carbohydrate needs, it is more reliable and practical to set guidelines in terms of a fixed amount of carbohydrate, rather than an energy percentage.

How can I check the amount of carbohydrate I eat each day?

The following chart provides information about the carbohydrate content of common foods.  Each food portion provides 50 g of carbohydrate. Use this information to plan a daily menu, or specific pre-event meals and post-exercise recovery meals to meet the carbohydrate intake targets provided in Table 1. These carbohydrate-rich foods should form the basis of meals and snacks, with other nutrient-rich foods added to round out the meal.

Food Portions Providing 50 g of Carbohydrate

(Source: Peak Performance: training and nutritional strategies for sport J. Hawley and L. Burke. Sydney: Allen & Unwin, 1998).