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Marathon Fueling Techniques: Physiologic Understanding and a Proposed Intake Schedule

Bennett, John R MS, CSCS; Kehoe, Michael P PhD

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Strength and Conditioning Journal: October 2008 - Volume 30 - Issue 5 - p 56-65
doi: 10.1519/SSC.0b013e318187f1b5
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Abstract

MARATHON FUELING TECHNIQUES: PHYSIOLOGIC UNDERSTANDING AND A PROPOSED INTAKE SCHEDULE

Understanding an athlete's metabolic profile may be an individual's blueprint for success. Athletes do not intentionally neglect race nutrition (13,29); many simply do not understand the need for a nutrition schedule in prolonged events, whereas others attempt nutrition but may not fully understand the need. When an athlete lacks an understanding, the accuracy and effectiveness of any nutritional schedule attempted will likely be inconsistent or ineffective. This discussion provides a resource for performance specialists and athletes alike to learn and understand competition nutrition, avoiding diversion into training nutrition, an important but different subject. The foundation of analysis is for endurance athletes specifically focusing on the marathon.

General intake recommendations and guidelines exist for prolonged aerobic activities, including the marathon (3,18,32,44,52,69). Existing principles are intended to cover the entire performance spectrum, from first-timers to world champions. The intention is to focus specifically on the upper-echelon marathoner, or those with performances of less than 3 hours, with our focus primarily towards the elite. This parameter was established for 2 purposes. Primarily, top-end athletes are most concerned with the finite points of performance rather than simply race completion. Here, the interest lies in advancing performance by even the smallest fraction for advantage over the next athlete, rather than life-achievement endeavors. Second, elite athlete performance is less variant. Although each individual is unique, science inarguably agrees that athletes with a strong training history have set adaptations that have brought them thus far (6,8,22,58,76,81), whereas recreational athletes have not necessarily maximized their potentials to the last percentages. Additionally, elite athletes have demonstrated their commitment and focus on achieving maximal capacity-they exist in a state of readiness to breach the next threshold.

ENDURANCE PHYSIOLOGY

In the sports world, intensity mostly relates to near or supramaximal short-duration bursts associated with power athletes. For aerobic activity, as compared with an instant effort, the force put forth is substantially less. Marathoners, for example, compete at a pace between 75% and 90% of their maximum intensity (6,7,27,40,53,76). By comparison, 100-meter sprinters perform almost exclusively in a state of oxygen debt at 110-120% (8,83). Intensity has several measures, including heart rate (HR), lactate, and power, but oxygen consumption (V̇O2) is the most popular aerobic power measure. For the context of this review, intensity is relative only to that of prolonged aerobic performance.

An athlete may start running at a 5-minute mile pace and maintain a heart rate reserve (HRR) of 75%. However, that continual relative effort shifts as performance continues, increasing the heart rate to as much as 95% HRR before the end of the event (7,26,52). On the other hand, if an athlete were to perform solely based on intensity with 85% HRR being the accepted elite marathon performance norm (7,64), the pace would have to decline to maintain a consistent heart rate (35). At some point, the heart rate may continue to increase unless the activity is ceased altogether (21,33,34).

Aerobic intensity truly exists at any level that uses oxygen for work. However, the focus herein is competition, a rate rarely performed at below 50% of maximal effort, even in week-long ultra-endurance events (29,52,55). During aerobic competitions lasting 90-180 minutes, top athletes perform at 80-90% V̇O2max (6,7). Even a trained athlete's submaximal effort for a 5-minute run becomes near-maximal exertion by the end of the second hour. This absolute work rate refers to the total fuel necessary for the task, whereas relative work rates suggest the substrate portioning, which will be discussed later in the review (58).

With this suggestion, it becomes apparent that the energy systems have shifted. Regardless of activity, the systems steadily progress through a fixed path to eventually end up at the final state most dependent upon intensity-duration is secondary only to the body's ability to maintain a given relative intensity (19,56,78). The 2 primary factors affecting duration at controlled threshold intensity are energy metabolism and hydration.

