Eating a meal three hours or less before exercising can prolong your endurance and improve your performance. This applies whether you are going on a long walk, cycling, rowing, jogging, playing golf or tennis, lifting weights, or even if you are running a marathon or racing long distances on your bicycle. You have only a limited amount of sugar in your muscles and liver and when you run out of stored muscle sugar, you feel tired and have to slow down. You can extend your endurance when you eat a high-carbohydrate meal two to three hours before you exercise intensely for more than 70 minutes (International Journal of Sport Nutrition, 2000;10(2):103-113).

What Happens If You Don’t Eat Within Three Hours Before Long Exercise?
Your brain gets its energy almost exclusively from sugar in your bloodstream and when blood sugar levels drop, you will feel exhausted. Also, when your muscles run out of their stored sugar, they will not contract effectively. There is only enough sugar in your bloodstream to last about three minutes, so to keep sugar levels from dropping, your liver releases sugar from its cells. But your liver stores only enough sugar to last 12 hours at complete rest and far less when you exercise. Eating fills your liver with sugar. You want to fill your liver with sugar as close to your exercise session as possible and leave your stomach empty. That’s two to three hours before you start. You do not need to eat sugar because your body converts all carbohydrate foods to sugar for energy. If you eat six hours before you start to exercise, your liver will already have used up a major portion of its stored sugar.

During exercise, your muscles use primarily fat and sugar for energy. The limiting factor to how fast you can move when you exercise is the time it takes to move oxygen into muscles. Since sugar requires less oxygen than fat does, you move faster over distance when your muscles burn sugar for energy. However, you have only a limited amount of sugar stored in your muscles and liver. If you run out of liver sugar, your blood sugar will drop and you will feel weak and dizzy and can even pass out. Bicycle racers call that “bonking”. When your muscles run out of stored sugar, they hurt and feel weak so you have to slow down. Runners call that “hitting the wall”.

What Happens When You Eat a High-Sugar Pre-Exercise Meal?
When you eat foods that cause a high rise in blood sugar, your pancreas releases insulin that drives sugar from your bloodstream into cells. A pre-exercise meal that raises your blood sugar too high can cause excessive blood insulin levels. Then when you start to exercise, the combination of high blood insulin levels and your contracting muscles drawing sugar from your bloodstream can drive sugar into your muscles so rapidly that you can suffer from low blood sugar. Low blood sugar levels will make you feel exhausted and limit your ability to exercise (American Journal of Clinical Nutrition, August 2006; Journal of Applied Physiology, 43: 695-9, 1977; Medicine & Science in Sports and Exercise, 11: 1-5, 1979). You can prevent the drop in blood sugar from high insulin levels by taking sugared drinks, gels or foods soon after you start exercising. The sugar that athletes take during prolonged exercise prevents blood sugar levels from dropping so they do not feel tired from their pre-exercise meal, even if it caused a high rise in blood sugar (Medicine & Science in Sports & Exercise, 1998;30:S82, Abstract 471).

High-Sugar Foods Most People Should Avoid in a Pre-Exercise Meal
The Glycemic Index classifies foods by how high they cause your blood sugar to rise right after you eat them. Your pre-exercise meal should not contain high-glycemic foods such as white bread, ice cream, high-sugar energy bars, sugared drinks (including fruit juice), and all sugar-added foods. These are the foods that are most likely to cause high insulin levels and the low blood sugar response that can follow when you begin to exercise.

You can eat anything you want for your pre-exercise meal as long as it:
• can pass from your stomach before you start, and
• is not full of sugar.
A healthful pre-exercise meal should contain some protein in addition to unrefined carbohydrates. You can include foods such as oatmeal, eggs, oranges and other whole fruits, peanut butter, nuts, beans and so forth. Avoid sugar-added drinks and foods.

Eating and Drinking for Vigorous Exercise or Competition
If you plan to exercise vigorously for more than an hour, the best time to take sugar to help you prolong intense exercise is 30 minutes or less before you start. Researchers in Scotland showed that taking a sugared drink 30 minutes before exercise allowed the subjects to exercise at 90 percent of their maximum capacity for 12 percent longer than when they took the same sugared drink two hours before exercise (International Journal of Sport Nutrition and Exercise Metabolism, November, 2013). You can take chocolate bars and drinks because they contain both sugar and caffeine. The researchers showed that taking sugar two hours before exercise does not help you to sustain intense exercise any longer than taking nothing at all. Taking sugar within 30 minutes before exercising will not cause a low blood sugar response (International Journal of Sports Medicine, 1991;12:180-186).

Should Your Pre-Exercise Meal Contain Caffeine?
Caffeine in chocolate, coffee and tea helps you to exercise longer. You can take a caffeine source from a half hour before exercising, through the entire exercise session. Taking caffeine after you finish exercising helps you recover faster. Caffeine specifically drives sugar into muscles faster during exercise to keep you going, and during recovery to help refill your muscles with sugar (International Journal of Sport Nutrition, 1995;5:S84-99; Sports Medicine, 1996;21: 393-401; Journal of Applied Physiology, 1998;85:1493-1501; International Journal of Sport Nutrition, 1999:229). Older texts claimed that caffeine increases endurance by causing fat cells to release large amounts of fat into the bloodstream, but this has been disproved because caffeine increases endurance even in doses that do not raise blood fatty acid levels (Journal of Applied Physiology, 1995;78:867). Caffeine is a diuretic when you are resting but does not increase urine output during exercise (J Sci Med Sport, 2015 Sep;18(5):569-74). Caffeine prevents reabsorption of water in the kidney tubules. Exercise decreases blood flow to the kidneys so that less fluid passes through the glomeruli and less urine is produced (International Journal of Sport Nutrition and Exercise Metabolism, 2005;15(3), 252-65).