Meanwhile, numerous studies have shown traditional carbohydrate loading enhances performance in time trials lasting longer than 90 minutes. Whereas, fat loading reduces the amount of glycogen the muscles burn during exercise, carbo-loading does the opposite. However, carbo-loading increases the supply of glycogen even more and, as a result, that supply lasts longer, and endurance is increased. In other words, carbo-loading achieves the same effect as fat loading, but in a completely different way.
Wouldn't it be great if you could fat load and carbo load before marathons and other long races so that you could take advantage of both ways of making your muscle glycogen supplies last longer? Actually, you can. In 2001, Vicki Lambert tested the hunch that a period of fat loading followed immediately by a short carbo-loading phase would allow endurance athletes to have the best of both worlds in a subsequent endurance test. The fat-loading period would increase the capacity of the muscles to burn fat during prolonged exercise. The carbo-loading phase that followed would then reverse the depletion of muscle glycogen stores that was caused by the fat-loading period. The only question was whether carbo-loading would also negate the boost in fat-burning capacity that resulted from fat loading.
Turns out it didn't. Five trained cyclists were recruited to serve as subjects in the study. All were required to complete a 2.5-hour stationary ride followed by a simulated 20K time trial on two separate occasions: once following a three-day carbo-load that was preceded by a 10-day fat load, and once following a three-day carbo-load preceded by their normal diet. On average, the cyclists completed the time trial 84 seconds faster when their carbo-load was preceded by fat loading. They burned more fat and less muscle glycogen in that trial as well, even though carbo-loading had succeeded in restoring their muscle glycogen levels to normal.
Now, a 20K cycling time trial that comes right after a 2.5-hour ride is a little different than a marathon. Fat loading before carbo-loading is likely to yield a smaller performance boost for marathon runners, and may offer no advantage for some. But if you want to give yourself every advantage in your next marathon, fat loading may be worth a try. In Lambert's study, the subjects got 65 percent of their calories from fat for 10 days before switching to carbo-loading for three days, when they got 70 percent of their calories from carbs. I recommend that you try to match those numbers in your personal experiment. For detailed guidelines on how to do it, check out my latest book, The New Rules of Marathon and Half-Marathon Nutrition.marathon.