Carbohydrates and The Distance Runner: A Scientific Perspective

by Jason R. Karp, M.S.

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Jason Karp provides a summary of what the latest scientific research tells us about glycogen storage and synthesis, and the most effective amounts, types, and frequency of carbohydrate ingestion for endurance running training.

    A classic view of muscle fatigue is the limitation in energy supply. During prolonged endurance exercise, muscle glycogen (the stored form of carbohydrates) represents the greatest limitation, with fatigue coinciding with glycogen depletion .
    It has been known since the late 1960s that the ability to perform endurance exercise is strongly influenced by the amount of pre-exercise glycogen stored in skeletal muscles, with muscle glycogen depletion becoming the decisive factor limiting prolonged exercise . With the well-documented decrease in muscle glycogen content that accompanies endurance exercise, an empty-refill cycle becomes evident. When muscle glycogen is depleted by prolonged exercise, muscles respond to the empty tank by synthesizing and storing more than what was previously present. Empty a full glass, and you get a refilled larger glass in its place. Much like college fraternity parties.
    For most individuals, the synthesis of muscle glycogen and subsequent endurance exercise performance can be supported by eating a normal diet. There just simply isn't enough of a chronic drain on muscle glycogen stores to concern oneself with a scientific strategy for optimal recovery.
    Distance runners, however, are a unique breed. Doing two workouts a day and running 15 miles on a Sunday morning places a chronic strain on their storage of fuel. Recognizing the importance of carbohydrates as a fuel source, distance runners may be the only people who regularly gather for pasta parties.
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    Over 30 years ago, Costill and colleagues showed that running 10 miles a day at 80% VO2max for just three consecutive days was enough to markedly decrease muscle glycogen, despite eating a mixed diet (e.g., 40-60% carbohydrates, 30-40% fat, 10-15% protein). While early investigations reported that it takes nearly 48 hours to replenish glycogen stores to pre-exercise values, more recent evidence shows that glycogen can be restored within 24 hours when using an optimal dietary strategy.
    It is not uncommon for elite distance runners to train twice a day, as recent research from our laboratory on U.S. Olympic Marathon Trials qualifiers supports. Surely the performance of two workouts a day would necessitate an even greater need to recover quickly. Not only do the adaptations to training occur during the recovery period from the training rather than during the training itself, the rapidity with which athletes recover from a long or intense workout will dictate how often they can perform other long or intense workouts, which may ultimately influence their ability to reach their athletic potential. Therefore, strategies for optimal recovery are needed for competitive distance runners.

GLYCOGEN SYNTHESIS
    The human body responds rather elegantly to situations that threaten or deplete its supply of fuel. A metabolic priority of recovering muscle is to replenish muscle glycogen stores. And the more the tank is emptied, the faster the rate of refilling.
    For example, Zachwieja and colleagues found that glycogen was synthesized significantly faster during six hours post-exercise in a leg that was exercised to elicit a large degree of glycogen depletion compared to the opposite leg that was exercised to elicit only a small degree of glycogen depletion. The rate at which muscle glycogen is replenished depends primarily on the hormone insulin and the availability and uptake of glucose from the circulation. Using two different muscles extracted from mice, Bonen and colleagues observed that glycogen synthesis significantly increased when both muscles were incubated with insulin and increased linearly with increasing concentrations of glucose.
    Examining muscle glycogen synthesis in exercising rats, Johnson and Bagby observed that glucose infusion for three hours post-exercise resulted in a greater glycogen synthesis in soleus, gastrocnemius, and quadriceps muscles compared to lactate or saline infusion. The infused glucose following exercise was directly used by the oxidative muscle fibers (slow-twitch and fast-twitch A) for glycogen synthesis, while the non-oxidative fibers (fast-twitch B) also used lactate through an indirect pathway.
    Ingestion of carbohydrate elevates blood glucose concentration, thus providing a substrate for the synthesis of new glycogen, and increases insulin concentration, stimulating cellular uptake of glucose. Insulin, which is secreted from the pancreas, is the primary signal for glycogen synthesis. Doyle and colleagues reported that the glucose and insulin responses to carbohydrate ingestion together accounted for 94% of the variance in glycogen replenishment, highlighting the importance of this substrate and this hormone. Muscle glycogen concentration is a powerful mediator of insulin sensitivity, with sensitivity greatest within the first two hours after glycogen-depleting exercise when carbohydrate is available.

