First off, you should always hydrate first thing in the morning, and again before your workout. This means drinking pure water, not culinary water filled with chlorine and fluoride and sediments. Keeping hydrated is important for property body and brain function. When dehydrated you'll experience fatigue, headache, foggy thinking, and dry lips. Keep well hydrated long before any of these every happen. You can brew green or black tea in your water, if you want. There's just enough caffeine in these to help charge you for a work out.
Whether or not you should eat depends mainly on three things: what your goals are for the workout session, how long you'll be working out, and how long it's been since you last ate.
What your goals are for the workout session: There can be many goals for working out. Is your main goal to lose weight, to build muscle, to increase endurance or VO (volume of oxygen) capacity, to keep your body supple and limber, to keep your body healthier and feeling better, to improve your sleep at night, to get the endorphin surge that makes one feel good from physical activity, to keep or improve capacity to perform, to gain weight and mass, etc.
For many people the primary goal they have is to lose weight (or maintain low weight), with secondary goals of keeping healthier and feeling better. Because the body uses glucose first and then goes into fat storage to move, then it seems obvious that one would want to not eat before working out, so that the body has to tap into fat storage for fuel. But actually what happens is that when the muscles are called upon to work out after a long fast, such as after 12 hours of not eating and then sleeping, the final end result is greater fat storage. The body taps into fat storage for the workout, but the stress on the body is so great from the previous fasting period, that it releases various hormones that increase hunger and increase fat storage - to be prepared for this kind of stress again. So the way to prevent this is to fuel up with a small amount of healthy foods that will start the body into the workout, with a slow change over to fat storage usage. Steel cut oats or old fashioned oats (the less processed the better) seems to be a prime food for this. Because of the high amount of resistant starch, starch that is formed in such a way that it resists being digested in the small intestine therefore is able to move on to the large intestine (colons) before being digested. This makes the amount of fuel released from the oats a constant even product. Yet the caloric amount will be small enough, in most cases, that the body will also tap into fat stores. The stress amount to the body will be much lower with this fuel system, and the workout will also be much easier to do, because it's fueled.
Other foods can be consumed. Many running coaches suggest honey as a fuel source, though this gives a quick surge of sugars and can actually cause a release of insulin, which pulls out more sugars from the blood stream than were put in. Sports drinks are loaded with sugar so they are even worse.
Many long distance biking coaches recommend honey and peanut butter sandwiches on whole wheat bread. Again, the honey can give a quick release of sugars, and the wheat bread breaks down into sugars pretty quickly, while the fats in the peanut butter take a lot longer to be tapped into for fuel, so they are usually put into fat storage.
Many muscle building coaches recommend protein drinks and bars. If there are sugars in them, they can help give fuel for the workout, while the protein is adding to muscle mass. The same issue can happen for the sugars with insulin production and fat storage processes. Choosing a protein bar that is low in sugar, and not made from whey protein, which is a cheap (both in price and in quality) protein source, then this can be a good fuel for working out. One complaint of many people that consume protein pre-workout is with heartburn issues. While calcium can be consumed also, to help balance the blood pH, to lower acid, there is still an increase in stomach acid to digest the protein so exercising will bounce this acid up when working out. Non-jumping movements such as lifting weights can also reduce this problem.
Bananas have long been recommended for fueling a workout. They break down into sugars fairly quickly, though because of the fiber in them, they are actually a better alternative to honey. Apples, apricots, strawberries, kiwi, papaya, oranges, and many other fruits can work good, as well.
In all cases, whether wanting to lose weight or just fuel a workout, or improve health, be sure to start your workout within 20 minutes of eating. A team of researchers, led by Dr. Kenneth Cooper, at BYU, discovered that beginning the exercise within 20 minutes of working out will actually help lower the body weight set point. Twice a day, at about 12 hours intervals, is optimal for achieving this.
In almost all scenarios, it's optimum to give your body great nutrition and a steady stream of low amounts of starches, with a small serving of oats (1/3 to 1/2 cup dry). Bring water (about 3/4 to 1 1/4 cup) to a boil, or near boil, then add to the oats, and let it steam for five to ten minutes. Add cinnamon or cacao and a little bit of liquid stevia, and you're ready to begin a great workout that will bring your body to greater health in many ways!