ATP and Fitness
The ATP Cycle
ATP and Its Importance in Fitness
ATP (full name Adenosine Triphosphate) is a substance used in the body as an energy carrier; in other words, it is an intermediate energy source “recharged” by energy-producing reactions in the body. The two main energy-producing processes in the body, both of them actually a complex chain of reactions, are oxidative phosporylation and glycolysis.
In both cases another substance called ADP (adenosine diphosphate) is converted, by the addition of the energy produced by these reactions and a phosphate (PO4) group, into ATP. The ATP is then used to power energy-consuming reactions and is in the process broken down into ADP once more, thus closing the cycle.
Energy-consuming reactions are virtually all the reactions in the body except the oxidative and glycolytic ones; examples would be DNA synthesis (essential for cellular reproduction and hence tissue repair), the synthesis of proteins (important for tissue repair and growth and also for digestion) and, important from the point of view of an exerciser, muscular contraction.
Oxidative phosphorylation, also known as the aerobic process, involves the breakdown of energy-containing molecules such as fatty acids and glucose into carbon dioxide by a rather long chain of events that involve various vitamins (mostly in the B Complex group) and a substance called Coenzyme Q (also called ubiquinone). The form of Coenzyme Q found in humans is Coenzyme Q10 (also known as CoQ10). The number refers to the length of a molecular side chain, and varies between different organisms. All organisms that can use oxygen contain some form of Coenzyme Q.
The end result of this process is that the fatty acids, glucose and oxygen are used up and converted into carbon dioxide and water and large numbers of ADP molecules are converted into ATP, which can then be used for any process in the body that requires energy.
There are a few further complications. One is that fatty acids cannot be directly used in this process; they have first to be converted into a number of molecules of a substance called Coenzyme A. Also, various substances derived from the breakdown of amino acids can be used in the oxidative process.
One might ask the reason for all this complication. The reason is that directly using oxygen and energy molecules to produce energy is impossible; the direct reaction of glucose or fats with oxygen is extremely energetic and would cause massive damage to the tissue in which it was occurring. In fact, many micro-organisms cannot use oxygen to produce energy and are actually poisoned by it. They are called anaerobic bacteria, and they are found in many places including the bottoms of stagnant ponds, where there is no oxygen.
In other words, the process is complicated because that’s the only way it can work.
Glycolysis is a rather simpler process, although still quite complex. It involves the conversion of glucose into lactic acid, using no oxygen but producing smaller amounts of energy than the aerobic process. Glycolysis is also called the anaerobic process. The reason why your muscles cannot use this process indefinitely to produce energy is precisely because lactic acid is produced; this disturbs the acid/alkali (pH) balance of the tissue (usually the muscles) and eventually they can no longer do their job. (All biochemical reactions can only occur within a small range of pH). The reason why this process is ever used at all is because it uses no oxygen, and also because it is faster than aerobic respiration. (Technically, in biochemistry “respiration” refers to the use of oxygen to create energy, not to breathing.)
If you are working harder than the oxygen-carrying capacity of the blood can support, then the anaerobic process is used. This usually happens during periods of intense activity; examples would be weightlifting for muscle growth, and sprint-type events such as the 100 metres.
After stopping the intense activity, the body is capable of using up the lactic acid (and also producing some energy from this) using extra oxygen. This process is essentially the reason why people breathe hard for some time after ceasing intense exercise.
To sum up so far, the oxidative pathway is most important during aerobic exercise, which is exercise that can be carried on for a reasonable length of time such as long-distance running, cycling and swimming. The anaerobic pathway is most important for short-duration, high-intensity exercise such as sprinting and weightlifting. And the phosphagenic pathway is the means by which the energy from these two processes is delivered, within the cells, to the systems that need it; in the case of exercise this is the muscle fibres.
All these various pathways require, as well as energy-producing food, a rather large collection of nutrients involved in the reactions constituting those pathways. These include mostly the B vitamins, but also various minerals including zinc and iron. Iron is also needed to form haemoglobin, which is the molecule in the blood that gets oxygen to the tissues in the first place. These facts imply that athletes have a particularly strong need to eat food with high levels of nutrients – which means fresh food processed as little as possible. It might also be a good idea for an athlete to take vitamin supplements, particularly B vitamin supplements, for the same reason.
Another system involved in this process is the creation and use of creatine phosphate. Creatine, despite its recent fame as a sports supplement, has always been in everybody’s body. The process here is that some of the energy from either of the energy-producing pathways is diverted into converting creatine into creatine phosphate. Creatine phosphate then acts as a small, fast-acting store of energy which is used to reconvert ADP into ATP. This process is a major part of the process of short –term recovery between bursts of heavy activity, such as the minute or so between “sets” in a weights workout.
The existence of this process is the reason why creatine is of use in high-intensity exercise. Supplementing with creatine increases creatine phosphate levels in the muscles beyond the level in someone not taking it, which in turn increases the size of the short-term energy store in the muscles.