Sports Nutrition Explained
There are various forms of carbohydrate, which are dependent on the number of sugars they contain. They are classified as follows:
• Monosaccharides are the simplest forms, containing only one sugar
• Disaccharides are are made up of two monosaccharides
• Polysaccharides are made up of several monosaccharides and are commonly called complex carbohydrates
Carbohydrates are one of the most important sources of fuel for an athlete. They are the predominant fuel source for high intensity exercise and help to monitor fat and protein metabolism as well as providing energy for the nervous system.
It is therefore essential that an athlete consumes an adequate amount of carbohydrate prior to training and post-training to restore and maintain depleted glycogen stores.
Several studies have demonstrated the detrimental effects of a low carbohydrate diet on performance and time to exhaustion. Inadequate carbohydrate or glycogen stores will limit the amount of energy that can be produced both aerobically and anaerobically, since most exercise consists of the use of both energy systems.
During extreme endurance events glycogen stores in the muscles and the liver will be depleted before the completion of the race. It is therefore necessary for athletes to consume simple sugar gels, drinks and snacks to increase blood glucose to provide energy rapidly. Complex carbohydrates are more suitable for carbohydrate loading prior to an event or in the process of recovery as they take several hours to digest.
Research has suggested that consuming a sugary snack 15-45 minutes before exercising may have detrimental effects on blood glucose levels. The secretion of insulin in response to excess glucose will result in reduced blood glucose values. Preparation is key; stores should be replenished several hours before an event and then afterwards.
Prior to exercise - A combination of complex (rice, cereal, oats, pasta, potato, lentils) and simple carbohydrates (juice, fruit, sweets, gels) can be consumed at least 2 hours before exercise. Foods high in fat and protein should be avoided as they take a longer duration to digest.
During exercise - Simple sugars, in the form of gels or energy drinks can be consumed due to their rapid gastric emptying, however, the higher the percentage glucose the longer the digestion will take.
Following exercise - A high carbohydrate meal to replenish the depleted glycogen stores should be consumed, immediately after a simple or complex snack can be eaten.
However, carbohydrate is not the only essential nutrient required for a balanced healthy diet.
Fat consists in our body in a variety of forms; fatty free acids, cholesterol, triglycerides, phospholipids etc. It is vital for the protection of internal organs, cell membranes and nerve fibers and for insulation. It is also the predominant fuel used by the body at rest. Fat provides a huge amount of energy in comparison to carbohydrate. The main consumption of fat should come from unsaturated sources such as fish, nuts, olive and sunflower oil.
Protein is also important nutrient for growth, repair and maintenance of the body’s tissues. Hemoglobin, enzymes and antibodies are made from protein, which reinforces its importance within an athlete’s diet.
Vitamins and minerals are also vital components of an athlete’s diet. Vitamin A is vital for growth with its importance in bone growth. Vitamin D plays an important role in the absorption of calcium in the intestines, as calcium is important for bone growth and muscle contraction, its importance is reinforced. Vitamin K is important in the electron transport chain and therefore plays an important role in the oxidative phosphorylation. B-complex vitamins are of uttermost important for the cellular metabolism and the production of energy. Vitamin C functions to aid absorption of iron in the intestines.
Another important feature of an athlete’s diet is the role of antioxidants. There has been much interest recently among athletes in vitamin C and E, which have been shown to have antioxidant properties, and which may be involved in protecting cells, especially muscle cells, from the harmful effects of the highly reactive free radicals that are produced when the rate of oxygen consumption is increased during exercise (Kanter, 1995). Many studies have shown that unaccustomed exercise, particularly if it involves eccentric exercise in which the muscle is forcibly lengthened as it is activated, results in damage to the muscle structure and post exercise soreness. Because it normally peaks 1-3 days after exercise, this is often referred to as delayed onset muscle soreness. It is believed that free radicals, highly reactive chemical species, may be involved in the damage that occurs to muscle membranes. Alleviating or avoiding these symptoms would allow a greater training load to be sustained. An increased generation of free radicals is also associated with damage to cellular DNA and to a variety of lipids and proteins. If the post exercise damage can be reduced by an increased intake of antioxidants, then recovery after training and competition may be more rapid and more complete.
Nutritional antioxidants include vitamins A, C and E. Other dietary components including selenium, which has a structural role in glutathione peroxidise, and ubiquinone (or co-enzyme Q10) may play important roles. These nutritional antioxidants can be consumed in the foods we eat such as colourful fruit and vegetables, such as berries, broccoli, spinach, pomegranate, oranges, carrots, peppers and sources of vitamin E and selenium can be found in nuts, seeds and fish. However, there is the option to consume these antioxidants through supplements. The dosage to take would be dependent on the daily amount one consumes through their diet. It may be recommended to have a low dosage such as 300-500mg of each vitamin C and E to support dietary consumption and to reduce the extent of exercise induced oxidative damage to tissues.