METABOLISM

Throughout the majority of the race, an athlete's aerobic metabolism taps into stored glycogen and intramuscular triglyceride (IMTG) sources, requiring continued glucose availability for both efficient fat metabolism and glycogen sparing (9,23,44,47,81,84). A growing shift to stored glycogen signifies a decreasing ability of aerobic metabolism to process anaerobic waste. This may be demonstrated as an increase in lactic acid, but before its sharp increase to the point of detriment (27,38,50,80). Performance in this range for the elite athlete requires a fine tuned carbohydrate-fat usage. Both have benefits and downfalls. Fatty acid (FA) oxidation is limited by the availability of carbohydrates (CHO); however, carbohydrate alone is a direct measure of energy reserve potential to a marathoner (Figure 1) (4,38,48,53).

Figure 1
Figure 1:
Benefits and drawbacks to metabolic sources.

If the athlete were performing in a low-intensity event, CHO would be of much less importance. Figure 1 illustrates the benefit of providing a CHO energy source for high-intensity aerobic events. Training allows the elite athlete to burn fat more efficiently and at a greater intensity (38,52,53,81). The idea seems antithetical to the rapid energy source benefit from CHO. The key for the athlete is to shift energy reliance from CHO to fat as much as possible without affecting performance. Even minimal shifts will have tremendous savings with regards to sparing glycogen sources (4,17,47,81,84). The elite athlete must determine a point of diminishing returns for success.

Glycogen versus glucose

The body has a limited capacity to perform at high-intensity levels. Muscle glycogen, the primary fuel source for marathoners, provides approximately 2 hours of energy. Performance will decline before muscle glycogen stores are truly depleted. Decrements may occur as early as 50% capacity, although some runners may be able to maintain performance without decrement until 70% depletion (14,19,53,76,84). Beyond this point, athletes are unable to maintain marathon intensities, or often even intensities greater than 70% V̇O2max.

The liver stores approximately 20% of the body's glycogen, which is much less than the muscular system, but the primary hepatic function is blood sugar maintenance (3,26,38). Jeukendrup and Jentjens (48) clearly demonstrated that the liver has a rate limitation to spare substrate over time rather than overly rapid use. An important point to consider is that glycogen yields 4.2 kilocalories (C) per gram, whereas glucose yields 3.7 C/g (55,83). Research strongly demonstrates a preference for muscle glycogen over endogenous blood glucose for several well-understood reasons (14,25,46,47,78,82):

  1. Intramuscular glycogen is immediately available, not requiring transport to the working muscles.
  2. Although glycogen is a more complex substance, the process to yield energy is more efficient than with glucose.
  3. The amount of circulating blood glucose is negligible (approximately 5% of the energy potential from glycogen) for long-duration activities. It is intended more for vital functions, mental awareness, and quick energy bursts.

These comparisons also illustrate that a relatively greater lean mass aids in athletic performance (5,24). Women tend to have a proportionally lower glycogen store simply as the result of anatomical averages of lower lean body mass percentages than men. Women, however, are more efficient users of lipids (40); whether this factor or others affect their maximum pace is beyond this discussion.

Why lipids alone don't work

Although a lean athlete has greater than 25 times the amount of calories available from fat as compared with CHO, fat usage actually has several disadvantages (3,9,18,38,81):

  1. Intensity must be low enough to prevent cellular oxygen depletion.
  2. Fat metabolism requires more oxygen usage per C expended as compared with CHO.
  3. Fat metabolism requires one CHO molecule for every FA molecule metabolized.
  4. The most immediate source of fat for muscles is IMTG. However, IMTG must first be broken down to its usable FA parts.
  5. Fat metabolism requires nearly twice as long to provide energy as aerobic carbohydrate metabolism.

The body adapts well to training stimuli placing the body at threshold levels. This teaches the body to use a slightly greater percentage of fat at increased intensities (4,6,13,18,23,76,81,84). Although the percentage is relatively minimal, with the high-energy yield from lipids, the result is both statistically and practically significant. Consider the following example.

