Despite oxidative phosphorylation's large capacity for energy production, the rate of electron transport and therefore ATP generation is limited by oxygen, the final electron acceptor/oxidizing agent in the chain. Oxygen is readily available to the cell through circulating hemoglobin but can only be used when the energy requirements of the cell do not exceed the rate of ATP production via oxidative phosphorylation. This is often the case because the rate at which electrons can be transferred to oxygen is relatively slow. In its search for a more readily available energy production pathway, the body will switch to anaerobic glycolysis as its primary provider. Despite requiring tendifferent reactions to form its end product pyruvate, glycolysis occurs quite rapidly. During these ten reactions, two net ATP are formed per molecule of glucose, which help working muscles satisfy energy requirements quickly. However, upon inspection of the Krebs Cycle figure above, it is clear that ATP generation through this process would stop abruptly if NAD+ were not regenerated to act as an electron acceptor in the conversion of glyceraldehyde 3-phosphate to 1,3 biphosphoglycerate. The body has adapted to this requirement by using pyruvate to oxidize (remove electrons) from NADH, thereby forming NAD+ and lactic acid in an oxidation-reduction reaction. An enzyme known as lactate dehydrogenase carries out this conversion, allowing NAD+ to be recycled quickly for repeated use in glycolysis.
Like most things that seem too good to be true, there is a problem with anaerobic glycolysis. The production of lactic acid during this process can decrease the pH of active local tissue enough to cause cramping and fatigue. At moderate exercise intensities, the body does have a limiting mechanism to cope with this entirely new problem. While working muscle releases lactic acid into the bloodstream during anaerobic exercise, the liver is busy absorbing and converting lactic acid back to pyruvate by the same enzyme found in anaerobic glycolysis. Pyruvate is then recycled into circulation to feed working tissue. In this way, part of the metabolic burden is shifted to the liver. If oxygen is available and aerobic conditions persist, pyruvate can enters the mitochondria to be used in the Krebs cycle and electron transport chain if oxygen is available. This relatively simple process is known as the Cori cycle.
During aerobic conditions, the level of lactic acid can be maintained at a low level in the blood for a significant period of time through the Cori cycle. The greater the fitness level of an individual, the longer the lactic acid level can be maintained. However, even the best of active athletes reach a point where their bodies begin to produce more lactic acid than their liver can metabolize and it begins to accumulate in the blood. This point is known as the lactic acid threshold for that individual and is dependent mostly on cardiac performance if aerobic conditions persist.
The metabolism of carbohydrates can also be considered in the context of exercise strategies. For example, what kinds of foods should be eaten before a particular type of exercise? For almost any activity, most dieticians suggest a diet high in starch and other complex carbohydrates. Since this is also recommended for most diets in general, eating a diet high in carbohydrates should not be difficult. Furthermore, dieticians also recommend that people avoid meals high in fats and proteins before exercise, since these foods inhibit gastric emptying and require longer to digest than other foods. Since the sympathetic nervous system inhibits the gastrointestinal tract during exercise, any remaining quantities of food in the stomach may lead to cramping and acid reflux into the esophagus. Lastly, simple sugars that are readily absorbed by the intestine should be avoided because they cause rapid fluctuations in blood glucose, thereby affecting the circulating energy supply. Unlike simple sugars, starch must be broken down into glucose before being absorbed into the blood. In this way, the energy supplied by starchy foods such as pasta and vegetables can be absorbed more slowly and be available for longer than other simple sugars.
How long before the onset of exercise should a meal be consumed? For most people, a meal high in carbohydrates will be emptied from the stomach after three hours of fasting. As mentioned earlier, exercising on an empty stomach is physiologically beneficial and eating three hours before an exercise bout will usually give the stomach plenty of time to empty. Although the circulating glucose provided by starchy foods is used as energy for working muscle to some extent, most of the energy used comes from glycogen stores in the muscle itself. Therefore, it is important to be eating high carbohydrate meals days before an event or exercise bout in order to build up glycogen stores in the muscle and liver. Because of glycogen's branched structure and hydrophilic hydroxyl groups, it is able to absorb significant quantities of water, which helps keep the body hydrated during exercise.
Losing weight can also be related to carbohydrates. Fluctuations in hormones caused by the ingestion of carbohydrates make the burning or utilization of fats more difficult. For example, eating a meal high in carbohydrates will cause the release of insulin, a hormone that allows cells to uptake glucose. However, insulin also acts on adipose tissue to inhibit the release of fatty acids and promote their synthesis. Since most people are trying to become leaner by burning fat through exercise, eating a meal before exercise may not be the best idea for losing weight. The best time of the day to exercise with the intention of losing weight is in the morning. Since no food has been consumed since the last meal of the previous day, levels of insulin in the blood will be low. Furthermore, levels of glucagon will increase due to fasting and this hormone stimulates the release of fatty acids from adipose tissue while inhibiting the breakdown of glycogen from the liver. Epinephrine and Norepinephrine are also secreted in response to exercise and these hormones mimic the effects of insulin by increasing availability of fatty acids in the blood. Of course, fatty acids may only be utilized by working muscle under aerobic conditions. Therefore, in order to burn fats effectively, the ratio of insulin to glucagons must be low and the body must be performing exercise at a relatively low intensity for a prolonged period of time.