The International Association of Athletics Federations recognizes various distance events, with current World Championship and Olympic Games hosting the 10,000-m track event and road marathon (42.2 km) in running and 20 and 50-km events in race walking. In addition, there are separate International Association of Athletics Federations Road Race Label events spread throughout the year in half marathons, marathons, and other distance races, a half-marathon World Championship, cross-country World Championships (10 km), and various Race Walking Cups and Challenges. Many events are held as national or continental titles and include competitions for junior athletes (e.g., under 20 or under 18 years) over shorter distances.
Table 1 summarizes the characteristics of key distance running and race walking events, noting the duration and intensity of races for top competitors and elements that contribute to the physiological and nutrition challenges of these events. Meanwhile, opportunities to address these challenges via within-event nutrition strategies are summarized in Table 2. As in middle-distance events, there are tactical, technical, and physiological components to successful outcomes. This paper focuses on knowledge that has emerged over the past decade on nutrition strategies to support the training and competition goals of distance runners and race walkers, translating race nutrition principles into practical recommendations.
Characteristics of Key Distance Events in Athletics
| Event | 10,000-m track race | Cross country | 21.1-km half marathon | 20-km race walk | 42.2-km marathon | 50-km race walk |
|---|---|---|---|---|---|---|
| World record: male (hr:min:s) | 26:15:53 (Kenenisa Bekele) | ∼12 km (no records) | 58:18 (Abraham Kiptum) | 1:16:36 (Yusuki Suzuki) | 2:01:39 (Eliud Kipchoge) | 3:32:33 (Yohann Diniz |
| World record: female (hr:min:s) | 29:17:45 (Almaz Ayana) | ∼8 km (no records) | 1:04:51 Joyciline Jepkkosgei) | 1:24:38 (Hong Lui) | 2:15:25 (Paula Radcliffe) | 4:04:36 (Rui Liang) |
| Approximate intensity (%VO2max) | 90–95% ≥Critical speed | 90–95% ≥Critical speed | 85–90% ≤Critical speed | 80–90% ≤Critical speed | 80–85% <Critical speed but above lactate threshold | 75–80% ≤Lactate threshold unless during higher intensity pieces |
| Surface | Track | Natural terrain, with undulating topography and variable surfaces | Road—may include changes in elevation | 1- to 2-km circuit on road—typically flat | Road—may include changes in elevation | 2-km circuit on road—typically flat |
| Physiological and nutrition limitations to performance | Fatigue related to peripheral factors (metabolic acidosis, Ca2+ handling, and low phosphocreatine), plus possible localized fiber-specific glycogen depletion | Fatigue related to peripheral factors (metabolic acidosis, Ca2+ handling, and low phosphocreatine), plus possible localized fiber-specific glycogen depletion | Fatigue related to glycogen depletion, central fatigue, and some peripheral factors | Fatigue related to glycogen depletion, central fatigue, and some peripheral factors | Fatigue related to glycogen depletion, hypoglycemia, possible dehydration, hyperthermia depending on environmental conditions, and central fatigue, possibly muscle damage | Fatigue related to glycogen depletion, hypoglycemia, possible dehydration, hyperthermia depending on environmental conditions, and central fatigue |
Nutrition Strategies for High-Performance Athletes in Key Distance Events in Athletics
| Issues and general guidelines | 10,000-m track race | 10-km cross country | 21.1-km half marathon | 20-km race walk | 42.2-km marathon | 50-km race walk |
|---|---|---|---|---|---|---|
| Prerace refueling • Normalization of glycogen = 7–12 g·kg−1·day−1 for 24 hr • CHO loading = 10–12 g·kg−1·day−1 for 36–48 hr | Glycogen normalization | Glycogen normalization | Glycogen normalization | Accentuated glycogen normalization | CHO loading, especially with a low-residue diet | CHO loading, especially with a low-residue diet |
| Prerace meal • 1–4 g·kg−1 CHO in 1- to 4-hr prerace • Reduced fat, fiber, and protein intake according to the risk of gut issues during the race | Familiar prerace meal | Familiar prerace meal | Familiar prerace meal + CHO after warm-up | Familiar prerace meal + CHO after warm-up | Familiar prerace meal + CHO after warm-up | Familiar prerace meal + CHO after warm-up |
| Opportunities for in-race nutrition (availability of drink stations) | Nil (if extremely hot, water stations may be provided on an outside lane of the track if extremely hot) | Nil | Typically, every 5 km in elite races Frequency differs in large city races | Every lap of 2 km loop course (sometimes course = 1-km loop) | Typically, every 5 km in elite races. Frequency differs in large city marathons: may be every 2–3 km | Every lap of 2-km loop course |
| In-race fueling goals • 45–75 min: mouth rinse/small CHO amount • 1–2.5 hr: 30–60 g/hr • >2.5 hr: up to 90 g/hr | N/A | N/A | Trial CHO mouth rinse up to an intake of 30–60 g from CHO drinks or gels/confectionery | Trial CHO mouth rinse up to an intake of 30–60 g from CHO drinks or gels/confectionery | 30–60 g/hr CHO; consider a trialing intake of up to 90 g/hr from the mix of CHO drinks and more concentrated gels/confectionery | Target an intake of 60–90 g/hr from the mix of CHO drinks or more concentrated gels/confectionary according to the fluid goals in race plan |