If an athlete typically uses 90% carbohydrate and 10% fat at 85% V̇O2max, proper training will allow the athlete to use 85% carbohydrate and 15% fat at the same 85% V̇O2max pace. At 120 C/mile, that's a savings of 37 g or 156 C of glycogen, enough to complete 1-1.7 miles at the same 85% intensity without depleting carbohydrate energy reserves. Most elite marathoners that have hit the infamous wall experience their crash within the last 6 miles of their race (32,43,49,53,64). Fats cannot be metabolized anaerobically and, as performance intensity approaches the lactate threshold, the likelihood of fat being used decreases (79). Hawley (38) cites traditional concepts suggesting that this shift comes from a decrease in plasma FA as the result of a decreased blood flow to adipose (and an increased flow to muscles for glycogenolysis). Decreased flow to adipose means less delivered albumin to transport free fatty acids (FFAs). Hawley references an argument for mitochondrial efficiency. The evidence suggests that increased glycogen breakdown may “inhibit the entry of long-chain fatty acid… into the mitochondria” (38). This inhibition is cyclical: it results in increased acetyl CoA and pyruvate, which in turn also inhibit a particular fatty-acid transferase for FFA mitochondrial entry. Nonetheless, no effect has been shown on medium-chain FA.

HYDRATION

Aside from fueling, fluid availability is the other ultimately limiting factor for performance. Hydration research on performance has a longer-standing history than sports nutrition and deals with only one fixed factor: water. Dehydration can begin by several methods. The main concern in sport is the hypotonic water loss through sweat. A dehydration flowchart demonstrates the interrelation of any single response to all others when losses are not offset (Figure 2) (12,44).

Figure 2
Figure 2:
Dehydration flow chart showing direct and indirect effects of low blood volume.

As a direct result of the sweat lost to cool the body, blood volume decreases (11,12,34). With a lower absolute volume of blood returning to the heart, ventricular filling volume is decreased, thereby decreasing stroke volume. With this decreased stroke volume combined with a total decrease in absolute volume, the HR increases to compensate. A concern with the compensation is that it has limited success and creates an increased workload (oxygen demand) on the heart. Additionally, the increased respiration rate and often loss of rhythmic pattern is barely adequate to meet the demands of the respiratory musculature, let alone the heart and peripheral system (12,21,33,69,80). The absolute amount of oxygen delivered is decreased simply because its medium, the blood, is not circulating to the working cells in as high a volume. Combined with the heart requiring more oxygen for itself first, the peripheral system must shift to an increase in anaerobic pathways to meet the energy demands. Such a shift also causes an increase in HR. The overall impact on the HR and relative blood volume compromises the body's ability to cool itself by shunting blood to the skin for air contact.

The body becomes unable to keep up with the mitochondrial heat output, increasing core temperature from fuel catabolism, up to 80% of the energy released. Needing blood for continued work, muscles compete for a greater percentage of the already-low volume shunted to the skin for cooling. The result is less blood to serve the needs of oxygen and nutrient delivery, as well as waste elimination. From here, the previously described pyruvate cycle begins (3,55,83). As time and intensity progress, lactic acid levels increase, with less oxygen available to convert the pyruvic acid (79). The concomitant hydrogen ion accumulation inhibits aerobic metabolism by way of displacing calcium ions. Once below 6.9 pH, a resulting increase in citrate levels (53) affects the efficacy of phosphofructokinase, the aerobic rate-limiter. At this point, myoexcitation becomes inhibited. Thus, once again, intensity is the primary controlling factor.

CALORIC COSTS

A variety of methods exist for calculating caloric expenditure, ranging from research-based mileage expenditure to individual metabolic assessment. However, most methods are not appropriate for the population or purpose at hand. Most formulas are typically for sedentary to average individuals, although some formulas do account for levels of fitness (42). No equation found is specifically appropriate for the elite athlete, let alone one that specifies between aerobic, anaerobic, or mixed-power athletes.

