As shown in Table ​Table2,2, during the event, solid foods provided 73% (4058 kcal) of the total energy, and the remaining 27% (1513 kcal) was provided by fluids such as sport drinks. Carbohydrates were the main macronutrient he ingested (1102 g; 13.1 g/kg). Overall consumption of fluids and sodium during the event was 20.7 L (862 mL/h) and 16,182 mg (34.0 mmol/L), respectively. Fluids comprised 86% (13,878 mg) of the total sodium intake, and solids comprised 14% (2355 mg). During the second half of the event (7–19 h), the cyclist increased consumption of caffeinated drinks, with total caffeine intake of 231 mg (2.7 mg/kg); consumed low amounts of branched chain amino acids in pill form during the rest periods; and ingested one ibuprofen pill after 9 h of competition and two aspirin pills at 18 h.

However, it is worth mentioning that the method used in this study to estimate energy expenditure (relationship between HR and VO₂) has several limitations. For instance, during longer events, HR can be influenced by environmental conditions such as temperature and humidity, which can favor dehydration and an increase of HR without associated changes in VO₂ (7). Currently, the method of doubly labeled water is considered the reference method to estimate energy expenditure. Another feasible method to estimate energy expenditure in cycling is the analysis of power output (8). However, neither of these methods was at our disposal during the current study. For this reason and similar to other recent studies (6, 9, 10, 11), we estimated energy expenditure using the HR-VO₂ method. Compared with the doubly labeled water method, this method is inexpensive and easy to perform. Additionally, monitoring of HR also provides information on the amount of time spent at different levels of exercise intensity, which may also be useful for the assessment of physical activity rather than energy expenditure. Furthermore, it has been reported that energy expenditure estimated using the HR method compared with the method of doubly labeled water is overestimated by ~10% (12). If we accounted for this by reducing the energy expenditure estimated in this study by 10%, the energy deficit would be decreased only 4%, from 64% to 60%. Therefore, although the doubly labeled water method could be used under field conditions, the high cost and the inability to obtain an activity pattern does not always make it ideal.

Based on the athlete's average intensity of 69% HR_max, it is estimated that approximately two thirds of the total energy required was met by fat oxidation, with carbohydrate oxidation providing one third (13). However, fat oxidation is not a limitation for providing fuel during longer events (13, 14). The estimation of anthropometric characteristics in the current athlete indicated that he had ~9.8 kg of subcutaneous adipose tissue that could provide >88,000 kcal. Based on that, the athlete should consume a high amount of carbohydrates during the event due to his limited glycogen stores (13). The recommended amount of carbohydrate intake to optimize the oxidation rates has been reported to be between 1.0 and 1.2 g/min (15). The current athlete ingested amounts of carbohydrates below these recommendations during three-quarters of the event; only during the last 6 h, when fatigue symptoms were more pronounced and the glycogen stores were possibly depleted, did he meet the carbohydrate consumption threshold of >1.0 g/min.

Additionally, although protein is not considered a primary energy source for athletes, it has been suggested to play an important role during longer events. An adequate ratio of carbohydrate/protein may reduce a negative protein balance (16, 17) and may enhance aerobic endurance performance (18). An optimal rate (g) between carbohydrate and protein intake seems to be 4:1 (18). Applying these recommendations in the present case study, and assuming that the athlete had ingested the recommended carbohydrate rate (~1.1 g/min), protein intake would have had to have been ~400 g (4.7 g/kg of body mass), representing more than threefold the actual amount of protein intake by the cyclist during the event. Accordingly, this amount of protein seems to be excessive and, independent of the supposed benefits of carbohydrate and protein combination, it should also be taken into account that protein intake is associated with greater satiety and a reduced ad libitum energy intake in humans. Thus, higher protein consumption during longer events can be associated with a reduction of food intake, as well as an increase of the risk of gastrointestinal disturbances. Further studies are needed to analyze whether an increase of protein intake above the current recommendations (1.2–1.7 g/kg of body mass/day) may induce benefits in longer and high-intensity sport events.

Furthermore, the hydration pattern is one of the nutritional keys in ultraendurance events. While the current athlete ingested the high amount of 20.7 L of fluids during the race, the hydration strategy was not in agreement with current recommendations (19, 20). He should have prioritized the consumption of isotonic fluids containing carbohydrates (sucrose, maltose, or maltodextrins) at ~3% to ~8% weight/volume during the race (21). Thus, the strategy of hydration followed by the cyclist substantially reduced the amount of carbohydrate intake. If he had prioritized the consumption of isotonic fluids (7% of carbohydrates), he would have obtained ~900 g extra carbohydrates, reaching values within the carbohydrate recommendations for longer events (15).

Related to the hydration pattern, one of the most common medical complications during long-distance events is exercise-associated hyponatremia (22), defined as a serum plasma or sodium concentration <135 mmol/L^-1. To prevent exercise-associated hyponatremia, the athlete ingested higher amounts of sodium, mainly during the second half of the event when the environmental conditions were harsher. Nevertheless, although some hydration guidelines recommend consuming fluids with a high content of sodium (30–50 mmol/L) (21), currently there is insufficient evidence to determine whether sodium intake prevents or decreases the risk of exercise-associated hyponatremia (23). On the contrary, some risks of excessive sodium supplementation in combination with overhydration have been documented (24). There are at least two ways to reduce the risk of excessive fluid retention: 1) drink only according to thirst and 2) monitor body weight so as to avoid weight gain during exercise. In the present study, the cyclist showed no weight gain; he lost 2.6 kg of body mass over the race. However, in ultraendurance events such as an Ironman triathlon, it has been reported that part of fluid losses, at least 2 kg, could be derived from reduction of fat stores, skeletal muscle mass, glycogen, and the metabolic water stored in glycogen (25, 26).