| In-race hydration goals • Aim to keep net fluid deficit <2% to 3% body mass, especially in hot weather | N/A | N/A | Cost–benefit analysis may show that time cost of drinking may negate benefits in elite runners | Drink stations allow plentiful opportunities for frequent small intakes of CHO-containing fluid toward a race plan | Fast runners will find it difficult to drink large volumes | Drink stations allow plentiful opportunities for frequent small intakes of CHO-containing fluids toward a race plan |
| Caffeine supplementation • 3 mg/kg before/during race | Prerace caffeine | Prerace caffeine | Caffeine prerace and/or during race | Caffeine prerace and/or during race | Caffeine prerace and/or during race to 3–6 mg/kg target | Caffeine prerace and/or during race to 3–6 mg/kg target |
| Special issues for hot weather events | Consider prerace precooling with ice slurry in addition to external cooling strategies if a significant thermal challenge is anticipated | Consider prerace precooling with ice slurry in addition to external cooling strategies if a significant thermal challenge is anticipated Consider prerace hyperhydration if a large fluid deficit is anticipated Adjust fluid intake during an event where possible in view of increased sweat losses | Consider prerace precooling with ice slurry in addition to external cooling strategies but take care with pacing strategies Consider prerace hyperhydration if a large fluid deficit is anticipated Adjust fluid intakes during an event where possible in view of increased sweat losses | |||
| Special comments for nonelite competitors | Do not overdrink by consuming fluid in excess of sweat losses | Do not overdrink by consuming fluid in excess of sweat losses | ||||
Note: All strategies should involve a personalized and well-practiced plan that is suited to the specific needs of the events. General guidelines can be found in more detail in Thomas et al. (2016). CHO = carbohydrate; N/A = not available.
Bioenergetic and Physiological Determinants of Success in Distance Events
The distance events (from ∼26 min in the 10,000-m track race to >3.5 hr in the 50-km race walk) are considered “submaximal,” with mean energy requirements of ∼75–92% of maximal oxygen uptake (
A high
There is variability in running and walking economy, defined as the steady-state
Support for the Periodized Training Programs of Distance Athletes
“Periodized nutrition,” the strategic combination of nutrition and exercise to optimize training adaptations and competition performance (Jeukendrup, 2017a), is explained in relation to Athletics by Stellingwerff et al. (2019). In distance events, a variety of strategies, often in apparent conflict with each other but nevertheless targeted at enhancing the specific session or training phase, should be integrated into the annual, meso, and microcycles of training according to the athletes’ individualized and changing goals. For example, workouts/phases targeting an enhancement of oxidative capacity should exploit the superior adaptations in fat metabolism and mitochondrial biogenesis following exercise with low CHO availability either at a whole-body level or muscle level (Burke et al., 2018c; Jeukendrup, 2017a). Conversely, quality/high-intensity training should be performed with high CHO availability, as it is during the races (Jeukendrup, 2017a). Furthermore, when intake during the event is beneficial, it may be possible to prepare the gut to optimize and tolerate this by practicing strategies with adjusted intakes of CHO and fluid within the training sessions (Jeukendrup, 2017b).
It is likely that the heavy training loads and habitual dietary practices of high-performance distance runners/walkers, including the remarkable East African athletes (Commentary 1), already create periodization of CHO availability across a training cycle. Whether more deliberate planning can improve the outcome is of interest. In this regard, although subelite endurance athletes performed better after undertaking 3 weeks of training with strategic manipulations of CHO availability (Marquet et al., 2016), a study of elite race walkers failed to find evidence of superior race performance after an intensified training block supported by periodized CHO availability compared with constant high CHO availability (Burke et al., 2017). Meanwhile, both dietary approaches were associated with better race outcomes than chronic (3.5 week) exposure to a ketogenic low CHO, high-fat diet, despite its achievement of substantial (∼2.5 ×) increases in muscle fat oxidation (Burke et al., 2017). This contrasts with claims that a low CHO, high-fat diet is the “future of elite endurance sport” but is supported by empirical (Krogh & Lindhard, 1920) and theoretical (Leverve et al., 2007) evidence of better exercise economy with CHO-dependent generation of adenosine triphosphate than fat oxidation pathways (see Commentary 2 for modeling of this effect). This is especially important at exercise intensities that are typical of the distance events (Hawley & Leckey, 2015). Further investigation of periodization of fuel support strategies in elite athletes is warranted, although it is clear that some areas are controversial or confusing. This is at least partly attributable to different definitions or inaccurate descriptions of the implementation or goals of these strategies. A recent commentary has promoted the case for a common terminology and understanding of this theme (Burke et al., 2018c). In the meantime, it appears that elite athletes include various versions of “train high,” “train low,” and “gut training” within their training programs, both accidentally and intentionally (Heikura et al., 2018; Stellingwerff, 2012).
