Show Me the Bagels

Glycogen As a Metabolic Fuel for Runners.

he many proponents of diets like Atkins and South Beach would have the public believe that carbohydrate is some kind of poison. Don’t listen to them. Carbohydrate is a marathoner’s best friend.

Carbohydrate is stored in our skeletal muscles and liver as glycogen and is also found as sugar (glucose) in the blood. When we run, our bodies use a combination of blood glucose and glycogen as fuel to regenerate adenosine triphosphate (ATP) through a process called glycolysis. (This would be a good time to break open your high school biology book.) ATP is a molecule that, when broken down, liberates energy that is used for muscle contraction and other cellular functions. It is well known that endurance performance is influenced by the amount of stored glycogen in skeletal muscles and that intense endurance exercise decreases glycogen stores (Ahlborg et al. 1967; Costill 1991; Costill and Hargreaves 1992; Evans and Hughes 1985; Green et al. 1995; Hermansen et al. 1967; Ivy 1991).

What the low-carb diets seem to ignore is that running (and all exercise) is largely a carbohydrate activity. The intensity of exercise is the main determinant of which fuels are used, with the reliance on carbohydrate increasing with increasing intensity. When you run at a pace slower than the lactate threshold—the fastest speed that can be maintained almost solely by aerobic metabolism and above which acidosis results in muscles and blood—energy is derived from a combination of fat and carbohydrate. At a pace faster than lactate threshold pace, your muscles use only carbohydrate as a fuel.

The goal of endurance training is to push the lactate threshold to a faster pace so that you’re running faster before carbohydrates become the only fuel source.

Elite or highly trained marathoners run a marathon at slightly below their lactate threshold pace, which means that carbohydrate is the main fuel used during a marathon. In an ultramarathon, however, there is a greater reliance on fat as a fuel because the intensity is much lower. Biochemically, carbohydrate still must be available for fat to be used effectively as a fuel, since the final product of glycolysis is used in combination with the final product of fat oxidation to produce ATP. Physiologists and biochemists refer to this concept as how “fat burns in the flame of carbohydrate.”

CARBOHYDRATE AS A FUEL

The human body has enough stored glycogen to last slightly more than two hours of sustained exercise at a moderate intensity (Robergs and Roberts 1997). Thus, running a marathon or ultramarathon presents the unique situation in which muscles can become depleted of glycogen during the event. This acute glycogen depletion coincides with feelings of fatigue and hitting the infamous Wall. However, fatigue can be delayed if you ingest sufficient carbohydrate during the marathon to maintain blood glucose levels.

Two major reasons why long-distance runners do long training runs are to increase endurance by depleting muscle glycogen on a regular basis and to increase the muscles’ reliance on fat metabolism at the same running pace. In the presence of ingested carbohydrate following the long run, the skeletal muscles respond rather elegantly to the empty tank by synthesizing and packing in more glycogen and thus increasing endurance for future efforts.

GLYCOGEN DEPLETION

If not enough carbohydrate is ingested between workouts to replenish the lost glycogen from training, chronic, low muscle-glycogen levels result. Glycogen depletion leads to blood acidosis (through the development of ketosis) and is characterized by low blood insulin, hypoglycemia, and increased amino acid (protein) metabolism with an associated increase in blood and muscle ammonia, which can be toxic to muscle cells. Glycogen depletion can also impair exercise tolerance and performance, increasing the perception of effort. In other words, you don’t want to go out for a training run when you’re glycogen depleted; in the best of cases, you won’t feel good, and in the worst, you may just fall down.

NUTRITION BEFORE A WORKOUT

What you eat in the few hours leading up to a workout greatly affects which fuel is used. For example, fat oxidation during exercise is very sensitive to the interval between eating carbohydrates and the start of exercise. Eating a meal high in carbohydrates stimulates the pancreas to produce insulin. Since insulin inhibits the breakdown of fat, the body will be forced to rely more heavily on carbohydrates as a fuel during your run. It appears that the body relies greatly on carbohydrates and less on fat when carbohydrates are available. Therefore, if the training session is going to be long—during which it is important to delay the use of carbohydrate—carbohydrates should not be consumed within a couple of hours of your workout.

STRATEGIES FOR MAXIMIZING GLYCOGEN RESYNTHESIS AND STORAGE FOLLOWING TRAINING

Muscles are picky when it comes to the time for synthesizing and storing glycogen. Glycogen resynthesis between training sessions occurs most rapidly if carbohydrates are consumed within a half hour to an hour after exercise (Costill 1991; Evans and Hughes 1985; Friedman et al. 1991). Indeed, delaying carbohydrate ingestion for two hours after a workout reduces the rate of glycogen synthesis by more than half (Ivy 2001; Ivy et al. 1988a).

