An excellent study from the United Kingdom shows that eating before intense exercise causes your body to:
• burn more carbohydrates during exercise, and
• digest, absorb, and metabolize more carbohydrates that you eat after exercising (American Journal of Physiology: Endocrinology and Metabolism, August 14, 2018). The researchers had 12 healthy males cycle for an hour, two hours after eating cooked oatmeal with milk . On another day, the same men cycled for an hour after skipping breakfast.
All athletes learn sooner or later that they need to eat within three hours before an intense workout or competition. There is some evidence that exercising after fasting may help your muscles retain and process sugar during controlled non-intense recovery exercise (Sports Med, Mar 2017;47(3):415-428), but you cannot do a substantial intense workout after fasting.
Eating Breakfast Helps You Move Faster and Exercise Longer During exercise, your muscles get their energy from carbohydrates, fats and, to a lesser degree, protein. The limiting factor to how fast you can move over distance is the time it takes to move oxygen into muscles. Since carbohydrates require less oxygen than fats do to power your muscles, you go faster over distance with the greater percentage of carbohydrates your muscles use for energy. When you lie in bed not moving, your muscles get most of their energy from fat, but when you are running at 100 percent effort, your muscles obtain almost 100 percent of their energy from carbohydrates (sugar).
Most people have enough fat in their body to supply them with energy for weeks, even if they are very thin. However, you have only a very meager supply of sugar in your body, stored primarily in your liver and muscles. After you take an intense workout, your muscles and liver will have used up a lot of the sugar stored in them. The faster you replenish the sugar in your muscles and liver, the faster you will recover to be able to take your next intense workout. Eating breakfast before a workout helps you process the sugar in your after-work out meal faster, which will help you recover faster.
Bonking and Hitting the Wall Your brain needs a constant source of sugar to function. However, there is only enough sugar in your bloodstream to supply your brain with energy for three minutes, so your liver has to constantly supply your blood with sugar to maintain blood sugar levels. However, there is only enough sugar in your liver to last about 12 hours if you lie in bed and do not move, and when you are exercising intensely, there is only enough sugar in your liver to last about 50 minutes. That is why athletes in all competitions lasting more than 50 minutes should take a source of sugar during their events, or they are at risk for suddenly passing out from low blood sugar levels. Endurance athletes, particularly cyclists, call this "bonking".
There is only enough sugar in your muscles to run about 18 miles and the faster you run, the sooner your muscles run out of stored sugar so they will feel stiff and heavy. Runners call this "hitting the wall". Recent research suggests that "hitting the wall" may actually be caused by low blood sugar, not low muscle sugar. However, the rules remain the same; you should take a source of sugar during any competition lasting longer than 50 minutes.
My Recommendations During sustained high-intensity sports lasting less than an hour, you can benefit just by rinsing your mouth with a sugared drink. A sugary taste in your mouth sends messages to your brain to make your muscles work harder. For 1-2 hour competitions, you should take at least 30 to 60 grams of sugar per hour in food, gels or drinks. For events lasting more than 2.5 hours, you may need to take in at least 90 grams of sugar per hour (Journal of Sports Science, June 9, 2011;29(sup1):S17-S27).
All serious exercisers and competitive athletes use some form of stress and recovery in their training. You will achieve a much higher level of fitness by alternating more intense workouts on one day with slow and easy recovery workouts on the next day. You should always eat before intense workouts and competitions. There is weak evidence that fasting before slow recovery workouts may help your body process sugar better, but I do not recommend doing this because it can make your workouts fatiguing and uncomfortable.
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