Periodization of body composition provides another example of strategic integration of different nutrition strategies within the training schedules. A recent science-based case study of an Olympic female middle-distance runner argued that it is not sustainable from a health and/or performance perspective to be at ideal race body composition year-round (Stellingwerff, 2018). Instead, an assessment of anthropometric, hematological, and performance metrics over a 9-year career demonstrated a periodized approach. During the general preparation phase (September–April), the athlete was ∼2–4% over ideal race body mass (BM) and body fat (%), with optimal energy availability being prioritized. Body composition optimization for competition (May–August) included an individualized time frame and energy deficit with various feedback metrics (BM, performance, and hunger) to guide the process. This approach supported targeted peak performances and minimized risk of injury while maximizing training adaptation and long-term athlete health through management of energy availability. Although this concept has, arguably, been understood for many years, the concept and calculated practice is a contemporary update (Jeukendrup, 2017a). Importantly, it helps the athlete to integrate the inevitability or benefits of brief periods of controlled low energy availability within the endurance training framework. Problems associated with chronic or severe low energy availability, known as relative energy deficiency in sports, are well known (Mountjoy et al., 2018); specific issues in relation to Athletics are covered by Melin et al. (2019).
Race Preparation
Race preparation should include strategies to store muscle glycogen in the amounts commensurate with the fuel needs of the event. For races <90-min duration (e.g., 10,000-m track race, cross country, half marathon, and 20-km walks), it is sufficient to normalize the superior glycogen concentrations associated with endurance training; this is typically achieved with CHO intakes of 7–10 g/kg BM for 24 hr (Burke et al., 2011). In the marathon and 50-km race walk where glycogen can become limiting for race performance, the protocols that supercompensate glycogen are beneficial. Indeed, investigations of the original protocol devised in the 1960s involving distance runners (Karlsson & Saltin, 1971) and race walkers (Hyman, 1970) were largely responsible for the popularization of this strategy within the sports world; these studies showed that CHO loading improved performance by attenuating the decay in speed in the last part of the race. The contemporary CHO loading protocol is an abbreviated version of the original involving 36–48 hr of CHO intakes targeting 10–12 g·kg−1·day−1 (Burke et al., 2011). This is often undertaken in conjunction with a low residue (fiber) diet (Table 3), which may not only reduce the risk of gut issues during the race but also achieve a small reduction in BM to partially offset the mass of the additional muscle glycogen and stored water.
Summary of a “Low-Residue” (Low-Fiber) Diet
| Characteristic | Comment |
|---|---|
| Background | • Used as a more acceptable alternative to a “clear liquid” diet or pharmaceutical preparation to reduce intestinal fecal matter and secretions prior to bowel investigations or surgery (Vanhauwaert et al., 2015). • Although the terms “low residue” and “low fiber” are often used interchangeably to describe this dietary practice, it has been argued that it is best described as a low-fiber diet, with daily intakes <10-g fiber (Vanhauwaert et al., 2015). |
| Application to athlete practice | • The implementation and outcomes have not been subjected to rigorous scientific investigation in sporting scenarios. However, the acute use of low-fiber diets is often observed in weight division sports (Reale et al., 2017). Here, the athletes suddenly reduce their fiber consumption in the days before weigh-in, in the belief or experience that a reduction in bowel contents contributes a small but potentially valuable loss of body mass, with fewer disadvantages to the dietary preparation for competition than food restriction. • The reduction in body mass associated with this dietary practice in athletes is highly variable and individualistic (Reale et al., 2017), but an average response of ∼500 g might be expected (L.M. Burke, personal observations). • Additional benefits of a prerace reduction in intestinal fiber content for distance runners and walkers include a lowered risk of gut discomfort/upset during the race and simplification of the logistics of bowel evacuations in the hours prior to a race. • The optimal period of implementation of the prerace low-fiber diet is also highly variable and ranges from 24 to 72 hr depending on individual gut transit times (Reale et al., 2017). • Disadvantages of the prerace low-fiber diet include a lack of food variety, a (short-term) reduction in dietary quality/micronutrient density and discomfort due to lower satiety/hunger. |
| Suggested implementation of prerace low-fiber diet | • The distance athlete should experiment with the duration of the low-fiber diet to determine an optimal plan according to their usual fiber intake, gut transit time and personal tolerance of limited food variety, and reduced satiety/hunger. • The diet can be integrated with a carbohydrate-loading protocol and may even assist with the achievement of targets for large amounts of carbohydrate intake due to the increased energy density of food choices. • Meals and snacks should be based on low-fiber, CHO-rich foods and the avoidance of significant sources of resistant starch. Suitable foods include the following: ○ “White” bread ○ “White” breakfast cereals (e.g., rice puffs) ○ Sweetened dairy products ○ “White” rice, pasta, noodles, and potato: these should be well cooked and consumed hot to avoid the creation of resistant starch with cooling ○ Pulp-free fruit juice and sugary drinks (e.g., soda) ○ Confectionary, jelly preserves, and honey ○ Cakes and desserts based on white flour (e.g., cakes, puddings) and sugar (e.g., jello) but the avoidance of dried or fresh fruit ○ Sports products (e.g., sports drinks, gels, confectionary) • Meat, milk, cheese, poultry, fish, eggs, and other protein-rich foods can be added to meals and snack menus. • Uncooked fruits and vegetables should be avoided, especially where they contain skin or pips. Cooked versions can be added in modest amounts to make up meals or menu items; these include the following: ○ Pureed fruit and apple sauce ○ Mashed/pureed vegetables with a preference for “ketchup” style sauces and canned/mashed vegetables |
Further contributions to fuel availability are provided by a pre-event CHO-focused meal and a small CHO-rich snack (e.g., sports gel or drink) during the race warm-up. This is particularly important for events undertaken in the morning where CHO intake can restore liver glycogen following an overnight fast as well as provide an ongoing supply of CHO from the gut (Burke et al., 2011). The timing, size, and food choices in the prerace meal will vary according to event characteristics and athletes’ preferences; these should be well practiced to develop an individualized protocol. Athletes should also consider fluid needs to achieve optimal hydration status for the event and specific race conditions (see Casa et al., 2019). As discussed in the section on racing in the heat, there may also be opportunities to address race challenges related to thermoregulation and dehydration by hyperhydrating and/or precooling in the hours prior to the event.
