The Ironman triathlon is a unique type of ultraendurance race in which performers participate in multiple events (swimming, cycling, and running) that can take up to 14 hours to complete . It is a very demanding event that requires considerable preparation, including specific training and equally important nutritional practice. With regard to nutritional preparation, it is important to consider the physiologic demands of such an event in order to calculate the required energy intake for energy balance and to align dietary strategy with energy substrate usage. To achieve this, triathletes need to have a structured, regimented lifestyle that often consists of specific routine and nutritional preferences, in which dietary change is sometimes resisted . Jeukendrup et al.  reviewed nutritional considerations in triathlons; this adds to the current body of knowledge by focusing on and critically evaluating the published literature of the past few years.
Glycogen Stores and Supercompensation
It is acknowledged that as reduced blood glucose concentrations  and glycogen depletion  during endurance exercise have been associated with fatigue, it is important for a triathlete to start such an event with high muscle and liver glycogen stores. The only research associated with this particular area of nutritional preparation in the past year is that of Arnall et al. , who tested whether muscle glycogen remained elevated 3, 5, or 7 days after supercompensation. Prior to this, the general recommendation for glycogen supercompensation was based on research of those such as Bussau et al. , who found that supercompensated muscle glycogen stores can be achieved within 24 hours after a day of physical inactivity and 1 day of high carbohydrate (CHO) intake (10.5 g of CHO/kg body weight [BW]/d). As Jeukendrup et al.  highlighted, this finding is of importance as it allows triathletes to follow their structured training regimes until the day before competition. However, triathletes are likely to taper down in the weeks prior to competition and therefore the findings of Arnall et al.  are considered important. The trained male cyclists used in this study carried out one bout of exhaustive exercise to deplete muscle glycogen followed by a 3-day consumption of a high-CHO, low-protein, low-fat diet. The cyclists were then randomly assigned to either 3-, 5-, or 7-day postloading maintenance diet of 60% CHO with limited physical activity. It was found that supercompensated glycogen levels remained higher than normal for up to 5 days after CHO loading. It was concluded that the CHO consumed, level of physical activity, and the magnitude of the glycogen supercompensation determine the interval during which the glycogen levels remain elevated.
Concern over injury or gastrointestinal-related illness after strenuous glycogen-depleting exercise and subsequent compensation periods has been highlighted . However, following the design set by Arnall et al. , there should be less trepidation as cyclists had only one session of glycogen depletion followed by a minimum of 6 days and maximum of 10 days glycogen compensation thereafter.
Although it is important that glycogen stores are maximized before competition to prevent fatigue, there is evidence that this may not be the only factor limiting endurance performance. Jeukendrup et al.  discussed findings of unpublished data reported by Noakes  in which a simulated Ironman triathlon model was used to predict that after 4.5 hours of cycling at 71% VO2max, a triathlete would begin the running component with very low or absent whole body CHO stores (based on predicted maximum body stores of CHO and fat). The triathlete would then be required to run at a speed of approximately 16 km/h for a further 160 minutes, which according to Noakes  represents an intensity of greater than 66% VO2max. This suggests that it is unlikely that muscle glycogen depletion is the single limiting factor for ultraendurance performance. Other studies have shown that total CHO oxidation during prolonged exercise of up to 6 hours in duration exceeds the estimated CHO stores in liver and muscle by up to 100% . This suggests that other sources of fuel are required for oxidation in such events.
Research carried out on a group of endurance-trained athletes found that maximal fat contribution was elicited at an intensity of 63% VO2max . Based on the assumption Noakes  made about the intensity of the last leg of an Ironman triathlon (> 66% VO2max), this predicates that fat may make a considerable contribution to overall fuel use at this particular stage of an Ironman triathlon. In addition, Jeukendrup et al.  postulated that long-distance triathletes may have an increased capacity to oxidize fat.
