Nutrition and athletic performance in the athlete without diabetes
Daily macronutrient recommendations for the athlete
Table 1 Guidelines for daily carbohydrate intake
|Very light training (low intensity exercise or skill-based exercise)
|Moderate intensity exercise for 1 hour per day
|Moderate to high intensity exercise for 1-3 hours per day
|Moderate to high intensity exercise for 4-5 hours per day
|³ 10-12 g.kg-1.day-1
Adapted from Burke, 2007 (2
Carbohydrate and fluid recommendations during training and competition
The primary goals for nutrient intake during training and competition are to replace fluid losses and provide CHO for maintenance of blood glucose levels. The choice of foods and fluids to be consumed during exercise is influenced by various factors, including the intensity and duration of the exercise, climatic conditions, the pre-exercise nutritional status, and the physiological characteristics of the athlete. Table 2 provides a summary of current recommendations.
Table 2 CHO and fluid recommendations for athletes during exercise
|Duration of event
|CHO and Fluid Recommendations
|Events of 30-60 minutes
|- Begin exercise well-hydrated
- Drink at a rate that is comfortable and practical to replace most of the fluid lost by sweating or replace fluid losses between sessions for shorter duration exercise
- CHO intake is beneficial for the performance of high-intensity exercise of about 1 hour duration (7)
|Events of 1-3 hours
|- Primary goal is fluid replacement + CHO intake
- 30-60g CHO per hour of exercise
- Drink at a rate that is comfortable and practical to replace most of the fluid lost by sweating
|Events of greater than 3 hours
|- Primary goal is fluid replacement + CHO + sodium intake
- 30-60g CHO per hour of exercise; when greater than 70g CHO per hour are required use a mixture of CHO sources (i.e. 2:1 ratio of glucose and fructose) as higher rates of CHO oxidation are possible (8)
- Drink at a rate that is comfortable and practical to replace most of the fluid lost by sweating
Adapted from Burke, 2006 (9)
Strategies to promote carbohydrate loading
Carbohydrate loading, also known as glycogen loading is often used by endurance athletes before competition to increase muscle glycogen stores with the goal of improving performance by delaying fatigue (10, 11). The performance benefits of CHO-loading for events lasting longer than 90 minutes are widely accepted. Typically a CHO loading regimen involves 3 days of a CHO intake between 8-12 g.kg-1.day-1, which may contribute 70-85% of total energy intake (12). Various carbohydrate loading strategies exist: Sherman et al (10) showed that muscle glycogen stores could almost double by undergoing a 3-day exercise taper, and ingesting a high CHO diet (70% of total energy intake) for the 3 days prior to the event with 20 minutes of low intensity exercise for the first 2 days and no exercise on the day before the event. Bussau et al (13) showed that increases in muscle glycogen stores of 90% can be achieved after only 1 day of inactivity along with ingesting a high CHO and high glycaemic index diet (10 g.kg-1.day-1). Studies with 1-day loading protocols have used high-GI foods; therefore the glycaemic index may be of relevance when rapid glycogen restoration is required (13, 14). There is some evidence to suggest that the increase in muscle glycogen is greater when CHO loading takes place after glycogen depleting exercise compared to lower intensity exercise (15).
Carbohydrate loading clearly leads to improved performance in endurance athletes, especially in those who habitually consume less than 8g CHO g.kg-1.day-1 but it may not indicative of improved performance in all groups of athletes
The impact of Type 1 Diabetes on current nutrition recommendations for athletes
No specific daily macro nutrient recommendations exist for athletes with Type 1 diabetes. McKewen et al (16) compared the effects of two diets differing in percentage CHO (50% and 60% CHO of total energy intake) in a group of trained males with type 1 diabetes. An increased carbohydrate intake for three weeks in this group was associated with deterioration in glycaemic control, increased insulin requirements, decreased muscle glycogen and reduced exercise performance. The authors hypothesized that increased insulin levels could inhibit liver and adipose lipolysis, leading to a greater oxidation of glucose within the tissues. It is worth noting that subjects in this study were on older insulins. Whether skills learnt from programs such as DAFNE (matching insulin dose to carbohydrate intake) and other forms of treatment e.g. CSII and MDI (using rapid-acting analogue insulins) will give similar results is not known. This subject area clearly warrants further investigation.
CHO requirements during exercise and the Diabetic Athlete
As noted in table 2 a general recommendation for CHO requirements during exercise is 30-60g of CHO per hour of exercise. CHO requirement during exercise can also be based on body mass. Intense exercise at the time of peak insulin activity is associated with glucose disposal into muscle of approximately 1 g glucose.kg-1.hour-1 of exercise (17). Fuel needs during exercise is determined by the type, duration and intensity of exercise. For more specific estimations of CHO requirements for different types of exercise of differing intensity, standardized tables have been developed (18).