Race Feeding: Fueling and Hydration Update
Some distance events offer an opportunity for athletes to consume fluid and fuel during the race to address the physiological limitations of these factors (Table 2). CHO ingestion during longer distance events (e.g., marathon and 50-km race walk) can improve performance by delivering additional substrates to maintain high rates of CHO oxidation in the face of dwindling endogenous stores (Coyle et al., 1986). A systematic review of the literature on CHO ingestion during endurance protocols by Stellingwerff and Cox (2014) concluded that 82% of studies (50 of 61 studies, involving 679 subjects) showed statistically significant benefits from this practice. Older guidelines (Coyle, 1991) recommended that distance athletes should experiment with hourly CHO intakes within the range of 30–60 g to find a beneficial strategy. More contemporary recommendations (Burke et al., 2011; Thomas et al., 2016), however, suggest smaller amounts for shorter duration events and higher rates of intake for longer events (>2.5 hr), based on the mechanism of likely benefits to performance as well as the recognition that higher amounts can be tolerated and utilized than previously considered.
Early investigations of CHO ingestion during exercise concluded that maximal oxidation rates plateaued at 60 g/hr, even when larger amounts were ingested (120–180 g/hr; Jeukendrup, 2014). The limiting factor was subsequently found to be intestinal absorption, particularly the sodium-dependent glucose transporter, rather than gastric emptying, hepatic glucose extraction, muscle glucose uptake, or muscle glucose oxidation (Jeukendrup, 2014). However, as reviewed by Jeukendrup (2017b), sodium-dependent glucose transporter abundance and activity in animals is increased by a CHO-rich diet; furthermore, chronic exposure to higher CHO intakes by athletes, including exercise intake, increases gut tolerance, intestinal absorption, and muscle oxidation of CHO consumed during exercise (Costa et al., 2017; Cox et al., 2010). Combining glucose-based CHO sources with fructose (transported in the intestine by GLUT5) increases total exogenous CHO oxidation during exercise, with rates as high as 1.75 g/min (Jeukendrup & Chambers, 2010). Narrative review (Stellingwerff & Cox, 2014), meta-analysis (Vandenbogaerde & Hopkins, 2011), and dose–response (Smith et al., 2010) approaches to this topic have found that higher amounts of CHO promote better performance in longer events relevant to the marathon and 50-km race walk, with optimal intakes of ∼75–90 g/hr. However, in shorter events (half marathon, 20-km race walking), performance benefits may be associated with the intake of smaller amounts of CHO, including central nervous system activation associated simply with mouth exposure to CHO (the so-called “mouth rinsing” effect). Indeed, there is robust evidence that the detection of CHO by receptors in the oral cavity, independent of sweetness, activates certain centers in the brain to enhance perceptions of effort and pacing decisions (see Jeukendrup & Chambers, 2010). A range of sports drinks, gels, and confectionery is available to meet various targets, both in training and racing, around taste, practicality, balanced intake of fluid and CHO, inclusion of multiple transportable CHO sources, electrolyte replacement, and supplementation with caffeine, while other everyday foods and drinks may also be used.
Fluid intake to address sweat losses is important during longer events and in the heat because a fluid deficit equivalent to >2–3% BM loss is typically associated with increases in perception of effort and core temperature and reductions in performance, especially in hot environments (Sawka et al., 2015). However, plans for fluid intake in events that permit it (as indeed for CHO intake) should involve a cost–benefit analysis, where “costs” include the availability of supplies at drink stations during the race (see Table 2), the time lost while slowing down to obtain and consume drinks/sports products, and the risk of gut upsets. Furthermore, the associated BM reduction may partially compensate for the disadvantages of dehydration. High-performance runners are less able to consume fluid/CHO during the race than racewalkers because of the higher speed of movement and the lower number/increased time between drink stations; for example, ∼15–18 min for marathon runners with stations every 5 km versus 8–10 min for racewalkers with stations every 2 km. Furthermore, the impracticality of drinking large volumes despite high sweat rates explains BM losses of up to 10% in race winners in hot-weather marathons (e.g., Beis et al., 2012). We recommend that athletes develop a personalized and practiced race plan that optimizes fluid and CHO status within the prevailing conditions and opportunities of each event. Despite the practical challenges, the authors of this paper have assisted several elite athletes to break Olympic and world records/marks using aggressive hydration and CHO feeding plans (targeting 90 g/h); some of which have been described independently ( Caesar, 2017; Hutchinson, 2017). Indeed, some recent elite marathons, including the 2018 Berlin event in which the most recent world record was set, have increased the frequency of feed zones (every 2.5 km) to provide greater opportunity for race feeding. A personalized drinking plan can be adjusted to all levels of runners, including recreational competitors who may drink in volumes exceeding their sweat rates and who should be warned about the dangers of developing hyponatremia (Almond et al., 2005).
