On race day, the single biggest limiting factor in a long event is the relatively tiny amount of glycogen (a form of carbohydrate) that can be packed in the muscles and used to fuel movement: about 2,000 calories in the normal ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼adult male. (Don't count on blood sugar to help much; not much more than 40 calories are floating in a healthy person's bloodstream, according to well-known low-fat author/researcher Stephen Phinney.)
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While everybody burns a mix of glycogen and fat, there will be problems if the glycogen is used up too quickly. Once the 2,000-calorie glycogen tank goes low, usually in 90 minutes or 2 hours of sustained effort, you bonk. Glycogen rules several vital functions, like running your brain, and when it's low, you get lightheaded—i.e., bonk.
By contrast, everybody has an almost unlimited supply of body fat—as much as 80,000 calories' worth, even on skinny people. If you train your body, through eating fewer carbs and more fat, to become a "fat-burning machine," it'll use more fat and less glycogen, thereby delaying the bonk. Tapping your 80,000-calorie fat-fuel tank more often has other benefits, too: You won't need to refuel with a gel or energy bar every 5 or 10 miles to top off your glycogen tank, as high-carb-eating athletes must do, and you won't suffer gastrointestinal problems at mile 75 from eating sugary stuff all day. A high-carb racer might eat 6,000 in-race calories; a fat-adapted low-carb racer might eat a quarter of that (follow these rules for a successful low-carb diet). Running on fat and eating less, you can theoretically go strong all day long.