Therefore, to optimize glycogen resynthesis by the muscles, a couple of things have to be considered. First, since glycogen is composed of a collection of glucose molecules branched together, glucose is the most important type of carbohydrate to consume immediately following a training session to provide a substrate for the synthesis of new glycogen. Any other type of carbohydrate (for example, sucrose, galactose, and complex carbohydrates) will have to be first broken down or converted into glucose before being synthesized into glycogen. Second, since nutrients from fluids are absorbed more quickly than from solid foods, athletes should initially consume carbohydrates in the form of fluids. Despite all of the highly touted commercial sports drinks, any beverage that contains a large amount of carbohydrate will serve this purpose. Indeed, chocolate milk, with its high carbohydrate content, has been found to be effective as a postexercise recovery drink (Karp et al. 2004).

For maximal glycogen resynthesis, you should ingest 50 to 75 grams of carbohydrate within 30 to 45 minutes after the workout (American Dietetic Association et al. 2000), and while earlier studies have suggested ingestion of 0.7 to 1.0 gram of carbohydrate per kilogram of body weight per hour for the next few hours (Berning et al. 1998; Blom et al. 1987; Ivy 1991; Ivy et al. 1988b; Robergs 1991), more recent evidence suggests that ingestion of 1.2 to 1.5 g/kg/hr is optimal, especially in highly trained endurance athletes (Ivy 1998; Ivy 2001; van Loon et al. 2000). For most commercial sports drinks, such as Gatorade, this comes out to be about 5 1/2 glasses every hour for a 65-kilogram (145-pound) athlete. For a beverage like chocolate milk, which has more than twice the carbohydrate of most commercial sports drinks, the same 65-kilogram athlete would need to drink about 2 1/2 glasses every hour to meet the above recommendations.

Ingesting protein along with carbohydrate may also increase the rate of glycogen resynthesis, but only when not enough carbohydrate is ingested (Ivy 2001; Ivy et al. 2002; van Loon et al. 2000; Zawadski et al. 1992), however, not all studies have found a benefit of the co-ingestion of protein (Carrithers et al. 2000; Tarnopolsky et al. 1997; Van Hall et al. 2000). Full muscle glycogen replenishment requires a high dietary carbohydrate intake (greater than 60 percent of total calories) over one to two days.

Interestingly, glycogen resynthesis following high-intensity interval training occurs faster than following prolonged endurance exercise, regardless of whether carbohydrate is ingested immediately after the workout (MacDougall et al. 1977). This difference in glycogen resynthesis following different types of workouts may be due to the elevation in blood glucose and insulin that occurs following high-intensity work, but a depression in blood glucose and insulin that occurs following prolonged endurance exercise (Sutton et al. 1972).

Although carboloading the night before an important race gained popularity in the 1980s, racing performance for events shorter than the marathon and ultramarathon is not limited by the amount of stored glycogen. Simply maintaining a high-carbohydrate diet all the time will ensure you have an adequate amount of muscle glycogen. It has been shown that reducing training volume and intensity (by tapering) while further increasing the carbohydrate content of the diet (greater than 70 percent of total calories) during the week before the competition can elicit an even greater increase in muscle glycogen levels, which has been called glycogen supercompensation (Bergstrom et al. 1967; Sherman et al. 1981).

REFERENCES

Ahlborg, B., J. Bergstrom, L.G. Ekelund, and E. Hultman. 1967. Muscle glycogen and muscle electrolytes during prolonged physical exercise. Acta Physiologica Scandinavica 70:129-142.

American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. 2000. Nutrition and Athletic Performance. Joint Position Statement of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. Medicine and Science in Sports and Exercise. 32(12):2130-2145.

Bergstrom, J., L. Hermansen, and B. Saltin. 1967. Diet, muscle glycogen, and physical performance. Acta Physiologica Scandinavica 71:140-150.

Berning, J., E.F. Coyle, P.R. Garcia, H. O’Connor, S. Orbeta, and N. Terrados. 1998. Sports foods for athletes: What works? Sports Science Exchange Roundtable 9(2).

Blom, P.C.S., A.T. Hostmark, O. Vaage, K.R. Kardel, and S. Maelum. 1987. Effect of different postexercise sugar diets on the rate of muscle glycogen synthesis. Medicine and Science in Sports and Exercise 19:491-496.

Carrithers, J.A., D.L. Williamson, P.M. Gallagher, M.P. Godard, K.E. Schulze, and S.W. Trappe. 2000. Effects of postexercise carbohydrate-protein feedings on muscle glycogen restoration. Journal of Applied Physiology 88:1976-1982.

Costill, DL. 1991. Carbohydrate for athletic training and performance. Boletin de la Asociacién Médica de Puerto Rico 83(8):350-353.

Costill, D.L., and M. Hargreaves. 1992. Carbohydrate nutrition and fatigue. Sports Medicine 13(2): 86-92.

Evans, W.J., and V.A. Hughes. 1985. Dietary carbohydrates and endurance exercise. American Journal of Clinical Nutrition 41(Suppl 5): 1146-1154.

Friedman, J.E., P.D. Neufer, and G.L. Dohm. 1991. Regulation of glycogen resynthesis following exercise. Dietary considerations. Sports Medicine 11(4): 232-243.