Supplements for Distance Athletes
The term “supplements’ includes products that address a distance athlete’s nutrition goals in a specialized context: medical supplements used to prevent/treat a nutrient deficiency (e.g., vitamin D or iron supplements); sports foods providing energy, macronutrients, and fluid requirements in scenarios where whole foods are impractical; and performance supplements that directly improve training or competition outcomes. Characteristics of these products and scenarios in which they contribute to a distance athlete’s nutrition plan are summarized elsewhere (Castell et al., 2019; Maughan et al., 2018; Peeling et al., 2019). The specific needs of long-distance races raise potential new uses of sports foods and performance supplements, based on the specific physiological, biochemical, and central nervous system factors that limit performance in these races, as well as the opportunity to consume products within the event, at least for races of half marathon and longer.
Only a handful of the multitude of performance supplements marketed to athletes have a strong evidence base. Peeling et al. (2019) have separately reviewed these products (caffeine, nitrate, creatine, β-alanine, and bicarbonate) and their mechanisms of action in relation to Athletics, identifying only the first two of this group as likely to achieve a performance benefit in distance events; investigations of these products in relevant scenarios are summarized in Tables 4 and 5. We note the sparsity of specific studies and the variability in findings; this may arise from differences in supplement protocols as well as the underpowering of studies due to small sample sizes and/or reliance on performance protocols lacking sufficient reliability to detect small but meaningful benefits. Indeed, the evidence base for these performance products relies on summaries of the general endurance sports literature (McMahon et al., 2017; Southward et al., 2018a). However, the actual use of any performance supplement by endurance athletes requires its integration into a bespoke nutrition plan that accounts for the specificity of their event and/or training schedule and their experience of individual responsiveness to the plan (Burke et al., 2018a).
Summary of Caffeine Supplementation and Performance of Distance Events
| Overview (see Burke, 2008; Southward et al., 2018a; Spriet, 2014) | |||||
|---|---|---|---|---|---|
| Mechanism of action | • Adenosine receptor antagonist with a large range of effects • Major effects during endurance exercise include masking of perception of effort, fatigue and pain, and increase in vigilance and alertness | ||||
| Best practice protocol | • ∼3 mg/kg (up to 6 mg/kg) taken before and/or during distance events, with sources including food (e.g., coffee, cola drinks, energy drinks), sports foods (e.g., caffeinated gels), or pharmaceutical products (e.g., caffeine tablets) • Greater responsiveness to small amounts of caffeine (2–3 mg/kg) may be seen when it is taken during a race, around the onset of fatigue (Spriet, 2014) | ||||
| Issues for future study | • Individual responsiveness to caffeine supplementation in distance running, including genetic causes (Southward et al., 2018b) • Interaction with other supplements—CHO, nitrate • Effect of caffeine on heat storage and performance in hot, humid environments (Hanson et al., 2018) • Use of caffeine to support training capacity/quality, especially when training in a fatigued state (e.g., altitude training, training with low CHO availability; Lane et al., 2013) | ||||
| Investigations | Study design | Caffeine protocol | Performance protocol | Effect | Comments |
|---|---|---|---|---|---|
| Cohen et al. (1996) | • Competitive distance runners (5 M + 2 F) • Crossover design with different caffeine doses vs. placebo | • 5 and 9 mg/kg taken prerace | • Half marathon (field study) • Hot conditions • Water during run | No benefit detected | No effects on RPE or performance at either dose compared with placebo |
| Van Nieuwenhoven et al. (2005) | • Trained to well-trained runners (90 M + 8 F) • Crossover design with caffeine + CHO vs. CHO vs. water placebo | • ∼1.3 mg/kg in 7% CHO drink vs. CHO drink alone • 600-ml drink consumed in equal portions before and at 4.5-, 9-, and 13.5-km race | • 18-km road race (field study) • CHO drink during run | No benefit detected | No differences in performance of whole group between caffeinated sports drink (78:03 ± 8:42 [min:s]), sports drink (78:23 ± 8:47), or water (78:03 ± 8:30) or for 10 fastest runners (63:41, 63:54 vs. 63:50 for caffeine sports drink, sports drink, and water, respectively). |
| Bridge and Jones (2006) | • Distance runners (8 M) • Crossover design with caffeine vs. placebo vs. control | • 3 mg/kg taken 60-min prerace | • 8-km race on track (field study) • No intake during run | Benefit detected | Relative to the mean time of the control and placebo trials, caffeine supplementation resulted in a 23.8 s or 1.2% improvement in run time (p < .05) with individual results ranging from 10 to 61 s improvement. HR was significantly higher in caffeine trial, with a trend to lower RPE despite the faster running speed. |
| Potgieter et al. (2018) | • Well-trained triathletes (14 M 12 F); • Crossover design with caffeine vs. placebo | • 6 mg/kg taken 60-min prerace | • 10-km run at the end of Olympic distance triathlon (field study) | Benefit detected | Caffeine associated with a 1.3% improvement in race time (149.6 ± 19.8 vs. 151.5 ± 18.6 min, p < .05), with great effect in male subjects. No difference in RPE despite faster time. Caffeine associated with greater blood lactate and cortisol concentrations. |
| Hanson et al. (2018) | • Moderately trained distance runners (6 M, 4F) • Crossover design with caffeine vs. placebo vs. control | • 3 mg/kg or 6 mg/kg taken 60-min pretrial | • 10-km treadmill TT in hot conditions (30°C and 50% rh) • No intake during run | No benefit detected | No difference in 10-km time (53.2 ± 8.2; 53.4 ± 8.4; 52.7 ± 8.2 for placebo, 3 mg/kg, and 6 mg/kg doses). However, a greater increase in core temperature with higher caffeine dose suggests greater heat storage. |
Note. TT = time trial; M = male; F = female; HR = heart rate; rh = relative humidity; RPE = ratings of perceived exertion.
