On April 20, more than 30,000 runners will run the 2015 Boston Marathon. Which one of these 30,000 athletes will burn energy at the highest rate during the race?

That’s easy: the winner—probably. How do I know this? Because the rate of energy consumption during running is largely a function of speed. The faster an athlete runs, the more rapidly his or her body burns energy. And, of course, the athlete who runs the fastest in any given race is the winner.

Actually, it’s a bit more complicated than that. Body weight and movement efficiency also affect the rate of energy use during running. Heavier runners burn more energy at any given pace while efficient runners burn less. But these other factors actually cancel themselves out: heaviness by slowing runners down so they burn less energy and efficiency by allowing runners to go faster so they burn more. In the final analysis, I can’t guarantee that the first person across the finish line of the Boston Marathon will have burned energy at the highest rate during the race, but I can guarantee that someone very close to the front of the field will have this distinction.

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Burning more energy is not often thought of as a good thing in running, but becoming a better runner is in fact largely a matter of developing the ability to burn energy at higher rates on the run. It’s like putting a more powerful engine in your car. Sure, a bigger engine burns more gas, but it’s precisely because it burns more gas that it makes the car go faster.

Continuing the analogy, your muscles are like a hybrid engine. They run on two main fuels: fat and carbohydrate. Lately there is a lot of talk in running circles about the benefits of increasing the capacity of the muscles to burn fat. But it’s actually more important to increase the overall rate at which your muscles can transform fuel into forward motion. And the real potential for increasing this capacity lies in making your muscles better carbohydrate burners, not better fat burners.

It’s a simple fact: The best runners are the best carbohydrate burners. The latest proof comes from a study conducted by Italian researchers and published in 2014. Led by Federico Schena of the University of Verona, these researchers looked for correlations between numerous metabolic variables and half-marathon performance in a group of 43 recreational runners. One of the variables they looked at was the level of alpha-amylase in the blood at rest. Alpha-amylase is an enzyme that aids in the metabolism of carbohydrate. Schena’s team found a significant correlation between resting levels of alpha-amylase in the blood and half-marathon performance. In fact, the combination of this variable and aerobic capacity (or VO2max) accounted for 71 percent of the variation in half-marathon times among the 43 runners.

What does this mean? Resting levels of alpha-amylase in the blood indicate that a person is “good at” burning carbohydrate. So what this result means is that runners who are especially good at burning carbohydrate perform better in races than do runners who aren’t quite as good at burning carbs. This correlation persisted even when the researchers adjusted for possible confounding variables such as body mass index and VO2max.

The take-home lesson is clear: If you want to become a better runner, you need to become a better carbohydrate burner. So how you do that? Diet and training both play a role. As you might expect, a high-carbohydrate diet tends to enhance the ability of the muscles to burn carbs during exercise. A high-fat diet has the opposite effect. This is why study after study has found that high-fat diets reduce time-trial performance. Returning once again to our vehicle analogy, switching to a high-carbohydrate, moderate-fat diet is like replacing an old-fashioned carburetor with a fuel injector. Switching to a high-fat, low-carb diet, as many misguided runners are doing lately, is like doing the opposite.

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Exactly how much carbohydrate is optimal for you depends on how you run. My general rule of thumb is this: Start at a baseline of 3 grams of carbohydrate per kilogram you weigh (1 lb = 2.2 kg) per day. Add 1 g/kg/day for every 20 miles per week you run. So if you run 40 miles per week, aim for about 5 grams of carbs per kilogram you weigh daily.

Training also influences carbohydrate-burning capacity. In order to get really good at burning carbs during running, you need practice burning carbs, and that comes at high intensity. But high-intensity running is very stressful to the body, so it’s important not to overdo this type of training. I recommend that all runners do about 80 percent of their running at low intensity (60-75 percent of maximum heart rate), 5 to 10 percent at moderate intensity (76-90 percent of HR max), and 10 to 15 percent at high intensity (91-100 percent of HR max).

It’s important to keep in mind that carbohydrate-burning capacity is not the be-all and end-all of running performance. Nothing is. Everything matters. But these days there are a lot of nutritionists and coaches who are suggesting that fat-burning capacity is the be-all and end-all. They’re wrong. Carbohydrate-burning is more important, because speed is ultimately most important.