Simply put, carbohydrate is the most important energy source, or macronutrient, for runners who are training for marathons and half marathons. This fact has been known for more than a century, but lately it has been obscured by the ripple effects of a general decline in carbohydrate’s reputation. Many runners today do not consume the right amount of carbohydrate to support their training. Some of these runners have not been taught how important carbohydrate is and how much of it they need. Others consciously limit their carbohydrate intake because they have been misled to believe that carbs are bad. To appreciate how this unfortunate situation came about it’s helpful to review a little nutritional history.
In 1924 a team of researchers from Harvard University took blood samples from runners at the finish line of the Boston Marathon and found that those runners showing the most extreme signs of exhaustion invariably had blood glucose levels low enough to be classified as hypoglycemic. By this time it had already been shown by other scientists that endurance athletes (that is, cyclists, runners, and swimmers who compete in races lasting longer than a couple of minutes) performed significantly better in fitness tests after maintaining a high-carb diet than after maintaining a high-fat diet. The next year the same team of Harvard researchers encouraged some runners to train on high-carbohydrate diets in preparation for the 1925 Boston Marathon and also supplied some participants with sugar candies. Blood samples taken at the finish line this time revealed that runners hopped up on extra carbs had higher blood glucose levels than did those who had not trained on high-carb diets or eaten sugar candies during the race.
Subsequently a pair of Danish researchers, Erik Christensen and Ole Hansen, took the next step and actually quantified the effects of carbohydrate on endurance performance. They placed volunteers on either a low-carb, moderate-carb, or high-carb diet for one week, at the end of which all of the subjects were required to pedal a stationary bicycle to exhaustion at a fixed high intensity. On average the subjects lasted for just 81 minutes after a week of low-carb eating, compared to 206 minutes after seven days of carbohydrate feasting.
The reason for these effects remained a mystery for almost thirty years, until a group of Swedish researchers led by Jonas Bergstrom discovered that glycogen was the crucial link. In a landmark study, Bergstrom depleted the muscle glycogen stores of subjects with exhaustive exercise. As in the earlier study by Christensen and Hansen, the subjects were then placed on either a low-carb, moderate-carb, or high-carb diet and subjected to a stationary bike ride to exhaustion at a fixed intensity. The difference was that this time measurements of glycogen concentration were taken in each subject’s quadriceps muscles. The average times to exhaustion in the low-carb, moderate-carb, and high-carb groups were 59 minutes, 126 minutes, and 189 minutes respectively, and there was a significant correlation between initial muscle glycogen concentrations and time to exhaustion.
Bergstrom had shown that maximizing muscle glycogen stores was a key to pushing back the wall of fatigue in endurance exercise, and that a high level of carbohydrate intake was a key to maximizing muscle glycogen stores. After the publication of this study in 1967, high-carbohydrate diets became widely used by serious endurance athletes around the world. In 1972 Frank Shorter practiced “carbohydrate loading” before winning the Olympic Marathon in Munich and the method was subsequently emulated by American runners of all ability and experience levels.