Even if the marathon you're preparing for is your first, you've probably heard of carbohydrate loading and its potential to double your glycogen stores. While this is accurate to an extent, simply loading up on the starchiest food you can find is not the right way to do it. Potatoes may be loaded with carbs, but they're not the best thing to eat right before a marathon. They can be a legitimate part of a carb-loading phase, but even then, there is a better alternative.
A proper carb-loading course begins about a week before the race, when you drop your carbs to make up no more than half your daily calories. During this period, you continue to train on schedule -- the point is to deplete your glycogen stores as thoroughly as possible. Think of it as cleaning out the fridge before you load it up with fresh, new food. About four days before the race, bump up your carb intake to about 70 percent -- your training should be lighter at this point in preparation for the race, so the glycogen is less likely to get used up. Over the next few days, all of these carbs will restock your glycogen stores and keep them ready for those last 10 miles.
White potatoes are not unhealthy, but they are not the best choice when you're preparing your body for a strenuous event like a marathon. Although white potatoes are complex carbs, they break down more like simple carbs. They affect your blood sugar quickly, but the energy they give you is short-lived. This means relying on white potatoes for fuel may make you hit the wall not long after starting the race. The carb-loading tradition is to eat a big pasta dinner the night before the race, but this may not be the most effective method, for the same reason.
For a healthier high-carb food, eat sweet potatoes instead. They are higher in fiber than white potatoes, and they have a more gradual effect on your blood sugar, leading to a steadier, more sustained energy source. They are also packed full of vitamins, minerals, antioxidants and anti-inflammatory agents that can help your body during the race in other ways. If you can't give up your pre-race pasta dinner, switch to whole grain pasta, and toss it with roasted sweet potato cubes.
Star marathoner Grete Waitz religiously skipped a big pasta dinner the night prior to the race, and carb-loaded at breakfast the morning of the race instead. Race day is not the time for experimentation -- you must work out a nutrition plan that works for you during training. Eat different things at different times before your long runs to see what agrees with you and what doesn't. Everyone is different. Some people's stomachs would be too upset for them to run if they followed Ms. Waitz's method, while others graze on carbs right up until the starting gun. Wherever you fall in that spectrum, avoid fats and large amounts of protein the morning of the race -- they can slow digestion and weigh you down, and your performance will suffer.