A few general rules seem to be widely accepted in sports nutrition, much like the theoretical 220 beats per minute maximum HR for exercise calculations. Bookstore publications for the layperson suggest that running expends a confounding range of 90-160 C per mile (1,3,43,53,64). None of these citations, however, provide clear guidelines about intensity reference for the population studied-a 7:30 pace may be 60% V̇O2max for one athlete, yet 90% V̇O2max for another (5,70). The variation is likely the result of the wide range of performance in data collection, with 50-90% of maximal efforts considered valid to generalization (26,36).

A well-conditioned athlete expends the same amount of energy on a fixed distance (24,53) “regardless of whether the run takes just over 2 hours, 3 hours, or 4 hours!” (55). Although twice the duration is spent running at 5 mph as compared to 10 mph (12 min/mi and 6 min/mi, respectively), the net energy cost is nearly the same. For most elite marathoners who weigh less than 75 kg, this suggests the expenditure is well below the preceding 90-160C/mi recommendations. Additional research (5,57,76) supports V̇O2 and speed are in linear proportion. Nonetheless, the level of efficiency and expenditure is more weight and heat dependent (24) between athletes. Individually, it must be remembered that “speed increases heat more than it dissipates it” (24).

Although no method calculates precisely how much energy one individual athlete will expend during a race, 2 items narrow the variability: testing and experience. An athlete does not perform the same each time, and each season is different. The goal is to hone the estimate as best as possible without adverse effects (Table 1). Consulting an appropriate certified sports nutritionist may prove invaluable, especially when the level of expertise required exceeds the scope of practice of a certified trainer.

Table 1
Table 1:
Substrate expenditure per intensity

STRATEGIES

It is suggested that a marathon is not a 26.2-mile race, but rather a 6.2-mile race (32,43,64,69). The trained body undoubtedly has enough resources to perform maximally for about 20 miles (38,53,55) with the final 6.2 residing in strategy. As illustrated by Table 2, the human body in top form has enough energy to perform at maximal aerobic capacity for about 90-120 minutes (14,17,19,56,76). Most athletes never approach this level of intensity for such duration.

Table 2
Table 2:
Total caloric capacity in trained athletes

The elite and aspiring-elite athletes are most interested in the ability to maximize their capacities. Yet even with the best training minds, most endurance athletes still never succeed on physical ability alone. If the body only has limited fuel and no formula exists to internally amplify the energy, then how does one suggest crossing this threshold?

The key component to superseding ability is to think outside the body. Exogenous fuel during performance is the only way that triathletes, marathoners, tour cyclists, and other long-endurance athletes can thrive (9,28,38,50,78). Fuel intake during a marathon has 2 reflective purposes:

  1. To provide a continual fuel source for immediate consumption
  2. To provide aerobic fuel methods to spare resources for the last 6.2 miles

Although cited research is inconclusive on the precise intensity-expenditure relationship (26,36,70), it seems that, for a given athlete, the faster the pace, the greater the calorie burn per minute (but not per mile) at a constant training state. The more efficient the runner (24,70), the lower the calorie burn at a constant intensity (5,24,57).

FACTORS

Facts recapitulated:

  • CHO is the primary source of energy at high aerobic intensities, 80-90% V̇O2max, used during a marathon.
  • Muscle glycogen is the preferred source for muscular work.
  • The body only has enough muscle glycogen for about 20 miles if using 75-90% of its caloric expenditure from muscle glycogen.

Consuming 2,000 C during a race is simply not practical. Intake of a 6% carbohydrate solution as found in the most popular commercial fluid-replacement beverages (1,3,69,75) would require 8.3 liters intake to meet this requirement. Precompetition gastric fluid levels are recommended, if tolerated, to be as high as 600 mL (10,31,62,68,71). If an athlete consumed the recommended 180-240 mL 6-8% solution every 15 minutes during competition, a fluid quantity rarely met (25,41,63,67,74), the athlete would take in 167-223 C/h and only 2 of the 8.3 liters. Elite men complete marathons in less than 2.25 hours, and women 2.5 hours, respectively providing a caloric intake of 502 C and 582 C during the race. Adding one carbohydrate gel pack (usually 110-120 C) every 45 minutes still leaves the athlete far short of the supply goal.