High-fat Dietary Manipulation and CHO Loading
Humans can increase skeletal muscle fatty acid oxidative capacity through endurance training and by consuming a high-fat diet . Robins et al.  found that a 2-week high-fat diet enabled ultraendurance athletes (transatlantic rowers) to row faster and cover a greater distance in a simulated race environment (at 60% VO2max) despite having lower respiratory exchange ratios (RERs) and heart rates. The findings suggest that fat utilization was effectively manipulated through a 2-week dietary intervention. This corroborates earlier research  that found that a 4-week high-fat diet significantly lowered RER values and increased time to exhaustion (at 80%VO2max) and a 5-day high-fat diet significantly altered substrate utilization from CHO to fats during exercise at 70% VO2peak . This suggests that a high-fat diet prior to a period of carbohydrate loading may be good practice and along with exercise will train the muscle to utilize a greater percentage of fats during the race while preserving CHO stores. Lambert et al.  found that a 10-day high-fat diet prior to CHO loading increased reliance on fat and decreased reliance on muscle glycogen and improved time trial performance after prolonged exercise at 70% VO2peak. However, Havemann et al.  found that a 6-day high-fat diet followed by 1 day of CHO loading increased fat oxidation during a 100-mile cycling time trial, but compromised 1-km high-intensity sprint performance when compared with an isoenergetic 6-day high-CHO diet. This particular finding contributed to a critical examination of strategies involving fat loading  by noting that during “breakaway” or surges the athlete is working at higher intensities than expected. As a result of this, Burke and Kiens  believe that sports nutritionists should delete fat-loading and high-fat diets from the list of ergogenic aids for endurance and ultraendurance sports. Although there is merit in this observation, such comments fail to consider earlier evidence supporting high-fat dietary manipulation and subsequent gains in endurance and ultraendurance performance in which short bursts of high-intensity exercise are not required. Further research is necessary in order to investigate such strategies with differing methodologies before discarding high-fat dietary manipulation for improved endurance and ultraendurance performance.
CHO Loading Prior to Exercise
Glycogen stores can be partially depleted following an overnight fast, making it important for the triathlete to replenish these stores prior to a race. Some athletes do not tolerate CHO and fluid consumption during the race, so the prerace meal may be the last opportunity to consume CHO without concern over ill effects during the race, such as gastrointestinal distress. During an Ironman triathlon, endogenous fuel stores were estimated to supply over half of the energy expended , providing further support for CHO-loading to maximize endogenous fuel stores. One must also consider that there may be limited ability to consume adequate CHO and fluid during the race.
CHO 3 to 4 hours before exercise
CHO consumption 3 to 4 hours before exercise can enhance endurance performance  and rate of CHO oxidation during exercise . O'Toole et al.  found an Ironman triathlete could yield an energy expenditure (EE) of between 8500 and 11,500 kcal compared with an energy intake observed by Kimber et al.  of 3115 kcal (range, 1298–4235 kcal) in female and 3940 kcal (range, 2843–5269 kcal) in male triathletes. Thus, CHO consumption during the race may account for only 27% to 36% of EE for females and 34% to 46% of EE for males. It is recommended that triathletes consume 200 to 300 g of CHO 3 to 4 hours prior to a race .
CHO intake less than 1 hour before exercise
Controversy over detrimental effects of CHO consumption in the hour preceding the race remains based on reports by Foster et al.  and Keller and Schwarzkopf . The concern relates to rebound hypoglycemia, in which a rapid increase in blood glucose and subsequent insulin secretion occurs, followed by a rapid decrease in blood glucose 15 to 30 minutes after the start of exercise. In addition, hyperinsulinemia will inhibit lipolysis and the subsequent availability of fatty acids for oxidation. Although in theory this can be detrimental, only Foster et al.  and Keller and Schwarzkopf  have found reduced endurance performance, whereas others have found no change [24–26] or increased performance [27–29]. There is evidence to support the fact that rapid declines in blood glucose may not interrupt functional capacity. Although an hour may be sufficient time for blood glucose to normalize, the effects of hyperinsulinemia on fatty acid oxidation may last 3 to 5 hours . During the early stages of ultraendurance performance, the type of CHO consumed must be considered to prevent the detrimental effects of hyperinsulinemia on overall fuel utilization. The consumption of foods and/or fluids with low glycemic indices can reduce potential metabolic disturbances as a result of lower and more sustained blood glucose and insulin responses that occur postprandially and during subsequent exercise . Implementation of foods with low glycemic indices into prerace nutritional practice would prevent potential hypoglycemia and hyperinsulinemia; they can be consumed early yet still produce the required release of glucose for subsequent performance without altering fuel utilization in the early stages of exercise .
