If you’re gearing up for a big race day and plan on carb loading beforehand, you’ll want to be prepared with the proper fuel for your body so you can give your max effort and reap the benefits of all of your hard work. 

“Hydrating properly and consuming the right fuel leading up to race day can make or break your chances of crossing the finish line, or better yet, making it atop the podium,” James Oliver, certified sports nutritionist and the founder of Atlas Bar, says. "Having finished the 2017 Iceland Spartan Ultra World Championship myself, I know firsthand how hard it can be to go the distance, so be sure to fuel your body with the right foods to perform your best."

Carbohydrates should be your priority fuel focus in the days and hours leading up to toeing the start line.

“Carbohydrates are the quickest and most efficient fuel source for exercising muscles," Kelly Jones, MS, RD, CSSD, LDN, says. "Without adequate stores on race day, you'll fizzle out more quickly than you'd like, while also limiting intensity.

"Since carbohydrates are also the best fuel source for the central nervous system, lack of carbohydrate stores and/or low blood sugar can impact coordination and mental strength for your race as well."

Related: Avoid These 5 Race-Day Breakfast Mistakes to Finish Strong and Fast

So, you’ll want to get enough carbs in, but also be careful about which ones you’re choosing, as they weren’t created equal.

“You’ll want to eat foods that are easy to digest and loaded with electrolytes,” Oliver says.

Eating the wrong carbs before can backfire, leaving you feeling sluggish and weighed down (not to mention plaguing you with potential gut rot). These are the best and worst carbohydrates to fuel with pre-race, plus a few ways to enjoy them when you're carb loading.

What to Eat When You're Carb Loading Before a Race 

On race day, Jones says that some of the best carbohydrate sources to eat in advance are oats, bananas, wheat toast, bagels, and dried fruit.

"The first three options contain the types of carbohydrates that supply sustained energy without causing a blood sugar spike and crash,” she explains. 

You might have some plain oatmeal with chopped fruit and nuts, or you can spread nut butter on a bagel or banana.

“Oats are packed with carbs that will provide you with a good, sustainable pool of energy to draw on, and they’re also packed with magnesium and calcium — two electrolytes that have multiple performance-boosting health benefits,” Oliver says.

And bananas are great for portability, ease of digestion, and their high amounts of starchy carbs.

“They also have 10 times more potassium than eight ounces of Gatorade, which is a key electrolyte you’ll need to help prevent muscle cramps,” Oliver adds.

Related: The One Food You Should Eat Before Every Run

Sweet potatoes can provide a great source of starch the night before you hit the course, too. They’re filled with a powerful assortment of vitamins and minerals to keep the body rock solid come morning. 

“These delicious potatoes are a great ‘night-before’ meal, as they’re dense in starchy carbs and can be prepared in a multitude of ways,” Oliver says.

You can also ditch breakfast upon waking up and eat something right before the race.

“If you prefer not to eat a lot before the race, wait until about 15 minutes before and enjoy some Medjool dates," Jones says. "The small amount of fiber can help blunt sugar spikes from being too significant without weighing you down."

What to Eat During the Race 

If your race is longer than a Sprint — like a 10K Super, 21K Beast, and certainly a 50K Ultra — you'll need some extra fuel on-person to make it through the course without crashing.

Related: 10 Ways to Calm Your Nerves and Get to Sleep the Night Before a Race

“During the race, raw honey packets are a great single-ingredient quick energy source, since honey contains a mix of carbohydrate sources," Jones says. "It is also easy on the digestive tract and able to provide energy quickly."

Another option? Get out that trusty banana, which has potassium to help banish any muscle cramping and bring back electrolytes. 

What to Avoid When Carb Loading Before a Race

In the hours leading up to the race, say no to the “magical” fruit.

“The worst carbs to ingest prior to a race include legumes and vegetables," Jones says. "Both are nutritious most of the time, but due to high fiber content, they'll sit in the digestive tract, limiting the delivery of energy to the bloodstream and causing you to feel weighed down."

That means ditch cruciferous veggies and lentils, as they’re very high in fiber — one cup of lentils has 64% of your daily value — which could lead to digestive issues, and they are also high in protein, which may slow down the delivery of glucose to your muscles. 

The same goes for leafy greens, like spinach.

Related: The Pros and Cons of Your 5 Favorite Dark Leafy Greens

“This may be the only time where I would ever advise against leafy greens, but hear me out: Leafy greens are relatively high in fiber and low in carbs,” Oliver says. “That means that you’re not getting a lot of glucose from them, but you are getting a lot of fiber, which could lead to digestive issues on the course."

You might need to forego dairy, too, if that's something that generally upsets your stomach. In this case — like most others — you'll want to rely on weeks of nutritional training to make that decision. 

While a Greek yogurt with some granola might be a good pre-race snack for some, it could lead to major gastrointestinal distress in others. Again, don’t experiment on race day.

“For some people, dairy is a bad idea before a race, while for others it may be fine,” Jones says. “But even those who do not suffer from lactose intolerance or dairy allergies may be better off saving dairy for other times of the day. People have different gastric (stomach) emptying rates, and dairy is a food that may move more slowly for some."

The same goes for juices and fruit-based smoothies. You’ll also want to cut back on this come race-day morning.

“For someone with IBS or who has a sensitive GI tract, the high amount of fructose in these foods may not be able to be well absorbed while exercising, causing cramps and discomfort,” Jones explains.

Plus, both are super sugary, which could lead to a crash mid-race. Instead, cheers to your victory with a protein-packed fruit smoothie once you’ve crossed that finish line.