CARBOHYDRATE INGESTION
    Relatively little glycogen synthesis occurs when your athletes do not consume carbohydrates after they run. The synthesis of glycogen between training sessions occurs most rapidly if carbohydrates are consumed immediately after exercise. Indeed, delaying carbohydrate ingestion for two hours after a workout can significantly reduce the rate of glycogen synthesis within the first few hours.
    For example, Ivy and colleagues found that glycogen was synthesized significantly faster when ingesting carbohydrates immediately after exercise compared to delaying ingestion for two hours, as both the insulin and glucose levels were significantly reduced when carbohydrate ingestion was delayed.
    To maximize the rate of glycogen synthesis, your athletes should consume 0.7 gram of carbohydrate per pound of body weight (g/b) within 30 minutes after their workouts, and continue to consume 0.7 g/b every two hours for four to six hours afterward. It would be even better if they can eat or drink more often, since a more frequent ingestion (e.g., every 15 to 30 minutes) of smaller amounts of carbohydrates better maintains blood glucose and insulin levels.
    For example, a study published in Journal of Applied Physiology in 1993 found that when subjects ingested 0.2 gram of carbohydrates per pound of body weight every 15 minutes, glycogen was synthesized at nearly double the rate found in other studies in which carbohydrates were ingested every 1 to 2 hours. Another study published in American Journal of Clinical Nutrition in 2000 found that the rate of glycogen synthesis significantly increased when subjects ingested 0.3 compared to 0.2 gram of carbohydrates per pound every 30 minutes. Therefore, the rate of glycogen synthesis following exercise seems to be maximized when 0.7 g/b is ingested every two hours or when carbohydrates are ingested every 15 to 30 minutes at a rate of 0.5 to 0.7 g/b/hr.
    In addition to the amount of carbohydrates, the type of carbohydrates ingested may influence glycogen synthesis rate and subsequent exercise performance, since different types of carbohydrates may produce different blood glucose and insulin responses. Since glycogen is derived from glucose, it is not surprising that glucose has been found to be the most effective type of ingested carbohydrates at enhancing the insulin response and restoring glycogen immediately after exercise.
    For example, Blom and colleagues found a significant difference in the rate of glycogen synthesis between glucose ingestion (0.3 g/b) and an equivalent amount of fructose ingestion. Ventura and colleagues found higher plasma glucose and insulin concentrations when glucose was ingested 30 minutes before exercise compared to when either fructose or a placebo was ingested. However, subsequent endurance time to exhaustion at 82% VO2max was only significantly longer when comparing ingestion of glucose to placebo.
    Comparing glucose to sucrose tells a slightly different story, as Bowtell and colleagues found that the rate of glycogen synthesis was significantly greater with an 18.5% glucose solution (0.4 g/b) compared to either an 18.5% or 12% sucrose solution (0.4 g/b). However, two studies did not find significant differences in the rate of glycogen synthesis between glucose and sucrose ingestion. Casey and colleagues, who also used 18.5% glucose and sucrose solutions (0.45 g/b), admit that a lack of a significant difference was likely due to the large interindividual variability in their data.
    Using a more concentrated solution (30%), which may have slowed gastric emptying, Blom and colleagues found a small, non-significant difference in the rate of glycogen synthesis between glucose ingestion (0.3 g/b) and an equivalent amount of sucrose ingestion. Taken together, these studies suggest that, while glucose ingestion has a greater effect on insulin level and glycogen synthesis compared to fructose ingestion, it has a similar effect as ingestion of sucrose.

CARBOHYDRATE VS. CARBOHYDRATE. PROTEIN INGESTION
    While the amount and type of ingested carbohydrates for maximal glycogen synthesis have been identified, the effect of other macronutrients in combination with carbohydrate is less clear. For example, research that has examined protein ingestion along with carbohydrates on the rate of glycogen synthesis or endurance performance (Table 1) has yielded inconsistent results, as some studies have shown this strategy to hasten the rate of glycogen synthesis and improve endurance performance, especially when the amount of carbohydrates ingested is less than that needed for maximal glycogen synthesis, while other studies have reported no benefit with the simultaneous ingestion of protein.
    At least some of the discrepancy in the literature may be attributed to the use of post-workout beverages that contained different amounts of calories or different amounts of carbohydrates. It is possible that Carrithers and colleagues and Tarnopolsky and colleagues did not observe differences in muscle glycogen content between the treatments because the carbohydrate-protein beverages contained less carbohydrates than the carbohydrate-only beverages (0.32 vs. 0.45 g/lb/hr and 0.34 vs. 0.45 g/lb/hr, respectively). In the study of Rotman and colleagues, the carbohydrate and carbohydrate-protein beverages, which were ingested every two hours, already contained the recommended amount of carbohydrates for maximal glycogen synthesis, which may have obscured any added benefit of protein.
    The specific type of carbohydrates contained in the beverages has also varied between studies. Tarnopolsky and colleagues and Rotman and colleagues used an equal mix of glucose and sucrose in both their carbohydrate and carbohydrate-protein beverages, Van Hall and colleagues used sucrose, Zawadzki and colleagues used a mix of dextrose and maltodextrin, van Loon and colleagues used an equal mix of glucose and mal to dextrin, and Ivy and colleagues used a mix of sucrose and maltodextrin. Carrithers and colleagues did not even use the same type of carbohydrates between the different treatment beverages, as their carbohydrate-only beverage contained glucose while their carbohydrate-protein beverage contained fructose and dextrose.
    All three studies that reported a beneficial effect of combining carbohydrates with protein used maltodextrin, a complex carbohydrate, as one of the carbohydrate ingredients. Of the studies that reported no additional benefit with the co-ingestion of protein, three used sucrose and three used glucose as one of the carbohydrate ingredients. Despite some similarities in the type(s) of carbohydrates used between studies with similar findings, the type of carbohydrates used does not help explain the contrasting results in the literature.
    Another potential reason for the conflicting results may be due to differences in the frequency of supplementation. For example, studies finding a beneficial effect with the co-ingestion of protein have most often used feeding intervals of two hours, while studies reporting no benefit with the co-ingestion of protein have most often used feeding intervals of less than one hour (Table 1). It appears, therefore, that both a high carbohydrate content of the beverage (enough to elicit maximal glycogen synthesis rates) and the more frequent ingestion of carbohydrates negates any benefit of added protein.
    The only thing that seems to be clear, obvious as it may be, is that beverages containing carbohydrates or carbohydrates plus protein are more effective than plain water or a placebo at restoring glycogen after exercise and lengthening time to exhaustion during exercise. Despite the many highly-advertised commercial sports drinks like Gatorade, any beverage that contains a large amount of carbohydrates will be very helpful for recovery. For example, research from our laboratory has shown that chocolate milk, which has a high carbohydrate and protein content, is an effective alternative to commercial sports drinks for recovery from exhausting exercise.
    So, to get your athletes to fill their empty tanks and recover as quickly as possible after their next long run, tell them to drink 1 to 2 glasses of chocolate milk per hour for a few hours after running. And tell them to skip the fraternity party.
 

 

FROM: TRACK COACH 176

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