Summary of Nitrate Supplementation and Effect on Performance of Distance Events
| Overview (for review, see Jones et al., 2018; McMahon et al., 2017) | |||||
|---|---|---|---|---|---|
| Mechanism of action | • Improves exercise economy (reduces the oxygen cost of submaximal exercise) to improve endurance exercise performance • Enhances skeletal muscle contractile function to improve muscle power and sprint exercise performance | ||||
| Best practice protocol | • ∼8 mmol nitrate taken 2- to 3-hr prerace, especially with chronic intake for 3+ days pretrial • Usually taken in the form of beetroot juice concentrate | ||||
| Issues for future study | • Individual responsiveness to nitrate supplementation in distance running, including the effect of athletes’ caliber as it seems less effective in elite athletes (Jonvik et al., 2015) • Effect of protocols involving intake during distance events to maintain elevated plasma nitrite concentrations (Tan et al., 2018) • Interaction with other supplements—CHO, caffeine, etc. • Effect of nitrate on heat storage and performance in hot, humid environments (Kent et al., 2018) • Use of nitrate to promote training capacity/quality, including the use during altitude training | ||||
| Investigations | Study design | Nitrate protocol | Performance protocol | Effect | Comments |
|---|---|---|---|---|---|
| Shannon et al. (2017) | • Well-trained runners/triathletes (8 M) • Crossover design with nitrate vs. placebo | • ∼12.5 mmol (beetroot juice) taken 3-hr pretrial | • 10-km treadmill TT | No benefit detected | No difference in 10,000-m TT between nitrate vs. placebo (2,643 ± 324 vs. 2,650 ± 320 s, p > .05), although the same athletes performed better on a 1,500-m test in the same study. |
| De Castro et al. (2019) | • Recreational runners (14 M) • Crossover design with nitrate vs. placebo | • ∼8 mmol (beetroot juice) taken for 3 days and 2-hr pretrial | • 10-km treadmill TT | No benefit detected | No difference between nitrate and placebo (50.1 ± 5.3 vs. 51.0 ± 5.1 min, p = .391) for 10 km, although time to complete the first 5 km was lower in the nitrate group (p = .027). |
Note. M = male; TT = time trial; CHO = carbohydrate.
Table 2 summarizes the role of sports foods/drinks in achieving the goals for CHO and fluid intake during distance events. While the known benefits of these strategies provide a benchmark against which the magnitude of any effects from other performance products should be compared, these also provide a potential confounder of the effectiveness of other performance supplements. For example, a meta-analysis of a heterogeneous group of studies of caffeine supplementation and endurance performance (Conger et al., 2011) found that the margin of improvement when caffeine was consumed in addition to CHO was significantly reduced (but still worthwhile) in comparison to scenarios involving a water placebo (mean effect sizes = 0.26 vs. 0.52, p = .006). This illustrates why potential interactions between concurrently used supplements or nutrition strategies are of high priority for scientific investigation and specific consideration when developing race plans or training uses (Burke et al., 2018d). The efficacy of caffeine during endurance sports may be correlated with its role in masking fatigue (Spriet, 2014); therefore, in situations in which another strategy reduces the onset or magnitude or fatigue, a smaller effect on performance is logical. Conversely, in scenarios of increased fatigue such as “training low” with endogenous CHO stores, caffeine may provide a greater benefit in helping to attenuate the reduction in training capacity (Lane et al., 2013). Other issues associated with caffeine or nitrate use in distance Athletics are noted in Tables 4 and 5. Finally, the potential for enhanced glycogen storage following creatine supplementation (Roberts et al., 2016) merits further investigation in terms of increased CHO availability for the longer distance races; however, such benefits should be balanced against the likely increase in BM (Tomcik et al., 2017).
