If the race is early in the morning, is it unwise to skip breakfast beforehand? How long should you avoid eating before running a 5K?
I normally finish in twenty-one minutes, and the race is at 8:30AM.
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The main problem with eating before running is nausea and burping up your breakfast during your race. Once the food moves from the stomach to the small intestine, most people are fine (some people experience cramping, but this is uncommon in healthy runners). The stomach usually takes 30-60 minutes to empty, and it empties more quickly with a liquid meal than a solid meal (PubMed).
In endurance running, your main nutritional concern is carbs, because your body has relatively small stores of them. Runners often load up on carbs in the days before long races to fill up these stores as much as possible. Full stores can last 30-90 minutes, depending on intensity (about.com), so maybe that covers you for a 5k, maybe it doesn't, but why not give yourself a little extra fuel, just in case?
Before a race, I usually go for something like a banana smoothie made with yogurt about 45-60 minutes before the race. It has a low glycemic index, which means it will give you a sustained delivery of glucose throughout your run (rather that a spike and a drop). Bananas are also high in potassium, which is important to prevent muscle cramps (health911). If you want to really step up your game, caffeine is an effective performance enhancer, which is thought to work by increasing muscle contractility, mental alertness, and pain tolerance (Current Sports Medicine Reports). According to the article I cited its diuretic properties (makes you have to pee more) are not significant enough to dehydrate you.
You certainly don't want to be hungry right in the middle of your run, as that hungry feeling is quite distracting. Eating immediately before exercise though can lead to cramps, which are quite undesirable as well. Running.about.com suggests eating a light snack or meal an hour and a half to two hours prior to running. They also suggest that what you do eat be higher in carbohydrates for quickly available energy, and lower in fat, fiber, and protein.
My personal experience is that I'm fine to run in the morning provided I ate a sufficient supper the night before. If I didn't, I'll have something light like less than 300 calories about an hour before I go. I will say though, I'm not quite up to 5K yet either, and I can't guarantee that my personal experience will remain the same as I increase my running distances.
Another article on about.com agrees with this, stating that you should simply get up early enough to eat well before your run if you're going to be running for more than an hour. They also suggest a 300-500 calorie breakfast. As a less-preferred alternative, they suggest taking an energy gel about 30 minutes into your run.
Its perfectly fine (and common) to skip breakfast if your race is in the morning PROVIDED that you eat a good meal the night before with lots of healthy carbs (wholewheat pasta is a good one).
Also bear in mind that it is very subjective and what works for one person won't for another.
Something that I would highly recommend is to work training runs into your schedule that are at the same time as your race so that you are used to getting up and running in the morning already. Make sure to test whether you can eat breakfast or not during these tests and you'll have no problems.
For early runs I find there is no time to digest any foods for a comfortable run so I load up the night before so im not too hungry but the one thing I do recommend is spend some time on the lavatory when you wake up and 'empty' yourself. I find it helps knowing your as light as can be. It may just be a mental thing but whatever helps helps.
I usually eat breakfast before I run for races or training. For races, I usually eat before I leave the house. By the time the race starts, then it's been at least an hour because I get to the race early to register, etc.
For training, I run in the evenings before dinner and mid-morning on the weekend. On the weekend, I eat breakfast and run sometime after 1.5 hours. I eat the same breakfast on training and race days, so I know how I will react. I'll get up earlier on a race day so that I can eat my normal breakfast.