While the above recommendations serve as a guide for determining CHO needs there are various other factors that need to be taken into account when tailoring advice to an individual with diabetes:1) Form of treatment: CSII gives the athlete a greater degree of flexibility for making basal rate adjustments before, during and after exercise, unlike other regimes i.e. MDI and twice daily insulin regimes, therefore CHO needs during exercise are likely to differ. 2) The timing of exercise relative to an insulin bolus: if the exercise is within 90-120 minutes of an insulin bolus the athlete is able to adjust the meal insulin bolus and consequently CHO needs during the exercise will be reduced, otherwise exercising within 2 hours of a bolus may require taking more CHO than the recommended 30-60g her hour. Similarly, CHO needs when exercising in a fasted state will be less than when exercising at other times during the day. 3) Blood glucose when commencing exercise: if the blood glucose is below 7 mmol/l a 15-30g CHO snack may be required; in contrast to when the blood glucose is 10 mmol/l or above CHO feeding during exercise should be delayed. 4) The effect of antecedent hypoglycaemia and prolonged moderate exercise: Both these factors have been shown to cause similar blunting effects on counterregulatory responses during subsequent exercise thereby making the athlete more susceptible to hypoglycaemia. Davis et al (19) found that nearly threefold greater exogenous glucose infusion rates were required to preserve euglycaemia during exercise following a day of antecedent hypoglycaemia as compared to a day without hypoglyaemia. 5) Type of exercise: High intensity (e.g. sprints) and intermittent high intensity exercise (e.g. football) may result in hyperglycaemia during and/or after the exercise; therefore CHO feeding during exercise may likely exacerbate the problem. General CHO recommendations for athletes may therefore need adapting for these activities. The risk of delayed post-exercise hypoglycaemia is a reality due to replenishment of glycogen stores and is therefore something that the athlete needs to be aware of so that precautionary measures are taken.
It is common practice for athletes with diabetes who are prone to hypoglycaemia during exercise to preload with CHO to ensure that blood glucose is sufficiently raised at the start of exercise. Exercise in a hyperglycaemic state poses problems with regards to performance, but is also associated with a shift towards CHO oxidation as the main fuel source compared to when exercising in euglycaemia (20).The timing of CHO intake also effects fuel metabolism. In non-diabetic athletes when CHOs are consumed before exercise there is a greater reduction in the availability of fatty acids for oxidation compared to when CHOs are taken just before or 20-30 minutes after the start of the exercise (21). In the Diabetic athlete a pre-exercise blood glucose result which is below target often dictates the need for a CHO snack. It may therefore be advantageous for the athlete with diabetes to aim to commence exercise with a blood glucose of 6-10 mmol/l and delay CHO feeding to 20 minutes after commencing exercise to optimize fat oxidation.
Carbohydrate loading and the Type 1 Diabetic athlete
Tapering in training prior to an event is likely to result in significant increases in insulin requirements and therefore a challenge to maintain euglycaemia. This anecdotal finding along with lack of evidence for CHO loading practices in the diabetic athlete suggests that CHO loading should be discouraged. The diabetic athlete engaging in daily moderate intensity training should preferably aim to have a fairly consistent CHO intake of at least 7 g CHO kg-1.d-1. Daily management strategies for the Type 1 Diabetic athlete
In Table 3 is a summary of strategies, some of which have been discussed above, to prevent hypoglycaemia in the diabetic athlete.
Table 3 Strategies to prevent hypoglycaemia associated with exercise
|Reducing pre-exercise bolus (preferably when exercise is within 90-120 minutes of a bolus)
|Reduced hypoglycaemia during exercise; reduces CHO requirement; beneficial for weight management
|Requires planning; not helpful for spontaneous or late postprandial exercise; may result in starting exercise with raised blood glucose
|Adjusting pre-exercise and during exercise basal rate (relevant to CSII)
|Requires planning as basal rate adjustments may need to be made at least 60 minutes prior to the start of exercise
|CHO feeding during exercise
|Useful for unplanned or prolonged exercise
|Counter-productive when purpose of exercise is for weight control; not practical with all sports; gastrointestinal discomfort
|Pre-exercise or post-exercise sprint
|Reduces immediate post-exercise hypoglycaemia
|Effect limited to shorter and less intense exercise, no effect on hypoglycaemia during exercise
|Reducing basal insulin post-exercise (possible with CSII and MDI)
|Reduces nocturnal hypoglycaemia
|May cause raised fasting BG
|Taking caffeine before exercise
|??, reduced hypoglycaemia during and after exercise; reduced CHO requirements
|Impairments or alterations of fine motor control and technique, and overarousal (interfering with recovery and sleep patterns)
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Energy and macronutrient needs, especially carbohydrate (CHO) and protein, must be met to maximize training effects and maintain health. Current guidelines recommend 5 to 12 g of CHO per kilogram body mass per day (1). Table 1 provides a summary of daily CHO recommendations for athletes based on the intensity and duration of training (2). It is preferably to express CHO and protein requirements per kilogram body mass instead of using specific proportions (e.g. 60% of energy intake) (3). Protein recommendations for endurance athletes range from 1.2 to 1.4 g.kg-1.day-1 and strength athletes from 1.2 to 1.7 g.kg-1.day-1 (4). The timing of protein intake also needs considering, as it has been shown that protein or amino acids consumed near strength and endurance exercise can enhance maintenance of and net gains in skeletal muscle (5). Unlike recommendations for carbohydrate and protein, there are no reference values for fat intake in sport. The Food Pyramid for Swiss Athletes, which can be easily replicated for other nationalities, is a practical tool to translate energy and nutrient recommendations for athletes into food choices (6).