Strategies for Hot Environments
Major championships are often held in hot and/or humid environments, with the Doha 2019 World Championships and Tokyo 2020 Olympic Games being the immediate targets at the time of preparation of this review. There are multiple and circular interactions between the hot environment and nutrition; exercise in the heat creates extra challenges in terms of increased rates of fluid loss and glycogen use (Jentjens et al., 2002), with dehydration increasing the risk of gastrointestinal discomfort/upset (Rehrer et al., 1990) and further interference with nutrition status and goals. Meanwhile, fluid intake reduces thermal stress (Montain & Coyle, 1992), and CHO intake reduces gut damage (Snipe et al., 2017). The performance and health challenges associated with racing in hot weather should be addressed by strategies, such as acclimatization, appropriate pacing, and precooling activities (Racinais et al., 2015). Adjustment to race nutrition strategies, if practical, may also assist (Table 2). For example, a more aggressive approach to in-race hydration strategies to address greater fluid losses may be possible, while hyperhydration during the hours before a race via the consumption of large amounts of fluid together with an osmotic agent (e.g., glycerol or sodium) can reduce the net fluid deficit incurred over the race (Goulet et al., 2007; van Rosendal & Coombes, 2013). Here, we note that glycerol has been removed from the World Anti-Doping Agency’s list of prohibited substances and may be reinstated for use in hyperhydration/rehydration strategies. The intake of ice slurries within precooling strategies to reduce prerace core temperature via the “heat sink” created by the phase change from ice to water may also be beneficial (Jay & Morris, 2018; Ross et al., 2013). Where in-race fluid intake is practical, mouth sensing of cold water or menthol may provide a sense of cooling during a race to reduce ratings of perceived effort (Stevens & Best, 2017), while intake of reasonable amounts of cold/icy beverages might theoretically contribute to improved thermoregulation (Jay & Morris, 2018). The literature on the specific benefits of these strategies (see Table 6) in high-performance running or racewalking scenarios is sparse; an investigation is required, including the assessment of potential disadvantages such as an increase in BM or a greater risk of gut disturbances. In the meantime, athletes should practice the intended use of these strategies before implementing in a race.
Summary of Hyperhydration and Cooling Strategies With Relevance to Events in Distance Athletics
| Prerace hyperhydration (for review, see van Rosendal & Coombes, 2013; Goulet et al., 2007). | |
|---|---|
| Mechanism of action | • Fluid retention achieved by the use of an osmotic agent (glycerol or sodium) in fluids consumed in the hours before exercise increases body fluid stores; allows greater sweat losses during exercise to occur before the net fluid deficit becomes physiologically significant and impairs performance |
| Best practice protocol | • 25 ml/kg of fluid consumed ∼2-hr pre-exercise with ∼1-g/kg glycerol or 7-g sodium chloride; typically aids in the short-term retention of ∼600-ml fluid to add to body water stores |
| Issues for future study | • Which is the most effective osmotic agent? Can a combination of osmotic agents increase fluid retention? • Does the gain in body mass associated with fluid gain create a performance disadvantage? • What are the other side effects (e.g., headache, gut upsets) associated with hyperhydration strategies? |
| Ice slurry for precooling and within race cooling (for review, see Jay & Morris, 2018; Ross et al., 2013) | |
|---|---|
| Mechanism of action | • Internal heat transfer from cold drink or the enthalpy of fusion of ice (phase change from solid to liquid) may reduce total body heat content and allow greater duration or intensity of exercise before thermoregulatory challenges become significant and impair performance |
| Best practice protocol | • Ice slurry: ∼14-ml/kg of fluid consumed in two servings in the 30–60 min pre-exercise (i.e., immediately before abbreviated race warm-up) to allow time to excrete excess fluid if needed. Should be combined with external cooling strategies (e.g., cold-water immersion or ice towels/vest) to provide additional effects, which might be continued during/after warm-up |
| Issues for future study | • What is the most effective combination of internal and external cooling for each specific event, taking into account the logistical issues (timing of warm-up and race, facilities in race setting) as well as thermal challenges? • What is the effect of precooling on pacing strategies? Can precooling be detrimental if athlete misjudges perception of effort in the early party of race and chooses an unsustainable intensity causing a higher thermal load than can be tolerated? |
| Mouth sensing of “cool” during race with menthol (for review, see Stevens & Best, 2017) | |
|---|---|
| Mechanism of action | • Exposure of L-menthol in the oral cavity activates transient receptor potential channels eliciting a cold sensory perception in the brain • Offers the opportunity to reduce thermal sensation/discomfort without changing body heat load to improve performance in the heat |
| Best practice protocol | • Mouth rinsing or consumption of L-menthol in fluid or other agents (e.g., confectionery) • May be potentiated when combined with a cool fluid |
| Issues for future study | • What is the optimal concentration and vehicle for mouth rinsing with menthol? • Is the effect on thermal sensations repeatable throughout the race? • Is there a danger, to health or performance, of using artificial sensations of “cooling” during exercise in the heat if the athlete chooses a pace that leads to a higher thermal load |
Commentary 1: Dietary Practices of East African Runners
East African athletes have dominated distance running for decades, with their superior performance drawing speculation about a range of potential contributing factors (Larsen & Sheel, 2015), including their striking dietary practices and specific anthropometric features (Burke et al., 2018b; Mooses & Hackney, 2017). Although dietary surveys of Kenyan and Ethiopian runners have been limited to their home environments and training camps (Beis et al., 2011; Christensen et al., 2002; Fudge et al., 2006, 2008; Onywera et al., 2004), it appears that they maintain their eating practices on the competition circuit or in their Northern Hemisphere training bases because of the low cost and cultural familiarity, as well as self-belief that it might contribute to their success (M. Mooses, personal observations, Dec 10, 2018). A range of features, both consistent and in contrast to current sports nutrition guidelines, merit comment.
