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If you're a vegetarian training for a marathon, be sure to read up on your alternative proteins, because protein is important for muscle maintenance and recovery (see Protein Power tip). Vitamins and supplements won't hurt, but don't depend on them. Most nutritionists agree that it's best not to rely on vitamins or nutritional supplements at the expense of real food, because there are components of foods that interact with each other in ways we don't fully understand.
From a marathon training perspective, the most important thing is to eat enough, and try to eat a variety of foods. As you increase your mileage, you may find yourself craving more protein, whether it is peanut butter or something else, so have some! Your muscles will thank you. Runner's World is a great nutrition information resource for runners, and includes vegetarian recipes and snacks that appeal to non-vegetarians, too.