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Understanding Your Body's Energy-Producing Chemicals
and how they produce energy for sports training – Part 1

When we exercise, a myriad of physiological processes switch on and heighten.
In this two-part article we look at specific energy producing chemicals and reactions.



Lactate, lactic acid and glycolysis
Lactate

You might think that the body’s chemical lactate (particularly lactic acid – of which more later) is ‘bad’ – after all why would we want to warm down to clear our muscles of it, if it was good? Perhaps we think of it as a ‘waste’ product that causes muscle damage and that is only generated by the anaerobic energy systems. However, lactate and (lactate acid) is actually crucial for energy production for both short and long duration exercise. It’s far from being ‘bad’.



What is lactate?
Lactate is produced in muscles at very low exercise intensities (such as an easy pace run) as well as much higher ones (such as a series of flat-out sprints). It is actually present in the body at all times. It’s only when exercise intensity increases that lactate levels increase in response to ‘glycolysis’ (literally meaning ‘the breaking down of glucose’).

There are two types of glycolysis – ‘oxygen dependent’ and ‘oxygen independent’ (these can be equated to the aerobic and anaerobic energy systems). Glucose is derived from the carbohydrates that we ingest. Glycolysis kick-starts chemical processes within our muscles that produce the energy required for sustained muscular contractions. Without glycolysis, exercise could not last more than a few seconds.



Lactate is not…
Lactate was one of the first body chemicals that exercise scientists were able to analyse. Partly because of this, invariably it has been wrongly associated with a myriad of exercise-induced physiological responses, such as fatigue, cramp and even sprains.



Lactic acid

When lactate is released into the bloodstream it does not cause pain. If it did, we’d be hurting all over, perhaps all of the time, or at least after any form of exercise – even a stroll in the park (due to glycolysis and the production of lactate at very low exercise intensities)! Expanding on this, lactate begins to rise as a consequence of glycolysis in the untrained, at only about 55% of their maximum capacity for aerobic metabolism (VO2 max). So, if lactate is not the painful problem, what is?

To answer this we need to understand a bit of chemistry – any substance that ionises in solution and gives off hydrogen ions is an acid. When we exercise at a high intensity, we create the ‘right’ conditions for acid to develop in our muscles. Although we are still gulping down oxygen, it becomes insufficient to create enough aerobic energy – this alters the chemical equations taking place and, instead of lactate, lactic acid is produced.

Specifically, lactic acid is formed when pyruvic acid temporarily accepts two hydrogens (electrons).

Note: lactic acid becomes lactate again once it enters the blood stream.

Lactic acid can be regarded as lactate’s errant cousin. Lactate is the ‘goody-goody’, perfect, always able to get the energy creation job done (at least up to certain intensities – as high as 80% of VO2 max for the endurance trained athlete).


Unfortunately, lactic acid ends up getting the wrap – despite trying to emulate its cousin when it is involved in energy production at higher exercise intensities, it can’t. It wants to keep muscular contractions going – but its pace of production ultimately exceeds its rate of clearance, leading to loss of muscle power and eventual exercise cessation. The pain that results is believed to be from acidic muscles aggravating nerve endings, which ‘irritate’ the central nervous system. This can also lead to feelings of nausea and disorientation which can result from work outs that produce lactic acid.



No bad guys!
Lactic acid, like lactate, is not a waste product. During recovery, when there is a much more plentiful supply of oxygen, lactic acid loses its two hydrogen molecules, returns to pyruvic acid and is used as an energy source. In fact 50% of the lactate (remember that lactic acid returns to lactate when it enters the blood stream) produced during a tough work out actually goes on replenishing muscle glycogen stores during recovery. (Glycogen is premium grade muscle fuel. It can only be stored in the muscles and liver in limited amounts (approx 375g) and is derived from carbohydrate.) Glycogen is central to glycolysis.



The lactate shuttle

The contribution lactate makes to glycogen replenishment and post-exercise recovery occurs during what’s known as the ‘lactate shuttle’ (LS). When lactate is released into the bloodstream, the liver uses it to produce blood glucose and glycogen (the heart and other muscles use it for energy production). In terms of the LS and energy sustainability during exercise, its ability to distribute carbohydrate – as potential glycogen, through the metabolism of lactate – from muscles that are fully glycogen stocked, is key. The LS lifts glycogen from muscles that are not being used significantly, for example the arms while marathon running, to areas where glycogen is being drawn significantly – obviously the legs of the marathon runner. This process helps to sustain energy.