The diets of East African runners contain substantially different contributions of foods and macronutrients compared with Western practices; indeed, CHO supplies 60–80% of energy, with high reliance on vegetables (80–90% of diet) rather than animal food sources (10–20%), and limited food variety (staple foods: rice, pasta, potatoes, porridge, cabbage, kidney beans, ugali maize meal, and injera flatbread). Typical fluid choices include water (0.9–1.1 L/day) and tea (∼0.9 L/day) with brown sugar and (for Kenyans) milk (Beis et al., 2011; Onywera et al., 2004). Daily energy intake is distributed over a small number of meals, with prolonged moderate- to fast-paced morning runs being undertaken before breakfast and with nil/minimal intake of fluid (Beis et al., 2011; Fudge et al., 2006, 2008; Onywera et al., 2004). Meanwhile, meals are consumed soon after training sessions, and high-intensity track sessions are completed as a midmorning workout after breakfast. Indeed, many concepts of periodizing CHO availability according to the needs of the session (Burke et al., 2018c) appear within these traditional practices. Although supplements are rarely used, data from observational studies (Beis et al., 2012) and accounts of recent attempts on world marathon records by male runners (Caesar, 2017; Hutchinson, 2013) note personalized race nutrition plans including proactive intakes of fluid and CHO, often with the involvement of Western sports scientists.
Also of topical interest is the reported or suspected prevalence of acute or chronic periods of low energy availability among these athletes. Notwithstanding artifacts in dietary survey methodology and calculations of energy availability (Burke et al., 2018a), there are consistent reports of low energy intakes relative to calculated or expected exercise energy expenditures in various groups of East African middle- and long-distance athletes (Fudge et al., 2006, 2008; Onywera et al., 2004). Contributors to energy mismatches include cultural eating patterns (e.g., fiber-rich unvaried diet, few eating occasions in a day), food insecurity, and the interaction with high training loads (e.g., lack of intake during training hours, postexercise appetite suppression; Burke et al., 2018b). Although the role of deliberate manipulation of BM/composition for performance purposes is unclear, some involvement is likely because low energy intakes have been observed during precompetition training camps (Onywera et al., 2004), and the majority of African athletes report that their “ideal racing weight” is lower than their normal training BM (Mooses, personal observations, Dec 10, 2018).
Further study is needed to consolidate our understanding of the dietary practices of these highly successful athletes and how much they contribute to, or interfere with, optimal performance. It is likely that practices include both helpful and harmful features, as well as accidental and intentional elements. As for any group of athletes, an audit of practices may identify the potential for performance improvement, but various practical and personal issues need to be taken into account.
Commentary 2: Modeling the 2-hr Marathon Barrier: Is CHO a Tool?
Nearly 100 years ago, Krogh and Lindhard (1920) reported that energy derived from the metabolic consumption of O2 depends on whether fat or CHO is the primary source of carbon substrate. For example, increasing the respiratory quotient (RQ) from 0.85 to 0.90 (49% to 66% contribution from CHO) results in a 5% increment in released energy (4.967 vs. 4.921; Krogh & Lindhard, 1920). In the D.B. Dill lecture at the 2015 annual conference of the American College of Sports Medicine, Professor Ron Maughan identified the important implications of this finding for marathon performance; an increase in RQ improves metabolic efficiency by reducing the O2 cost of running at a particular speed or permitting a higher speed for the same absolute
With regard to the challenges of a sub-2-hr marathon, if we assume a running economy of 190 ml·kg−1·km−1 at 21.1 km/hr and BM of 55 kg, the total energy cost of running 42.2 km is calculated at ∼2,200 kcal (9,210 kJ). Theoretically, this could be provided by CHO (550 g) in the form of supercompensated muscle and liver glycogen stores supplemented by an aggressive approach to consuming CHO during the race. The total O2 cost of oxidizing CHO alone or fat alone would be 435 L versus 459 L, respectively (Krogh & Lindhard, 1920). In our hypothetical athlete with a
Conclusions
Distance athletes should adopt nutrition strategies that address specific physiological and biochemical factors that otherwise limit performance. These include periodized support for specific goals of workouts or phases within the training program and as summarized in Table 2, nutrient choices prior and/or during the race to maintain optimal fuel and fluid status. In-race nutrition is dependent on practicalities, such as the availability of aid stations as well as time and gut considerations of consuming CHO-containing fluids or other sports products. Finally, several performance supplements, particularly caffeine and nitrate, could be considered for likely and potential benefits, respectively.
Acknowledgments
All authors contributed to the preparation of this manuscript. The authors declare no conflicts of interest in the preparation of this review.
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