A key to remember is that the fuel cannot be consumed in high gradients throughout the entire race. If consumed in the beginning, blood glucose levels would maximize and glycogen stores would already be maximized, allowing little influx, raising plasma glucose (48,61). Continual, smaller feedings are more feasible for runners at high intensities. Coggan and Coyle (14) did not evaluate the marathoner but did provide a good model of single feedings. They performed cycle-based research evaluating single bolus mid-exercise. The single feeding took place after the 2-hour time period and used a 50% concentration. The consumption would be late for the sub-3-hour runner and a marathoner could not consume such a high concentration solution. Furthermore, the decline in physiologic (blood glucose) and performance (respiratory exchange ratio) measures in the study were already declined below desirable levels. Ivy et al. (45) demonstrated that carbohydrate consumption did not actually improve performance in the first 60 minutes of exercise but did improve fatigue resistance in the last 30 minutes of exercise. McConnel et al. (56) compared continual feeding of a 7% solution to a single-dose 21% solution administered at 90 minutes of exercise. Continual 7% ingestion allowed athletes to perform more total work and achieve a higher percentage of V̇O2max with no difference in respiratory exchange ratio. Similarly, athletes are better able to maintain constant velocities when consuming a 5.5% solution as opposed to water or a 6.9% solution (77). Hawley et al. (39) further supported the Jeukendrup and Jentjens (48) validation of how carbohydrate oxidation rates are limited to just greater than 1 g/min, regardless of how much fuel is consumed, whether in single or multiple feedings.

Coping with the declining capacity of endogenous carbohydrates is a challenge. The ultimate limitation falls with gastric emptying, usually around 1 L/h, decreasing significantly as intensity progresses to more than 70% V̇O2max (20,51,62,65,72). Athletes can undoubtedly train themselves to be able to improve ingestion rate. The solubility provided in a sports drink has 2 benefits: readily accessible fuel and rapid fluid uptake. The universal 5-8% concentration provides optimum fuel while not compromising water uptake by the small intestine (18,62,75). While tempting to use a greater-concentration fuel, most research has agreed that a solution greater than 8% slows gastric emptying (18,23,26,30,75). Furthermore, by pulling fluid into the intestinal lumen to facilitate absorption, high-concentration fluids can actually invoke a relative dehydration (46,69,73).

Carbohydrates are rapidly taken up in the duodenum, almost immediately after passing the pyloric sphincter (31,51). The glucose-sodium “facilitative diffusion” (55) alters the relative osmotic gradient, thus pulling water with it. The duodenum has a capacity limit; when this limit is reached, the stomach stops emptying to allow time for uptake. Furthermore, water shifts into the small intestine to aid in diluting the substance (46,48,69).

RACE INTAKE SCHEDULING

Regardless of what research demonstrates or how complex a formula may be, each athlete must be treated as unique. It would be ideal to have a formula to provide a precise intake schedule for each athlete. However, such charting would require extensive data, much of which changes with the season: age, gender, total mass, lean mass, lactic threshold, vV̇O2 (V̇O2 at a given velocity), V̇O2max, projected pace, projected acceleration, fatigue rate, temperature, humidity, sweat rate, gastric tolerance, and fuel product. Each factor is a key in determining an athlete's caloric expenditure per mile at separate stages of the race. Unfortunately, no clean universal formula exists to calculate such precise expenditure. Even the mild prediction equations that do exist have an average population and are not likely to be accurate for highly trained athletes.

Barring experience and the existence of such a precise running formula, the factors discussed herein can lead to a generic-yet-accurate intake method. Studies have shown continuous smaller feedings are as effective as a significant feeding shortly before fatigue, generally at the 2-hour mark (15,48,56,78). Although Coggan and Coyle (15) showed an improvement of a single feeding over placebo, there was no comparison to continual feeding. Before the CHO feeding, there was a decrement in performance and metabolism, an undesired undulation from steady-rate performance. The benefit is that the continuous feedings are likely to promote adequate intake of fluids and calories. Furthermore, less flux allows a more steady systemic response, decreasing risk for adverse reactions.