Fluid Intake Prior to Race
The focus thus far has related to the fact that glycogen depletion can cause fatigue and limit endurance performance. Fluid balance is the other major nutritional area that can compromise endurance performance . Although it is important to prevent dehydration, recent research has concentrated on the detrimental effects of hyponatremia due to excessive water consumption .
Triathletes are recommended to drink between 400 and 600 mL of fluid 2 hours before the start of exercise  to promote adequate hydration while allowing time for the excretion of any excess water. As Jeukendrup et al.  highlighted, consumption of this amount of water is of importance for those triathletes who have particular difficulty in drinking sufficient amounts of fluid during the race. Although attempts have been made to increase hyperhydration by “over drinking” water, this has been found to produce only short-term expansion of body water, as the majority of the overload is excreted by the kidneys. As glycerol has water-binding properties, the American College of Sports Medicine (ACSM) position stand for exercise and fluid replacement recommended that 1 g/kg BW/d of glycerol is consumed together with or followed by 2 L of water some 1.5 to 2.5 hours before exercise . However, more recent research by Goulet et al.  concluded that when compared with water-induced hyperhydration, glycerol-induced hyperhydration significantly reduces urine production but fails to improve cardiovascular endurance, thermoregulatory function, or exercise performance during 2 hours of cycling in a 25°C environment.
It is likely that triathletes will benefit from hyperhydration prior to a race, whether it is water or glycerol mediated, although it is recommended that experimentation with such practice is undertaken during training, because glycerol supplementation can cause potential adverse effects .
CHO Intake During Race
CHO intake will increase blood glucose concentrations and augment CHO oxidation during endurance exercise ; however, the precise requirements during ultraendurance performance are not well researched. Kimber et al.  found that the average CHO intake (94% total energy intake) during an Ironman distance triathlon was 1.0 g/kg BW/h in females and 1.1 g/kg BW/h in males, which was sufficient to support the previously proposed maximum rate of CHO utilization of 1.0 g × kg-1 × h-1 . Kimber et al.  found that this level of CHO consumption (1–1.1 g/kg BW/h) was achieved by consuming large amounts (∼1.5 g/kg BW/h) during the cycling phase of the race, approximately three times as high as that consumed during the marathon run. A significant positive correlation existed for total CHO intake and finishing time and cycle CHO intake and finishing time in female Ironman triathletes, although this relationship was not found in their male counterparts. Although such findings provide a gauge for triathletes in terms of how much CHO per se should be consumed during competition, consideration must be given to the type of CHO consumed. Some types of CHO are oxidized at high rates (eg, glucose, sucrose, maltose, maltodextrins, amylopectin), whereas others, such as fructose, galactose and amylase, oxidize at rates that are 25% to 50% lower .
Jentjens et al.  found that combined ingestion of moderate amounts of glucose and sucrose (144 g) during 120 minutes of cycling resulted in 21% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose. In addition, they found that when a mixture of glucose and sucrose was ingested at high rates (2.4 g/min), exogenous CHO oxidation rates reached peak values of approximately 1.20 g/min. This corroborates their earlier work, which has consistently shown that ingestion of large amounts of multiple transportable CHO, with an average ingestion rate of 1.8 to 2.4 g/min, results in high exogenous CHO oxidation rates (> 1.1 g/min). This does not occur when an equal amount of glucose is ingested [39–41]. Jeukendrup et al. [42•] further investigated whether exogenous CHO oxidation rates would rise above 1.0 to 1.1 g/min during ultraendurance exercise (5 h at 58% VO2max). The overall finding was that exogenous CHO oxidation did rise to 1.24 g/min although this “leveled off” after 120 minutes, suggesting gluconeogenesis plays an important role in ultraendurance exercise even when CHO is ingested at high rates.
