The morning of the race do not skip your meal. Many athletes have a nervous stomach and passing on breakfast seems like a logical way of getting around that. This is a bad idea. You need the energy your morning meal gives you.
Eat something light, low-fiber, and easily digestible like a banana, bread, or granola. Remember to avoid high-fiber foods like corn, brown rice, whole wheat pasta, or beans. Those speed up digestion and that is not a problem you want to have at the starting line. Or at mile two.
One of the most important race day commandments is "Thou Shalt Not Try Something New the Day of Thy Race". This is a vital point. The best way to cause stomach issues is to do something untested the day of your event.
Wake up at race time during training to experiment. This should show you what is going to work.You might also look into some specially-made products like gels for a right-before-the-start energy boost. Buy a few different flavors and brands to try.
For me, the GUs have better taste and texture, but everyone is different. Many of these products have a high-caffeine version, which might be good for a jumpstart as well.
Some athletes drink coffee the morning of for a pick-me-up. This too falls on the Not-Unless-You've-Already-Tried list. Coffee is a diuretic. That could be bad.
During the Race
A 5K is short enough that race nutrition will not come into play. Grab a cup of water or sport drink from a wonderful volunteer at the aid station and keep right on trucking. You're almost home at that point.
After the Race
After your race you'll want to refuel, and this is where protein comes back into your diet. Eat carbs for energy and protein for rebuilding muscles. Don't forget to rehydrate as well.
More: 6 Best Post-Race Beers
The post-race meal is also the perfect place to sport your new race shirt and tell your friends and family about how great you did.
Special thanks to Summer Blunden, ATC, CSCS