Understanding Your Body's Energy-Producing Chemicals - Part 2
By: John Shepherd


In part 1 we looked at lactate and lactic acid and how these two chemicals are crucial for energy production, now we move on to equally important energy producers

Pyruvic acid
Glycolysis is the process by which the body produces energy from carbohydrate (or more specifically glycogen – of which more later). During this process, pyruvic acid (PA) is created. Like lactate, PA is crucial to energy production – more than 10 different chemical reactions are involved in this process. PA is then used within the Krebs Cycle (of which more later) to produce energy. When PA begins to accumulate in muscles, through what can be a relatively minimal increase in exercise intensity, the enzyme lactic dehydrogenase converts it into lactate. Under moderate-to-high exercise intensities, lactate is converted back to PA and reused to continue producing energy. However, at high exercise intensities, muscles are unable to do this and lactate production outstrips PA production, with the result that exercise becomes increasingly difficult and lactic acid is formed.



The Krebs Cycle and Adenosine Triphosphate
The Krebs Cycle provides nearly 90% of the energy required for aerobic exercise and produces adenosine triphosphate (ATP). ATP is an energy-rich compound and is known at the body’s ‘universal energy donor’. This is because it can produce energy with and without the presence of oxygen.

Although you might think it, energy for fuelling the body does not come directly from the food we consume, rather it is converted into ATP (and other sources). It’s this that produces the energy. The Krebs Cycle allows for the regeneration of ATP under aerobic conditions. During the Krebs Cycle, PA changes to a form of acetic acid, which breaks down carbohydrate (glucose and glycogen), via the release of carbon dioxide and hydrogen ions, to produce aerobic energy and ATP within the working muscles.

Endurance training will increase the number of enzymes relevant to the Krebs
Cycle. Their numbers may actually be increased two to three times after a sustained period of this training. This equates to a better energy transfer system and therefore improved endurance performance.

The body’s cells can only store limited amounts of ATP (about 85g at any one time), therefore it has to be constantly resynthesised. Note: muscles are able to store creatine phosphate in greater amounts.



Creatine phosphate (CP)
Creatine phosphate, like ATP, is another high-energy compound that does not require oxygen to ignite it to produce energy. Even when combined with ATP, CP can only sustain flat-out effort for 5-8 seconds, as they provide fuel for the immediate anaerobic energy pathway. Interval training will improve the body’s ability to produce CP, thus improving power endurance.

Approximately 5 millimoles (mmol) of ATP and 15mmol of CP are stored within 1Kg of muscle.



Creatine supplementation and sports performance
Creatine is a well known sports supplement that can boost immediate and short-term anaerobic power. This is achieved by ‘loading’ muscles with more creatine concentrations. This allows the athlete to handle increased training loads, resulting in increased strength, power and speed gains. Basically, creatine supplementation puts more ‘high octane’ fuel into the power athlete’s tank. Numerous research findings indicate that creatine supplementation works for power athletes and has few, if any, proven side-effects. However, older athletes, or those with kidney problems, should consult their doctor before using it.

Glycogen
Glycogen, like CP, is also premium-grade muscle fuel. It can only be stored in the muscles and liver in limited amounts (on average about 375g) – enough for about a day’s activity if we were not to eat.

It’s converted in the body from carbohydrate. It’s argued that a tough two-hour
endurance work out would virtually deplete the liver and muscles’ glycogen reserves. Consequentially, glycogen must be replenished via carbohydrate consumption. A carefully constructed training programme, which includes rest days and training of varying intensity, will also maximise glycogen replenishment. If you don’t do this you could train on ‘empty’ and your performances will decline.

Liver glycogen stores (which are converted into the sugar glucose during exercise), although small, are crucial, as they fuel the brain as well as the muscles during exercise. If the liver fails to meet glucose demands, then the athlete will feel feint/light-headed and weak. This condition is called hypoglycaemia. For this reason (and for maintaining as much as is possible of the body’s overall glycogen content during endurance events that last more than an hour), athletes should take on carbohydrate as they race/train. This can be achieved in the form of energy drinks or gels.

It takes about an hour to restock up on 5% of the body’s glycogen stores, assuming optimal carbohydrate consumption. To fully restock depends on the training level of the athlete, rest, recovery and nutrition.

What happens when glycogen stores decrease?
Athletes ‘hit the wall’ – they feel incredibly fatigued and light-headed and their bodies may significantly switch to using fat (and even protein) as energy sources. Note: the trained endurance athlete is much more adept at using fat as an energy source, when compared to the untrained this off-sets their glycogen, extending their endurance.

The knock-on effect on protein…
You don’t want to use protein as an energy source as this has a catabolic effect. Basically, this means that your body begins to ‘eat’ itself to supply energy. This sounds drastic, and in many ways it is for the athlete, as valuable power producing lean muscle can be lost as the body searches for fuel and obtains it from its muscles.

Consequently, endurance athletes need to ‘think’ protein as much as carbohydrate when it comes to their nutrition. You should supplement post-work out with both carbs and protein to kick-start protein resynthesis and glycogen replenishment