As CHO oxidation rates from single CHO do not exceed 1.0-1 × 1 g/min , and because there is no further research to the contrary, triathletes who ingest single CHO are recommended to consume 60 to 70 g/h. In hot conditions, however, it is advised to consume slightly less CHO (50–60 g/h) as the oxidation of exogenous CHO has been found to be 10% lower in the heat when compared with oxidation in cooler environments .
The combined ingestion of glucose and fructose during exercise in the heat (31.9°C) has been found to increase oxidation rates by 36% when compared with ingesting glucose in isolation . It has been postulated that higher exogenous CHO oxidation rates after combined ingestion of glucose and fructose  and other combinations of CHO [40,41] is due to a higher intestinal CHO transport rate, which may increase the availability of exogenous CHO for oxidation.
Fluid Intake During Race
Athletes who gain more than 4% body weight during exercise have a 45% probability of developing exercise-associated hyponatremic encephalopathy [45••], which highlights the importance of fluid balance for the prevention of hyponatremia during endurance performance. This is of particular relevance to ultraendurance triathletes in whom fluid overload has been found to be the cause of most cases of severe symptomatic hyponatremia [46•] and in whom the use of nonsteroidal anti-inflammatory drugs has been found to be a risk factor for this particular condition .
With regard to assessing body fluid volume during prolonged exercise, body weight has been found to be an inaccurate surrogate, as not all of the weight lost during exercise represents a fluid deficit . Noakes et al. [45••] found that a linear relationship exists between the serum (Na+) after racing and the degree of weight loss in 2135 athletes competing in endurance events such as 42-km marathon, 109-km cycling, or 226-km Ironman triathlon. This proves that factors determining change in body weight during exercise are the principal determinants of serum (Na+) after exercise.
The ACSM guidelines suggest a fluid intake of 600 to 1200 mL/h, which is a range largely based on laboratory studies performed on elite male athletes . These guidelines did not appear to protect the diverse population that was participating in endurance events  and as a consequence, the International Marathon Medical Directors Association released new guidelines that lowered the acceptable range to between 400 and 800 mL/h . Montain et al. [46•] investigated the adequacy of these guidelines in races of more than 42 km in relation to the prevention of dehydration and hyponatremia. They provided predicted recommendations for water intake, using the 2001 fluid recommendations , for three body weights (Table 1).
It is clear that any single range of fluid intake is unlikely to suit the wide spectrum of conditions that characterize endurance events, as both Montain et al. [46•] and Hew-Butler et al.  refer to the fact that body weight, running speed, and ambient temperature govern fluid loss during exercise.
Montain et al. [46•] considered the influence of consuming a sports drink (containing 20 mEq/L sodium and 3 mEq potassium) in delaying hyponatremia. In both cool/temperate and warm condition simulations, the sports drink is predicted to slow the rate of plasma sodium reduction and hence prevent hyponatremia. Therefore, Montain et al. [46•] suggest there is merit in consuming electrolyte-containing drinks or food during ultraendurance competition as a preventative measure. Athletes should be encouraged to evaluate their individual fluid losses during practice sessions and develop personal fluid replacement programs that ensure sufficient fluid is consumed to prevent excessive dehydration without risking hyperhydration and subsequent hyponatremia. Mundel et al.  found that fluid at 4°C significantly enhanced fluid consumption and improved cycling exercise endurance in the heat when compared with a drink at 19°C. With the cold fluid, rectal temperature was approximately 0.25°C lower during the second half of the exercise period and heart rate was approximately 5 beats/min-1 lower, suggesting the greater volume of cold fluid acted as a “heat sink,” reducing the effects of heat stress and increasing the time taken to reach an exercise-limiting core temperature.
