Athletes are often confused about the extent to which nutrition can improve performance.
Nutrition is part of a balanced approach to athletic success.
Matching energy expenditure with increased energy consumption is crucial during strenuous training to prevent reduced training capacity and performance decline.
Elite athletes may have daily energy expenditures two to three times greater than untrained individuals.
This high energy expenditure may not be met by normal eating patterns, leading to nibbling among athletes.
Macronutrient intakes of well-trained runners show a mean weighted carbohydrate (CHO) intake of 48% of total energy consumption for both males and females.
Female athletes often have lower energy intakes than expected, with inadequate iron, zinc, vitamin B12, and calcium intakes.
The suggested optimum diet for endurance athletes is up to 70% CHO, 15% fat, and 15% protein.
However, the proportional contribution of CHO, fat, and protein may not differ significantly between athletes and non-athletes.
Carbohydrate Intake
The amount of dietary CHO should be based on the absolute amount consumed relative to the athlete’s body mass (BM), not just the proportion of total energy intake.
A lower percentage of dietary CHO from a higher-than-normal total energy intake can still provide adequate CHO.
Costill (1986) found that long-distance runners consumed a diet containing only 50% CHO, but their total energy intake was nearly 50% higher than expected, resulting in adequate CHO consumption (375g (\cdot) day–1 or 5.7g (\cdot)kg–1 body mass (\cdot)day–1).
Recent studies (Lamb et al. 1990; Simonsen et al. 1991; Sherman et al. 1993) suggest that athletes ingesting 5–6g CHO (\cdot) kg–1 body mass (\cdot) day–1 are likely not compromising their training capacity.
Acute supplementation with additional CHO elevates muscle glycogen stores and improves performance.
For rapid recovery, a CHO intake of 8–10g (\cdot) kg–1 body mass (\cdot)day–1 is recommended.
Muscle Glycogen Stores and CHO Loading
Untrained individuals on a mixed diet have a muscle glycogen content of around 80 mmol(\cdot) kg–1 wet weight muscle.
Endurance-trained individuals consuming a similar diet have a higher muscle glycogen content, approximately 125mmol (\cdot)kg–1 wet weight muscle.
After several days of a high-CHO diet (8 g (\cdot)kg–1 body mass) and reduced training, muscle glycogen can reach 175–200mmol (\cdot)kg–1 wet weight.
Trained athletes on a moderate- to high-CHO diet (approximately 6g CHO(\cdot) kg–1 body mass (\cdot)day–1) may not increase muscle glycogen as much as untrained individuals.
Muscle glycogen ‘supercompensation’ can occur daily in well-trained athletes consuming a moderate- to high-CHO diet.
Costill et al. (1981) reported no significant difference in muscle glycogen content when trained runners consumed either 525 or 650g CHO (\cdot) day–1, indicating that supercompensation is not increased by very large CHO quantities (>600 g (\cdot)day–1).
The ergogenic effect of CHO loading may be due to delaying fatigue from muscle glycogen depletion or slowing liver glycogen depletion by reducing the muscle’s demand for blood glucose.
Liver glycogen sparing depends on the rate of hepatic glycogenolysis, which is accelerated by high liver glycogen content after CHO loading.
Effect of Carbohydrate Loading on Running Performance
Studies show a positive relationship between pre-exercise muscle glycogen stores and endurance performance for both cycling and running.
Sherman et al. (1981) found that differences in pre-exercise muscle glycogen content did not influence half-marathon performance, and running times were slower with higher glycogen levels.
The absolute amount of muscle glycogen left at the end of the runs was similar regardless of the initial glycogen content.
Madsen et al. (1990) confirmed that higher starting muscle glycogen did not improve treadmill run time to exhaustion at 75–80% of maximal oxygen uptake (V . o2max.).
Muscle glycogen content was still relatively high at the point of exhaustion.
CHO loading may not benefit performance for athletes in moderate-intensity events lasting up to 90 min.
Elevating pre-exercise muscle glycogen may extend endurance time in events longer than 90 min.
Galbo et al. (1967) found that a high-CHO diet (77%) increased muscle glycogen content by about 150% and extended running times to exhaustion by approximately 66% compared to a low-CHO diet (10%).
Karlsson and Saltin (1971) reported that a high-CHO diet improved race times by about 6% during a 30-km event, with athletes maintaining a fast pace for longer despite higher starting glycogen.
Williams et al. (1992) observed that a high-CHO diet increased speed over the last 5 km of a 30-km treadmill time-trial and improved overall performance by approximately 2%.
Fluid and Energy Replacement During Distance Running
Pioneering studies in the 1920s highlighted the importance of CHO loading before and CHO ingestion during prolonged running, but these findings were initially ignored.
Early investigations also showed the importance of adequate fluid replacement during prolonged exercise in the heat.
The 1953 IAAF Handbook stated that refreshments should only be provided after 15 km, with no external refreshments allowed.
By 1967, refreshments were available after 11 km, but only water was provided by organizers.
