The answer depends on two factors: your overall diet and how intense your workout is. If you eat small, balanced meals every few hours while you’re awake, you don’t really need a preworkout meal strategy. Your body should have ample fuel to get you through any workout around an hour or less (longer workouts have specific needs).
However, if you go longer than three hours without eating, you should determine your needs based on a few things:
First, how hard is your workout? Easy aerobics and other work where your heart rate doesn’t exceed 140/150 bpm (easy yoga, slow jogging, cycling, hiking, etc.) don’t use a lot of fuel (blood sugar and its back-up, glycogen) and can be done effectively on a fairly empty tank. Good hydration (water only) should be all the fuel you need.
Harder workouts that have an anaerobic interval component will burn your limited stores of glycogen. Your body stores glycogen until you need it, but when your diet is very lean, you will almost certainly deplete these stores before the end of your workout if you haven’t eaten in a while. This condition, called “bonking,” causes your performance to instantaneously plummet.
Here are some general rules to avoid the dreaded “bonk.” We’re not suggesting you eat three separate meals in the hours leading up to exercise. Rather, pick the one that best suits your day.
Three hours prior to a workout: Eat a well-rounded, light meal. As long as it’s not too many calories (more than 500-ish), most of it will be turned into fuel by the time you begin and you have ample time to digest.
Two hours prior: Eat a light snack that’s mainly carbohydrates. Something that’s 4 parts carbs to 1 part protein with little fat will ensure there’s time to convert it into glycogen. “Energy foods,” something like granola with yogurt and fruit, is ideal.
One hour prior: Eat very light, no more than 200 or so calories at around a 4:1 carb to protein ratio. Low-fat, plain yogurt with a little fresh fruit thrown in is ideal.
Any extra protein and fat will hinder your workout. It’s similar to the two-hours-prior snack, but since your body can only convert 200 to 300 calories into energy in a given hour you’ll want to keep the portions smaller.
Less than one hour: Try not to eat during the last hour before your workout. If you haven’t eaten in hours, liquid fuels, like juice, or easily digestible carbs, like half of a banana, will digest fast enough to help you during the later stages of the workout when your glycogen would otherwise run out.