Food for Thought – Ultrarunning requires some mental stamina and the question, “Can we feed the mind with any special nutrition?” seems intriguing. Certainly lack of food can produce some pretty negative thoughts and a bad attitude, but is there some food that can help us think positive thoughts and improve our race strategies and therefore performance. Does the mind only require intellectual nourishment for sound decision-making and a good outlook?
The Science – Looking at this from the brain’s side, glucose is virtually the only fuel the brain uses, unlike our muscle tissue, which uses both glucose and fats. The brain also does not store any glucose – as glycogen, as the muscles do – and is totally dependent on blood glucose for its energy supply. In order to function normally, the brain requires a relatively normal blood glucose level. So what happens to an ultrarunner when he or she is running hours and hours with an increasing demand for glucose? If our ultrarunner does not ingest adequate amounts of carbohydrate (glucose) along the way, hypoglycemia (low blood sugar) sets in. And with prolonged hypoglycemia comes central nervous system fatigue – the progressive shutdown of the part of the brain that drives the muscles. Bad news for attitude and bad news for performance.
In one of Tim Noakes’s studies from The Lore of Running, he found that two percent of marathoners, six percent of those who ran 50 kms and 11 percent of those who ran 100 kms were hypoglycemic. He cites the symptoms as a “reduced ability to concentrate, a sudden feeling of weakness, and the intense desire to stop running. Typically, the athlete senses the impossibility of completing the race”. Sound familiar? Obviously foods containing glucose would have helped these athletes perform better, but an explanation of the mechanism of action might make this concept easier to understand.
The main supplier of glucose into the blood is the liver. This happens directly from its own store of glucose or indirectly by a process known as gluconeogenesis (the production of glucose by the liver from substances other than carbohydrates, for example, proteins). Training also helps the liver’s ability to produce glucose from circulating blood lactate. However, because the liver is so small compared to the body’s muscle mass and only stores about 250 – 500 calories of glucose, it could never keep up with the muscles’ demand for glucose when exercising hard or long. In a trained athlete, this amounts to three or four hours of running at a moderate pace. And the muscles are prevented from using too much circulating blood glucose in order to protect the brain. Fortunately our body has provided the muscles with other ways of obtaining glucose; namely a good supply stored in its own tissues and the ability to use fat as a source of energy. And remember, the supply stored in the muscles can be doubled if post-exercise carbs were consumed within thirty minutes or so. But even this generous supply will only last about five to six hours. The rate that the liver releases glucose and the muscles take it up during exercise is increased six-fold from a resting state. When tissues increase their rate of usage faster than the liver can produce it, blood levels fall and body function is impaired. In order to prevent brain drain and preserve our mind’s ability to think and strategize effectively in an ultra event, it becomes crucial to ingest carbohydrates during the race. If the muscles become fatigued, the brain will reduce the number of muscle fibers activated and in order to continue running, the ultrarunner will have to slow down. This is the body’s way of surviving. It may limit the runner’s physiological capabilities, but preserve the neurological function of the brain. Exhaustion, in this case, may be defined as a decrease in signals from the brain to the muscles, not a direct change in glycogen levels in the muscles – in other words, central fatigue (see “Demystifying the Bonk, UltraRunning, August 2008).
More recent research is pointing to central fatigue, due to neurochemical changes in the brain, as the most common reason that runners stop running. Aside from very highly motivated runners, most people don’t usually push themselves to complete muscle failure. J. Mark Davis, a professor of exercise science and the director of the exercise biochemistry laboratory at the University of South Carolina, explains that during prolonged exercise, the brain’s production of the neurotransmitter (a chemical that carries signals from one neuron, or brain cell, to another) serotonin increases steadily. Elevated levels of serotonin can cause feelings of tiredness, sleepiness and lethargy. The rising levels of serotonin are caused by increased delivery of tryptophan to the brain and what’s interesting, Davis says, is that the increase in free tryptophan in the blood is very much related to the increase in free fatty acids in the blood.
“While many people believe that the increase in free fatty acids is very important to delaying fatigue in the muscle,” says Davis, “we think it has a negative effect in terms of central fatigue.” To make matters worse for the ultrarunner, the brain’s production of dopamine (the neurotransmitter responsible for generating feelings of excitement, reward, motivation and pleasure) begins to drop even as serotonin levels are rising. Davis is beginning to investigate new nutritional approaches to prevent dopamine levels from dropping, but one thing runners have been using for years to delay fatigue is caffeine. Most scientists agreed this was due to caffeine’s ability to increase the blood level of free fatty acids available for metabolism. Recent research by Davis and others, however, indicates that caffeine plays another, perhaps more important role, in delaying fatigue by increasing the levels of dopamine in the brain.
The Solution – The time to ingest carbohydrates is before exhaustion occurs. Our intestine cannot process carbohydrates and deliver them fast enough to keep up with the high rate of usage by the muscles. The amount of carbohydrates to ingest is approximately one gram per minute of exercise, ideally every 15 – 20 minutes or so. Start with this amount and experiment. Our bodies cannot absorb more than 240 – 280 calories per hour so don’t try to replace all carbohydrates lost – however, athletes are able to increase the absorption rate with practice and training. Caffeine may help with brain fatigue later in the race. Read the labels on the sport drinks, gels and bars you consume. Feed your brain and improve your mental edge for a better performance in your next race.