Amino acids, from protein, can’t be stored like glucose, so instead, they are either synthesised into proteins, deaminated into ammonia and leave the body as urea or join the process of metabolism by entering the Krebs cycle or joins the process of glycolysis to form pyruvate, the process of amino acids joining glycolysis is known as gluconeogenesis. Glycerol from fat also joins the process of glycolysis, the process of glycerol joining glycolysis is also known as gluconeogenesis, and fatty acids from fat can jump into the Krebs cycle. Interestingly if you have too much glycerol and fatty acids they can recombine with one another to form triglycerides, in the process of lipogenesis, these triglycerides are stored in the liver and are broken down in the process of lipolysis to be used to help form ATP.

When we run out of glucose oxaloacetate can start to turn into pyruvate, which produces Acetyl-CoA, as oxaloacetate is depleting the newly produced Acetyl-CoA can’t enter the Krebs cycle and it accumulates, they therefore combine and turns to ketones, in the process known as ketogenesis, which can exit the liver and travel to the brain in the bloodstream and change back into Acetyl-CoA, in the process known as ketolysis, and be used to make ATP, this is great fuel for the brain, if the brain was to not get the fuel it will resort to breaking down our skeletal muscle, for amino acids and energy, the reason why the liver can release ketones but not other organs, is because the liver contains different enzymes to other organs meaning ketones are not broken down in the liver. Ketones are also very useful to reduce hunger, reduce oxidative stress (reducing ageing), reduce stress, reduce arteriosclerosis (the accumulation of fats, cholesterol and other substances in and on the artery walls), decrease inflammation, helps breathing, protects the heart and nerves and deprives cancer, as it can not live on ketones.

ATP alone can be stored in organs such as the muscle, and used during short bursts of energy lasting 4-6 seconds, and after that ATP which is stored with creatine phosphate is utilised mostly, which lasts about 15 seconds (9-11 seconds longer than ATP stored alone), this is known as the ATP-PCr system. After this, for longer-lasting energy, optimally lasting 1-2 minutes, the glycolytic system is utilised, which relies on glycogenolysis and then glycolysis to form only 2 ATP, but instead of pyruvate forming Acetyl-CoA, it forms lactate, the process of glycogenolysis and glycolysis produce hydrogen ions, this system does not use oxygen making it anaerobic. When hydrogen ions accumulate quickly they can’t be removed and they begin to denature the enzymes responsible for breaking down fuels for energy, this will cause fatigue and lead to failure of an exercise if the muscle is not given enough chance to recover. Finally, the oxidative system requires oxygen and it can use fat, carbohydrates and protein for fuel, this process utilises the Krebs cycle and the electron transport chain, making it the most effective and less tiring system, this system breaks down fats for fuel and this is why you should perform lengthy cardiovascular exercise within your aerobic capacity, which is usually up to 70-80% of someone's maximum heart rate. I also regularly only recommend performing lengthy cardiovascular exercises with a low intensity, as high-intensity cardiovascular exercise has been shown to increase the risk of atrial fibrillation (causing an irregular and often fast heart rate), right heart failure and myocardial fibrosis (which can lead to heart failure or death). Each of these systems can be improved to enhance performance and the utilisation of certain fuels.