Participation in an Ironman triathlon requires specific training regimens and in most cases specifically tailored nutritional practice. Although recommendations have been made in line with the latest research, changes in the dietary practice should be undertaken during training periods rather than experimenting with a radical dietary change during the race. As a result of a dearth of research relating to the nutritional practice in triathletes, and more specifically Ironman triathletes, this particular review has been created by predominantly evaluating and generalizing the findings of research data derived from studies on endurance and/or ultraendurance performance. This highlights the need for further work with this particular sporting population in order to create supportive recommendations based on comparable performance. Nonetheless, recent research has improved the body of knowledge in this area by highlighting new methods of glycogen compensation, the possibility of undertaking a combination of fat and CHO loading in order to improve performance, stressing the need to intake sufficient energy during an Ironman to satisfy energy expenditure, incorporating low glycemic indices into dietary preparation, emphasizing the concern about fluid balance and intake based on body weight, temperature and speed, and highlighting the benefit of combined CHO intake during the race to increase exogenous CHO oxidation rates. Practical recommendations based on the latest research are presented in Table 2.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
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8. Noakes TD: Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance. Scand J Med Sci Sports
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11. Robins A, Davies DM, Jones GE: The effect of nutritional manipulation on ultra-endurance performance: a case study. Res Sports Med
12. Helge JW, Wulff B, Kiens B: Impact of a fat-rich diet on endurance in man: role of the dietary period. Med Sci Sports Exerc
13. Goedecke JH, Christie C, Wilson G, et al.
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14. Lambert EV, Goedecke JH, Zyle C, et al.
: High-fat versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance. Int J Sport Nutr Exerc Metab
15. Havemann L, West SJ, Goedecke JH, et al.
: Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance. J Appl Physiol
16. Burke LM, Kiens B: Fat adaptation for athletic performance: the nail in the coffin? J Appl Physiol
17. Kimber NE, Ross JJ, Mason SL, Speedy DB: Energy balance during an ironman triathlon in male and female triathletes. Int J Sport Nutr Exerc Metab
18. Schabort EJ, Bosch AN, Weltan SM, Noakes TD: The effect of a pre-exercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc
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20. O'Toole ML, Hiller WDB, Douglas PS, et al.
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22. Foster C, Costill DL, Fink WJ: Effects of preexercise feedings on endurance performance. Med Sci Sports Exerc
23. Keller K, Schwarzkopf R: Preexercise snacks may decrease exercise performance. Phys Sportsmed
24. Febbrario MA, Stewart KL: CHO feeding before prolonged exercise: effect of glycemic index on muscle glycogenolysis and exercise performance. J Appl Physiol
25. Sparks MJ, Selig SS, Febbrario MA: Pre-exercise carbohydrate ingestion: effect of the glycemic index on endurance exercise performance. Med Sci Sports Exerc
26. Febbrario MA, Keenan J, Angus DJ, et al.
: Preexercise carbohydrate ingestion, glucose kinetics and muscle glycogen use: effect of the glycemic index. J Appl Physiol
27. Sherman WM, Peden MC, Wright DA: Carbohydrate feedings 1h before exercise improves cycling performance. Am J Clin Nutr
28. Speedy D, Kelly M, O'Brien M: The effect of pre-exercise feeding on endurance exercise performance. N Z J Sports Med
29. Francescato MP, Puntel I: Does a pre-exercise carbohydrate feeding improve a 20-km cross-country ski performance. J Sports Med Phys Fitness
30. Frayn KN: Insulin resistance and lipid metabolism. Curr Opin Lipidol
31. Stevenson E, Williams C, Nute M: The influence of the glycaemic index of breakfast and lunch on substrate utilisation during the postprandial periods and subsequent exercise. Br J Nutr
