Carbohydrate is typically the limiting energy substrate in exercise, meaning that it will run out before protein or fat runs out. Glycogen depletion is a term used to describe when carbohydrates are used up and no longer exist as a fuel source for working muscles. In other words energy stores are depleted, which can result in cells and muscle tissue being damaged and the immune system being stressed if exercise continues. Some of theses metabolic processes, which can lead to muscle soreness and damage, can be counteracted to a degree by dietary factors.
What and How Much should I eat
Before the workout
It is important to fuel your muscles before the workout begins. A good analogy to remember is that you do not drive your car to its destination, and then put gas in it – our bodies work the same way. Fill the tank by eating a meal comprising mostly of low- to moderate-glycemic carbohydrates two to three hours before exercise (e.g. oatmeal). A good preworkout meal should contain some protein, which in turn decreases the body’s reliance on muscle protein as an energy source, sparing the muscle to focus on the workout. It also enhances the recovery period by being available as amino acids for protein synthesis (e.g. add a scoop of whey protein powder to the oatmeal).
During the workout
The aim is to increase sparing of muscle glycogen and simultaneously improve endurance. Maintaining a continuous source of glycogen during intense training minimizes muscle tissue damage and further aid in post-workout recovery. There is consensus that 8 to 10g of carbohydrate per kg of body weight will maintain appropriate glycogen levels during heavy training. Recommendations for an endurance event include liquid carbohydrate right before the event, and a regular consumption of glucose polymers (e.g. maltodextrin) during the event, to delay fatigue. A 6 to 8% solution containing 15 to 20g of carbohydrate per 7oz of water is ideal (such as Gatorade or Powerade). Such sports drinks are easily absorbed during workouts, as well as provide water and electrolytes to prevent dehydration. New research shows that in addition to the glucose intake, a small amount of protein during workouts may also stimulate more insulin, which further aids the transportation of glucose to the muscles. Amount of appropriate consumption will depend on the type of athlete, the intensity and duration of the event, and the temperature.
After the workout
A first priority after a workout should be recovery in order to return to be fully prepared with a full tank of fuel for the next training bout, as quickly as possible. It is not possible to consume enough carbohydrate during moderate- to high-intensity exercise to replace the glycogen that has been depleted. Furthermore, the muscle degradation during exercise requires protein to fully recover. It is therefore important to consume additional carbohydrate and protein after the workout. This should be done as soon as possible, a period known as “the muscle recovery window” or “the glycogen replacement window” (the first two hours after exercise). This is because the enzyme (glycogen synthase) responsible for storing glycogen is highly elevated immediately after exercise. Again, a combination of carbohydrate and protein is the best source of macronutrients for post exercise recovery. Sports drinks are highly recommended for the recovery period due to their ability of rapid absorption, and their water and electrolyte content.
Consequences of unrecovered glycogen depletion
Failure to appropriately replete glycogen may result in chronic glycogen depletion. There is evidence linking muscle glycogen depletion with both fatigue and injury. Symptoms are very similar to those of over-training. Muscles that are fatigued lose their strength, and thus their ability to protect joints, with the unfortunate risk of injury. The purpose of a good recovery nutrition strategy is to avoid chronic glycogen depletion that can take place over several days of training and avoid injury. Furthermore, consistency in glycogen repletion results in effective and recovery between workouts and competitions and thus overall improved performance during competition.