Competition and climate factors can make consuming the appropriate amount of fuel and, especially, fluid difficult. Single feedings can lead to discomfort and gastrointestinal (GI) distress. Furthermore, larger feedings require more focus on the GI system, attenuating blood flow from the muscles (3,33,68). Athletes are safer at fast paces with continual fueling attributable to greater oxygen demands for the muscles (48). If any adverse event were to occur that prevented the athlete from taking in sustenance later in the race, they, at minimum, will have partially offset caloric fatigue by their earlier ingestion. A low steady fluid and fuel intake early helps with prehydration, which is proven to be more effective in combating dehydration than any other strategy (54,60,68,79).

With endocrine changes not resulting in a noticeable decline in energy reserves until 30 minutes or more, it is sensible for feedings to occur at some point thereafter. Race experience and training with fueling techniques is paramount. Once a bolus feeding has occurred, low-dose maintenance should continue. Maintenance allows the GI tract to remain primed (65,66) but, more importantly, it also allows the bolus intake to be of longer duration as opposed to a spike and drop-off in energy uptake (15,17,54,56,59).

APPLICATION

Table 3 provides a basic example of race fuel strategy based on the analyses provided in this review. Few racers maintain an exact pace throughout a race. Many strategies have racers progressing after mile 15, whereas others drastically accelerate after mile 20. Each racer's fuel chart must accommodate their strategic aerobic intensity at each race stage. Note that the latter stages of the race, however, have less focus on simple hydration. At this point, fuel stores are becoming scarce, and hydration effects are difficult to counter without disturbing the need for late, low gastric volumes but high−energy sources (2,66,78). Wilmore and Costill (83) validate this point by stating, “…in later stages of an endurance event, blood glucose may make a larger contribution.”

Table 3
Table 3:
Marathon fuel schedule

FUTURE DIRECTIONS

The proposed schedule is undoubtedly a pilot theory. Several other factors must be evaluated and accounted for in developing a true template for other influences, namely individual sweat rates (12,37), consumption tolerance (59,67), running speeds (18), and environmental conditions (16,28). Decreased CHO proportions must be considered in the first hour of exercise (18,45,47,50,78) and in hot conditions (16,28,30,65). Table 3 intentionally violates this rule based on anecdotal experience, but will be modified as necessary based on future research.

Controlled studies and training sessions allow for ideal time-based intake. Aside from select events, races are geared toward per-mileage set-ups to accommodate an array of paces; therefore, it becomes difficult for the fastest racers to establish a personal fuel and hydration resource set-up for the course. Most races have hydration and sports drink stops every mile, intermittently including foods and other fuels. Reasoning from there, race-day fueling and hydration must be founded on mileage as opposed to time.

Although in this review we address the main acceptance of a rough 1 g/min CHO oxidation rate, other authors suggest that oxidation rates up to 2 g/min may be possible (14,82). Coggan and Coyle used very high concentrations (20% and 50% solutions) of glucose polymers and sucrose, whereas Wallis et al. used a specific 11.25% maltodextrin-fructose combination to achieve these rates. Although both studies were conducted on cyclists who have a different tolerance, the results do hold hope for runners who generally tolerate lower volumes and concentrations (67).

A review of the literature seems to indicate a relationship between percent V̇O2max and percent carbohydrate oxidation (Table 1). The fact that these percentages mirror one another must be investigated further. For example, does 75% V̇O2max equal or approximate 75% carbohydrate oxidation in highly trained athletes? Jentjens et al. (46) calls for even more detailed understanding of carbohydrate type in fueling. Future expansion on this investigation will include total concentration and osmolality analysis for total race intake. A procedure must be set forth for developing the hydration and fueling schedule for each runner. A quality model must be developed that is simple to follow. With the amount of existing research, a guiding template is feasible, where coaches spend their time assessing the athlete and minimal time perfecting the tool.

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Table
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Keywords:

caloric cost; endurance; energy; glycogen sparing; hydration; nutrition; substrate

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