All energy originates from the sun as light energy. Chemical reactions in plants (photosynthesis) convert light into stored chemical energy. Humans then acquire energy by consuming plants, or animals that feed on plants. The energy is stored in food as carbohydrates, fats and proteins, and these can be further broken down to release stored energy in our cells. All energy is eventually converted to heat; the amount of energy released is calculated from the amount of heat produced. Energy in biological systems is measured in kilocalories (kcal). By definition, 1kcal equals the amount of heat energy required to increase 1kg of water 1 degrees Celsius at 15 degrees Celsius. The combustion of 1g carbohydrate can provide approximately 4 kcal compared to fat which provides 9 kcal per gram (Wilmore and Costill, 1994).
Energy must be released from chemical compounds at a given rate. This is determined by the primary fuel source being used. An ATP molecule consists of adenosine combined with three inorganic phosphate groups. The enzyme ATPase splits the last phosphate group away from the ATP and energy is released. The ATP is then converted to adenosine diphosphate (ADP) and phosphate. Phosphorylation then occurs to restore this energy through various chemical reactions; adding a phosphate group to the ADP which then forms ATP. ATP is generated through three energy systems: the ATP-PCr system, the glycolytic and oxidative system. The ATP-PCr system, involves a phosphate group being separated from the phosphocreatine by the enzyme creatine kinase. The phosphate can then combine with ADP to form ATP. This system is anaerobic, with its predominant role to maintain ATP levels. The glycolytic system involves the process of glycolysis of which glucose /glycogen is broken down into two molecules of pyruvic acid. Without oxygen the pyruvic acid is converted into lactic acid. Both the ATP- PCr system and glycolytic systems are the main contributors to energy during the early minutes of high intensity exercise.
The oxidative system involves the breakdown of fuels in the presence of oxygen. Overall this system can produce more energy than the two previously mentioned. Oxidation of carbohydrate involves glycolysis, krebs cycle, and the electron transport chain, resulting in the release of water, carbon dioxide and 38.39 ATP molecules per carbohydrate molecule. Oxidation of fat also follows the same pathway; however the oxidation of fat produces much more energy. The oxidation of protein is more difficult, because the nitrogen within protein cannot be oxidized. Protein does not offer a lot of energy in the body’s metabolic systems.
The importance of adequate fluid intake cannot be overemphasised. Water comprises approximately 60-70% of a males total body weight and 50-60% of a females. A reduction in body water of up to 10% could have fatal consequences.
Water is essential for maintaining plasma volume which carries red blood cells and nutrients including glucose, fatty acids and amino acids to your muscles. It also carries the waste products such as carbon dioxide away from the muscle to be disposed of.
The blood plasma also transports hormones that regulate metabolism and muscular activity during exercise. Blood plasma volume is essential for maintaining blood pressure and for thermoregulation. It is therefore imperative to maintain hydration status to sustain performance.
Water alone should not be the only focus. Electrolytes play an important role in fluid balance and retention. The four main sources of water loss are through skin evaporation, evaporation from the respiratory tract, excretion from the kidneys and excretion from the large intestine.
Water loss is rapidly increased during exercise. The body attempts to maintain optimum core temperature during exercise which leads to increased sweating and evaporation of fluid to dissipate the heat away from the body. This leads to a reduction in bodily water and sodium losses.
Sodium is important in maintaining water retention and hydration. Dehydration leads to a decrease in blood volume, which ultimately reduces stroke volume, causing strain on the cardiovascular system (Marieb, 2006). This may potentially reduce blood pressure.
It seems plausible to suggest that decreases in blood volume may therefore reduce blood pressure, reducing cardiac output. This will decrease the blood flow to the muscle and the skin and the core body temperature will gradually increase to the point that exercise has to be terminated.
When dehydration exceeds 2 % of body weight, prolonged exercise is severely impaired. Sweat losses can exceed a liter per hour, with each liter containing almost 1 gram of salt. Athletes should focus on maintaining both adequate fluid intake and also sodium chloride to retain the water.
Often athletes focus on replacing water alone, consuming large quantities of plain water and not addressing the electrolyte losses. This can have dangerous consequences.
Hyponatremia is becoming very common among endurance athletes. More often than not it occurs due to excessive water intake which dilutes the electrolytes in the blood plasma. Symptoms of hyponatremia include confusion, dizziness and weakness, more severely coma and death can occur. Sodium chloride replacement is imperative, whether it be in the form of a salty snack combined with plain fluid or an isotonic sports drink.
Preparation before the event and restoration after the event are critical to maintaining and preventing hyponatremia and dehydration.
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