Every endurance athlete has "bonked," or
"hit the wall" at some point. 30 years ago, scientists believed that
extreme fatigue, or bonking, was a result of lactic acid build up. However,
research has shown that the causes of fatigue are more complex. There is
general agreement that fatigue has two components: a muscle component and a
brain component. The question is whether they act in concert or independently.
The best evidence indicates that fatigue begins in the muscle. As muscles work harder to generate energy required for muscle contraction, there is a natural change in the level of four critical components:
- Free radicals
- Branched-chain amino acids
Glycogen is the form in which muscles store energy.
Glycogen supplies in the muscles are limited. When glycogen falls to a critical
level, the muscle begins to lose its ability to maintain energy output and we
bonk. To preserve glycogen and stave off the bonk, the muscle cell increases
its use of blood glucose as an energy source. Since glucose is the brain cells'
primary energy source, any drop in circulating glucose impacts the brain,
making us feel more tired. A drop in blood glucose also causes a release in the
brain of IL-6, an immune regulator, that has been shown to increase feelings of
We all know about free radicals. These are highly reactive molecules that can damage muscle proteins and are one of the causes of post-exercise muscle soreness. Free radical buildup is a natural consequence of exercise. The more oxygen we breathe, the more free radicals we produce. Free radicals cause fatigue in three ways. First, free-radical buildup interferes with the mechanism responsible for maintaining oxygen and nutrient delivery to the muscles. Second, free radical formation also interferes with the mechanism that controls muscle contraction. Third, the muscle damage generated by free radicals may release brain fatigue signals.
During strenuous activity, blood pH is lowered when the muscle is forced to generate energy in the absence of cellular oxygen. This process produces lactic acid, which at one time was believed to cause a lowering of blood pH and generate fatigue. We now know that the pH effect is a result of the energy-producing reaction, not lactic acid. Regardless, a lowering of blood pH produces multiple symptoms associated with fatigue such as muscle burning and reduced work capacity.
Branched-chain amino acids, of which there are three--leucine, isoleucine and valine-- are highly concentrated in muscle cells. Unlike most amino acids, branched-chain amino acids can be efficiently utilized by muscles as an energy source. In fact, during extended exercise, up to 20% of the muscles' energy may come from branched-chain amino acids, particularly leucine. As branched-chain amino acid stores are used up, their blood concentration falls, which sets in motion the release in the brain of a neurotransmitter called 5HT. 5HT is often called the "fatigue hormone" since it creates a feeling of tiredness.
Although more research is needed to complete the picture, it is certainly clear that changes in these four key constituents, which are a direct result of increased muscle metabolism, can cause brain fatigue and bonking. This unified model of fatigue also gives us an excellent blueprint for how we can use nutrition to combat fatigue.
- Carbohydrate in the form of high-glycemic sugars, such as sucrose, fructose and glucose (preferably two or more). High-glycemic sugars preserve muscle glycogen, maintain brain function during exercise and prevent the release of fatigue neurotransmitters.
- Antioxidants such as Vitamins E and C. Antioxidants recharge the body's natural antioxidant system, which is essential in combating the effects of free radicals on muscle function and the brain's perception of fatigue.
- Protein, particularly protein high in branched-chain amino acids. BCAA's serve as an additional source of muscle energy and prevent the release of fatigue signals from the brain. The ideal ratio of carb to protein is 4:1, which is found in sports drinks like Accelerade and gels like Accel Gel. Numerous studies have demonstrated that sports drinks and gels that contain a 4:1 ratio are far more effective in not only preventing mind and muscle fatigue, but also minimizing post-exercise muscle damage.