If you’re like many runners, you’ve spent the last few months training for a big race. And as your 5-K, half-marathon, or marathon approaches, you’re probably taking extra care with what you eat and drink. Maybe you’re loading up on carbs, drinking lots of water, ordering extra servings of broccoli and beans. But are you doing the right thing?

“How you fuel up before the race has a huge impact on your performance,” says Beth Jauquet, R.D., a nutritionist for Cherry Creek Nutrition in Denver. Unfortunately, runners tend toward extremes: Skimping on fuel, overdoing food or drink, or eating foods that cause digestive disaster. Here’s how to avoid common mistakes and ensure what you eat and drink in the week before your race will help you secure the PR you hoped for.

The Mistake: Eating a Box of Pasta

Many runners like to top off their glycogen stores by feasting on carbs the night before a race. And why not? You’re going to burn through them the next day. But flooding your system with more carbs than it can process may lead to digestive problems that will have you running to the porta-potty every mile.

The Fix: Consume moderate quantities–not huge portions–of carbs for several days prior. “Massive amounts of any food throw your system a curve ball,” says Jauquet. Have oatmeal for breakfast, potatoes at lunch, and pasta for dinner. “Eat just to fullness, so you don’t get indigestion or have trouble sleeping,” says Tara Gidus, R.D., a spokesperson for the American Dietetic Association.

The Mistake: Drinking Gallons of H20

Not only will chugging too much water before a race leave you feeling bloated, but it will also dilute your electrolytes–minerals responsible for optimum muscle contraction. Diluted electrolyte levels can cause muscle weakness or cramping and, in extreme cases, can lead to hyponatremia, a life-threatening condition triggered by abnormally low sodium levels.

The Fix: In the days leading up to your race, drink fluids as you normally would to stay hydrated. This can include water, sports drink, juice, even coffee and tea. On the morning of the race, Jauquet recommends drinking 16 ounces of water two to three hours before the start, giving your body time to process extra fluid; drink another one to two cups right before the gun goes off.

The Mistake: Loading up on Fiber

Normally, runners should make sure to eat lots of cruciferous vegetables, beans, and whole grains. And if you’re used to such foods, all that roughage right before a race may pose no problems for you. But if you’ve been living on pizza and burgers, now is not the time to become a vegan. Loading up on high-fiber foods can cause uncomfortable gas, especially if your stomach is plagued by prerace jitters.

The Fix: If you think fiber might be an issue, “cut back on those foods three days before a major race,” says Gidus. That includes beans and bran cereals-but not fruits and veggies, which you should eat in modest portions. Think one cup of pineapple, a handful of cherries, or a few broccoli florets. But, Gidus cautions, if you’re racing every weekend, reduce your fiber intake only on race day to make sure you don’t trim all fiber out of your diet.

The Mistake: Skipping Breakfast

Too nervous or worried about feeling full, some runners can’t face food on race morning. But without it, you’re likely to bonk in any race. Why? Because studies show that a pre-race meal keeps your blood sugar steady and provides energy to power you through. “There’s no way to get enough fuel mid-race to make up for the energy you missed at breakfast,” says Jauquet.

The Fix: If you know you get too nervous to eat before a race, wake up a few hours before the start-so you can eat breakfast slowly, letting each bite settle before taking another. If you can’t stomach solid foods, drink a smoothie with bananas, fruit juice, and milk. These ingredients are easy on most stomachs, provide energy, and won’t leave you feeling overly full.

The Mistake: Trying Something New

If you’ve never had spicy salmon sushi, don’t order it the night before your race. You won’t know how a food affects you until you’ve tried it-and last-minute experimentation could send you bolting for the bathroom and leave you dehydrated.

The Fix: Stick with what you know for a week before race day. Check the race Web site to confirm which drinks and gels (if any) will be offered along the course so you can test them out in advance. Don’t be afraid to skip the pre-race dinner or hotel breakfast: If you’re not used to downing sausage burritos pre-race, you’re better off sticking with a familiar bowl of pasta. As long as it isn’t huge.

Eat better

In the days before a race, vary your diet with non-grain carb sources, such as fruits and starchy vegetables, to benefit from a wider range of nutrients.