Between 1960 and 1970, water replacement gained popularity over CHO replacement, driven by studies showing that the most dehydrated runners had the highest postrace rectal temperatures.
The IAAF altered rules in 1977 to allow earlier and more frequent water ingestion.
The question of water vs. CHO replacement was not resolved until the late 1970s and early 1980s, when commercial interests revived research into CHO ingestion.
Studies confirmed that CHO-containing solutions enhanced performance and endurance during prolonged exercise.
Today, CHO–electrolyte beverages are advocated by the IAAF in all races of 10km and longer, but optimal amounts for fluid, CHO, and electrolyte replacement are still under investigation.
Fluid Loss and Replacement
Fluid loss depends on sweat rate, which is proportional to metabolic rate and ambient temperature.
A 1960s study reported low fluid intake (150ml (\cdot)h–1) during a 32-m race, leading to 2.4kg weight losses and elevated rectal temperatures (Wyndham & Strydom 1969).
It was recommended that runners drink ‘at least 900ml of fluid per hour’ to avoid heat stroke.
Modern studies show runners voluntarily consume around 500 ml(\cdot) h–1 during distance races.
Sweat rates are typically around 1.0–1.2l(\cdot) h–1 during events lasting 2h or more.
Runners may develop symptoms of ‘fullness’ when trying to drink fluid at high rates, possibly due to limited fluid absorption.
Duodenal and jejunal perfusion studies show a maximum water absorption rate of about 0.8l (\cdot)h–1 from isotonic glucose solutions (Davies et al. 1980).
Not all ingested fluid appears in extracellular or intracellular fluid pools, leading to ‘involuntary’ dehydration (Noakes 1993).
Sodium chloride (NaCl) losses in sweat may attenuate the rise in serum osmolality during exercise-induced dehydration, reducing dipsogenic drive.
Whether drinking plain water or NaCl solutions, humans tend to stop drinking before fully rehydrated.
Rapid alleviation of symptoms like mouth dryness may also cause premature cessation of drinking.
Carbohydrate Ingestion and Oxidation During Exercise
High CHO concentrations (>15g per 100ml) may impair intestinal fluid absorption.
Inadequate CHO ingestion limits CHO oxidation rates late in exercise, impairing performance.
Gastric volume, solute energy content, and osmolality are critical determinants of gastric emptying during exercise.
The maximum rate of CHO and water delivery to the intestine depends on the average fluid volume in the stomach, governed by the athlete's drinking pattern.
The amount of solution emptied from the stomach is at least double the amount oxidized by active muscles.
Peak rates of ingested CHO oxidation rise to approximately 1g (\cdot)min–1 after 70–90min of exercise.
Performance improvement coincides with a faster running pace during the latter stages of a race.
Additional CHO helps athletes resist fatigue and maintain a given pace for longer.
CHO ingestion during steady-state running results in muscle glycogen sparing, particularly in type I fibres.
Conclusions and Recommendations for Optimal Fluid Replacement
Improve performance by limiting dehydration-induced decreases in plasma volume and skin blood flow.
Limit any rise in serum sodium osmolality or serum osmolality.
Diminish progressive rises in rectal temperature.
Decrease the subjective perception of effort.
Supplement endogenous CHO stores.
The exact solution composition for optimal electrolyte and fluid replacement is not fully established.
Fluid ingestion rates needed to match high sweat rates (>1l(\cdot)h–1) may exceed maximal intestinal absorptive capacity.
Maximize fluid consumption by ensuring the drink is cool and palatable, and by adding electrolytes, especially sodium.
CHO ingestion is recommended for exercises of sufficient duration or intensity to deplete endogenous CHO stores. With the exception of fructose:
The type of CHO consumed does not greatly influence the rate of gastric emptying.
There are no significant differences in CHO oxidation rates from various mono-, di-, and oligosaccharides.
All ingested CHOs are oxidized at a maximum rate of approximately 1 g (\cdot)min–1 after the first 70–90min of exercise.
The prevailing concentrations of glucose and insulin set the upper limit for glucose uptake and oxidation by skeletal muscle.
Practical Guidelines for Prolonged Exercise (up to 6 hours)
Immediately before exercise or during warm-up, ingest up to 5 ml (\cdot) kg–1 of body mass of cool, flavored water.
For the first 60–75 min, ingest 100–150ml of a cool, dilute (3.0–5.0g per 100 ml) glucose polymer solution at regular intervals (10–15min). Consume no more than 30 g CHO during this period.
After about 90 min, increase the concentration of the ingested solution to 7–10g per 100 ml, adding 20 mEq(\cdot)l–1 of sodium. Small amounts of potassium (2–4 mEq(\cdot)l–1) may also be included.
Consume 100–150ml of this solution at regular (10–15min) intervals for the remainder of the race.
This regimen ensures optimal fluid and energy delivery, limiting dehydration and maintaining CHO oxidation at approximately 1g (\cdot)min–1 late in exercise.