Studies of the dietary practices of athletes report that nutritional supplements are commonly used. Supplementation practices vary between sports and individual athletes; however, there is evidence that at least some athletes use a large number of supplements concurrently, often in doses that are very high in comparison with normal dietary intakes. In exploring supplementation practices we propose a classification system separating the supplements into dietary supplements and nutritional erogogenic aids. The dietary supplement is characterised as a product which can be used to address physiological or nutritional issues arising in sport. It may provide a convenient or practical means of consuming special nutrient requirements for exercise, or it may be used to prevent/reverse nutritional deficiencies that commonly occur among athletes. The basis of the dietary supplement is an understanding of nutritional requirements and physiological effects of exercise. When the supplement is used to successfully meet a physiological/nutritional goal arising in sport it may be demonstrated to improve sports performance. While there is some interest in refining the composition or formulation of some dietary supplements, the real interest belongs to the use or application of the supplement; i.e. educating athletes to understand and achieve their nutritional needs in a specific sports situation The sports drink (carbohydrate-electrolyte replacement drink) is a well known example of a dietary supplement. Scientific attitudes towards the sports drink have changed over the past 20 years. Initial caution that carbohydrate-electrolyte fluids compromise gastric emptying during exercise has now been shown to be unjustified. Numerous studies have shown that 5 to 10% solutions of glucose, glucose polymers (maltodextrins) and other simple sugars all have suitable gastric emptying characteristics for the delivery of fluid and moderate amounts of carbohydrate substrate. The optimal concentration of electrolytes, particularly sodium, remains unknown. Most currently available sports drinks provide a low level of sodium (10 to 25 mmol/L) in recognition that sodium intake may promote intestinal absorption of fluid as well as assist in rehydration. The sodium level of commercial oral rehydration fluids (used in the clinical treatment of diarrhoea and dehydration) is higher than that of the present range of sports drinks. However, even if research indicates that intestinal glucose transport is optimally stimulated at higher sodium concentrations, concern for the palatability of sports drinks may impose a lower ceiling for sodium levels. Commercial viability of a sports drink requires that it provide a refreshing and palatable fluid replacement across a wide variety of sports and exercise situations. However, in specific situations, the appropriate use of a drink may offer special advantages in meeting physiological requirements. For example, during endurance and ultraendurance exercise, suitable intake of a sports drink to preserve hydration and to supply additional carbohydrate substrate for glycogen-depleted muscles has been shown to enhance performance. Additionally, the sports drink may provide a special advantage in recovery and rehydration after prolonged exercise, particularly where more rapid restoration of body fluid and fuel levels will enhance performance in future exercise bouts. High carbohydrate liquids at higher concentrations (20 to 25% carbohydrate solutions, typically from maltodextrin sources) represent another type of dietary supplement. These high carbohydrate supplements, often known as 'carbo-loader' supplements, have been manufactured to assist athletes in achieving a high carbohydrate intake by overcoming problems of dietary bulk or lack of nutritional knowledge. They may be useful in situations where a short term need for carbohydrate supersedes other nutritional goals, e.g. carbohydrate loading and prolonged competition over successive days. Where long term nutritional goals and requirements for a combination of nutrients need to be addressed, liquid meal supplements provide a low-bulk, nutritionally complete choice. Liquid meal supplements may be useful as a compact nutrient-dense high energy supplement for athletes with high energy requirements, particularly when it is not practical or comfortable to consume solid foods. They may be useful in preparation for exercise. both as a pre-event meal and as a low residue preparation. Athletes who consume low energy intakes and/or restricted food variety may consume sub-optimal intakes of micronutrients, particularly minerals such as iron or calcium. Adequate vitamin and mineral status is a requirement for optimal sports performance, since many of the micronutrients play a key role in exercise metabolism, recovery and adaptation. A heavy exercise programme may increase requirements for some nutrients, including the B-group vitamins and iron. Athletes at high risk of inadequate micronutrient intake or nutritional deficiencies such as iron deficiency should benefit from nutritional counselling. Supplementation may be required in cases of ongoing suboptimal dietary intake, or to rapidly restore a diagnosed nutrient deficiency state. While many athletes regard iron supplements as a low cost 'safety net' against iron deficiency, there are many disadvantages to unsupervised long term intake of high doses of iron. The debate surrounding biochemical and haematological markers of optimal iron status remains unresolved. In general. screening and individual treatment of athletes in high-risk groups is preferred to a mass treatment programme. There is a need for nutrition education of athletes to explain the appropriate use of these dietary supplements. In many cases, the information is specific to the individual athlete or sports situation and may require one-to-one counselling. In most situations, the use of the supplement will simply be a part of a larger plan of optimal sports nutrition or the clinical management of a nutritional disorder. Effective education will not only ensure that dietary supplements are used correctly, but will highlight the importance of optimal sports nutrition.
