Three runners head out for a noontime five-miler. Each had eaten a yogurt an hour before. Midway through the run, one runner says he’s feeling strong and asks if anyone’s up for a few extra miles. The two other runners grunt their response, one indicating fatigue, the other expressing a strong desire for a pack of Tums. 

Sound like a riddle? In a sense, it is. All of the runners followed what the experts advise: eating an easily digestible food an hour before a run. But save for one person, they battled fatigue or stomach trouble. Where did they go wrong?

“Runners know intellectually that food affects performance, but most don’t take time to really examine what works best for their running,” says Lauren Antonucci, R.D., marathoner, Ironman, and director of Nutrition Energy, a sports-nutrition consulting practice in Manhattan. “Once you have the basics for running, you need to individualize your approach to improve performance.”

Foods that leave one runner feeling strong may leave another feeling depleted, or worse. “Runners are different sizes and genders, and they run at different paces, all of which changes their calorie and carbohydrate needs,” says Antonucci. “Plus, some people have stomachs of steel, others have sensitive systems.” To find out what works best for you, you have to experiment–and take good notes.

Experiment of One

Runners are more than just what they eat. Training, how well you slept, stress, and the weather–not just what, how much, and when you ate or drank–affect how well you run. “Keeping detailed notes about all of these factors in your training log will help you determine what’s most affecting your running,” says Leslie Bonci, R.D., a nine-time marathoner and director of sports-medicine nutrition at the University of Pittsburgh Medical Center.

It’s impossible to control all these variables, but having them down on paper enables you to make informed decisions about your nutrition. Say, for example, early morning workouts leave you feeling beat, despite steady training and plenty of sleep. Nutrition is the likely culprit–and an easy fix. “Most of what we eat the night before is used up by morning,” says Bonci. “If you’re putting in an easy hour, that’s not a problem. But eating a few hundred calories before an intense or long run could be all you need to keep your energy up.” (For other common mistakes and solutions, see “Fast Food Fixes,” right.)

To ensure you’re seeing the whole nutritional picture, record how well you hydrate and what you eat before and after a run–the two meals or snacks that most affect your workout–as well as quantities and the time of your meals. Start by chronicling your usual eating patterns for a week, then “change only one aspect at a time, whether it’s when you eat, the kind of food, or the amount,” says Bonci. Test that change on three to seven runs. In doing so, you give yourself time to uncover fueling nuances, such as whether an extra half hour of digestion relieves side stitches, or whole bagels weigh you down. “If you’re eating an easily digestible carbohydrate like a banana or a granola bar, and it’s not working for you, try eating less or eating earlier,” says Antonucci.

Also take a look at the nutritional ratio on your plate. “Ask yourself, ‘Am I shortchanging myself on carbs, fat, or protein?'” says Bonci. Your breakfast of two scrambled eggs, a yogurt, and a glass of OJ might be fine on rest days, but replacing the eggs with oatmeal on days when you run at lunch would boost your carb supply and, therefore, your energy. Or adding protein like nuts to a prerun snack of pretzels could help you feel more satiated before an evening run.

Dedicate at least two weeks to your trial-and-error period, but be open to as many as eight. “How long you experiment depends on what you uncover,” says Antonucci. Explore your eating when you’re doing a variety of running workouts in order to see how changes impact various workouts. If you work on your fueling while preparing for a race, do so early in your training; the last four weeks before a race is the time to stick with what’s worked best up to that point.

Make analyzing your fuel as habitual as logging your miles, and you’ll likely find yourself with extra energy. Not to mention a happy stomach.

RunnersWorld Magazine