Under the Microscope/Dr William Reville: Human athletic prowess is unimpressive compared with other animals'. Many of them can run faster than humans, for example. Lions can reach speeds of 50mph, and cheetahs can sprint at up to 70mph. Most humans would have difficulty reaching 25mph.
We are improving, though, and since 1900 track-and-field records have continued to fall in every event from sprints to marathons. Sports physiologists are developing dietary supplements to enhance athletic performance. Is this a good idea, and how much further should we go?
The mile run is traditionally the best-known athletic event, and records here are a good yardstick of the improvement in athletic performance over the past century. The record for the mile in 1900 was four minutes and 12 seconds. It took more than 50 years to lower the record below four minutes, when Roger Bannister set a record of three minutes and 59.4 seconds, in 1954. More improvement has been made since then, but overall less than 17 seconds have been sliced off Bannister's time. Some experts predict the mile record may be down to three minutes and 30 seconds by 2050, but the limits of the human body may prevent faster times than that.
Improved understanding of the physiology and biochemistry of exercise and fatigue is guiding nutritional and other interventions in athletics to extend the limits of human performance. Athletic performance depends primarily on the speed and strength of contraction and the endurance capacity of our muscles, and in order to perform our muscles need fuel. This comes mainly in the form of carbohydrate and oxygen to burn the fuel and generate energy.
Two muscle fuels are used in short-term high-intensity events such as sprinting and weight- lifting. These are glycogen, a storage form of glucose, and phosphocreatine. Phosphocreatine is made in muscle from creatine, and oral intake of creatine increases the concentration of phosphocreatine in muscle. The unit of chemical energy currency used by muscle to power contraction is adenosine triphosphate (ATP). When it is spent it is broken down to adenosine diphosphate (ADP), which is immediately recharged to ATP by phosphocreatine. Increased concentrations of phosphocreatine enhance the resynthesis of ATP, thereby keeping up energy supply and delaying fatigue. There is evidence that dietary creatine supplements improve short-term high-intensity exercise, but there is no convincing evidence it enhances aerobic exercise performance.
In 2000, the American College of Sports Medicine declared there was no definitive evidence that creatine supplements harmed people's health. Reports of negative side effects have been published, however. Using creatine in the long term may damage the kidney.
As everyone who exercises a lot knows, fatigue limits performance - and anything that can delay the onset of fatigue will enhance perfor-mance. Apart from chemical fatigue in the muscle cell, fatigue also involves the central nervous system. It is strongly suspected that a brain chemical called serotonin is involved.
Serotonin is a neurotransmitter - that is, it facilitates transmission of signals from one nerve to another. High levels of brain serotonin are suspected to cause increased mental and physical fatigue. Exercise raises levels of brain serotonin, but research has shown that oral administration of certain amino acids lets them compete with the passage of precursors of serotonin from the blood to the brain. This could prevent exercise-induced increase in serotonin, thereby delaying mental and physical fatigue. This would be useful in endurance events, but further studies are necessary before dietary supplements could be safely recommended.
Better breathing could also enhance performance. To breathe we must use inspiratory muscles, and this steals blood from locomotor muscles that power athletic performance. If the amount of blood used by the inspiratory muscles could be minimised, this would free up blood for the locomotor muscles, increasing their power output. Special respiratory training techniques for athletes are being developed.
A mountain antelope can run a two-minute mile in thin air at an altitude of 7,000 feet. If you look at antelope muscle in an electron microscope, you see that its mitochondria are three times bigger than human mitochondria. Mitochondria are the little organelles in the cell in which food is burned in the presence of oxygen to produce ATP. It is estimated that if humans were genetically engineered to have bigger mitochondria, and bigger hearts and more blood vessels to get more oxygen to the mitochondria, we might be able to run at 40mph.
To genetically engineer in order to enhance performance would be to manufacture a product with performance specifications. To date we can claim that record-breaking performances are largely the product of training and skill. This could not be said of a genetically engineered performance. Neither could it be called sport. You can read more in BioMed Central News and Views, October 3rd, 2000.
William Reville is associate professor of biochemistry and director of electron microscopy at University College Cork