The short answer: It varies.
Most athletes and every day exercisers underestimate the importance of pre-exercise fueling. The benefits of a pre-exercise meal are multi-faceted. Pre-exercise fueling helps maintain blood glucose levels to avoid hypoglycemia (low blood sugar), which negatively impacts performance. The right food choices can also alleviate upper gastrointestinal (GI) discomfort by absorbing gastric juices, can fuel your muscles via glycogen production and storage, and fuel your brain, all of which allows for increased performance and a higher calorie burn.
What you should eat is highly dependent on the duration of exercise.
For activities lasting less than 90 minutes, carbohydrates are your friend. Having a ripe banana, blueberries in low-fat yogurt, or a granola bar and milk will allow your body to quickly digest the snack and turn it into useable energy. Consume this pre-exercise meal or snack at least 90 minutes prior to the event.
Note: Protein and fats should be avoided in these instances as they delay gastric emptying, delaying the time between digestion and increased blood glucose levels. High fiber foods should also be consumed with caution as they stay in the stomach longer, which can lead to upper GI distress, such as heartburn and vomiting.
For activities lasting longer than 90 minutes, think half marathons or century bike rides, adding fat and protein will support sustained performance. While the fact that protein and fat delay gastric emptying and, in turn, decrease the rate of digestion is a problem for those shorter bursts of activity, it actually helps in these situations because these meals fuel the body over longer periods of time. Try a bagel with 2 T of any nut butter and a latte, a cheese omelet with toast and milk, or granola with full fat yogurt and berries or a banana. Timing of this type of pre-exercise meal is a little more personal and depends on your tolerance and digestive rate. Aim for at least three hours prior to the event to give the stomach time to digest and empty.
Like most things in life, each athlete needs to learn what works best for them. Keep a food log as you train for a big event to be sure you fuel with foods that you tolerate best. Play with the timing of meals and snacks to see how your performance reacts with different amounts and types of energy available. And finally, don’t forget the pre-exercise hydration!