32. Convertino VA, Armstrong LE, Coyle EF, et al.
: American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc
33. Noakes TD, Speedy DB: Case proven: exercise associated hyponatremia is due to overdrinking. So why did it take 20 years before the original evidence was accepted? Br J Sports Med
34. Latzka WA, Sawka MN: Hyperhydration and glycerol: thermoregulatory effects during exercise in hot climates. Can J Appl Physiol
35. Goulet ED, Robergs RA, Labrecque S, et al.
: Effect of glycerol-induced hyperhydration on thermoregulatory and cardiovascular functions and endurance performance during prolonged cycling in a 25 degrees C environment. Appl Physiol Nutr Metab
36. Rehrer NJ: Fluid and electrolyte balance in ultra-endurance sport. Sports Med
37. Hawley JA, Dennis SC, Noakes TD: Oxidation of carbohydrate ingested during prolonged endurance exercise. Sports Med
38. Jentjens RLPG, Shaw C, Birtles T, et al.
: Oxidation of combined ingestion of glucose and sucrose during exercise. Metab Clin Exper
39. Jentjens RL, Moseley L, Waring RH., et al.
: Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol
40. Jentjens RL, Venables MC, Jeukendrup AE: Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol
41. Jentjens RL, Achten J, Jeukendrup AE: High oxidation rates from combined carbohydrates ingested during exercise. Med Sci Sports Exerc
42.• Jeukendrup AE, Moseley L, Mainwaring GI, et al.
: Exogenous carbohydrate oxidation during ultraendurance exercise. J Appl Physiol
This study found that the ingestion of 1.5 g/min of glucose and fructose enabled higher oxidation rates of exogenous CHO during ultraendurance exercise (5 h) when compared with glucose alone. Oxidation rates reached a peak of 1.24 g/min after 120 minutes, which highlighted the considerable contribution of gluconeogenesis to CHO oxidation late in exercise (> 3 h).
43. Jentjens RLPG, Wagenmakers AJM, Jeukendrup AE: Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise. J Appl Physiol
44. Jentjens RLPG, Underwood K, Achten J, et al.
: Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat. J Appl Physiol
45.•• Noakes TD, Sharwood K, Speedy D, et al.
: Three independent biological mechanisms cause exercise-associated hyponatremia: Evidence from 2,135 weighed competitive athletic performance. Proc Natl Acad Sci U S A
This paper highlights three independent mechanisms to explain why some athletes develop exercise-associated hyponatremia/exercise-associated hyponatremic encephalopathy during and after prolonged exercise: 1) overdrinking due to biologic or psychologic factors, 2) inappropriate antidiuretic hormone secretion, and 3) a failure to mobilize NA+ from osmotically inactive sodium stores or inappropriate osmotic inactivation of circulating Na+. As the mechanisms causing reasons 1 and 2 are currently unknown, the authors recommend that athletes should be encouraged to avoid overdrinking during exercise.
46.• Montain SJ, Cheuvront SN, Sawka MN: Exercise associated hyponatremia: quantitative analysis to understand the aetiology. Br J Sports Med
This study used a mathematic model to illustrate the interactions between intensity of exercise, temperature, sweat composition, total body water, drinking rate, and drink composition in relation to promoting hydration and preventing hyponatremia. On this basis, the authors recommend that athletes should be encouraged to develop customized fluid replacement programs through trial and error in practice sessions.
47. Wharam PC, Speedy DB, Noakes TD, et al.
: NSAID use increases the risk of developing hyponatremia during an Ironman triathlon. Med Sci Sports Exerc
48. Hew-Butler T, Verbalis JG, Noakes TD: Updated fluid recommendation: Position statement from the International Marathon Medical Directors Association (IMMDA). Clin J Sports Med
49. Noakes TD: Fluid replacement during marathon running. Clin J Sports Med
50. Mundel T, King J, Collacott E, Jones DA: Drink temperature influences fluid intake and endurance capacity in men during exercise in hot, dry environment. Exper Physiol