I have found that many people, whether they have eating disorders or not, do not have enough general knowledge about basic metabolism, and the physiology of nutrition for recreational activities. This includes myself! This section is designed for general informational use so people will know fundamental facts such as, "you must consume carbohydrates in order for the body to metabolize fat for energy." Now while I would love to list everything you'd want to know and more about nutrition and metabolism, I simply don't know all there is to know, nor do I have the space for an exhaustive discussion on the topic; therefore, I'm only going to touch on the subjects which I think are the most relevant to people who need basic information about how their bodies utilize food. For more detailed discussions of nutrition, you might want to consult your local library. Two books I found extremely accessible and helpful, were: Catherine G. Ratzin Jackson's, "Nutrition for the Recreational Athlete" (1995), and Steve Wootton's "Nutrition for Sport" (1988).
ATP: adenosine triphosphate . . . an energy-carrying molecule which directly supplies potential energy to the cellular machinery necessary for all functions. All other energy reservoirs must first transfer their energy to ATP before use by the cell. Each day, an active person turns over (breaks down and resynthesizes) an amount of ATP almost equivalent to double body weight.
Aerobic vs. anaerobic metabolism: ATP in cells is regenerated from ADP (adenosine diphosphate) by breaking down fuel molecules using aerobic or anaerobic catabolic processes. Simply put, the aerobic system (aka "oxidative") depends upon the presence of oxygen (O2) to break down food energy and build ATP molecules, whereas the anaerobic (or "immediate") system is not dependent on the presence of oxygen.
Glucose and Glycogen: Glucose is the simplest form of sugar that is used by the body for energy. Glucose can be broken down to form carbon dioxide and water (expelled air, and sweat, respectively). Glycogen is simply a long string comprising hundreds of glucose molecules which are linked together within the cells.
The three dietary sources of energy are protein (i.e., meats, nuts, cheese, egg whites), carbohydrate (i.e., breads, pastas, grains, fruits), and fat (both naturally-occurring, like vegetable fats [olive oil, cocoa, avocadoes] and additional [butter, mayonnaise] . At rest and during normal daily activities, fats are the primary energy source, providing 80 - 90% of the energy. Carbohydrates and protein provide 5 - 18% and 2 - 5% respectively. During exercise, however, these proportions change.
Highly intense anaerobic activities which stress the phosphogen (anaerobic) system do not directly use any of the three substrates during the activity. However, post-activity, this system can be restored using the glucose stored as glycogen in the muscle cells. Whenever glycogen is broken down to form water, it must be replaced through the diet. All ingested carbohydrates degrade in the digestive system to form either glucose, galactose (milk sugar), or fructose (fruit sugar), with glucose being predominant. The majority of galactose and fructose molecules are converted to glucose in the cells lining the small intestine and most of the remainder is converted in the liver. Glucose is absorbed into the blood and transported to the body tissues for use or storage in the liver and muscles as glycogen (long strings of glucose). Glucose must be brought into the cell and then strung together as glycogen. The process of storage or replacement of glycogen is slow, and can take up to two days to complete.
The situation may arise where there is insufficient carbohydrate stores to provide glucose to regenerate muscle glycogen. If glucose supplies are low, the muscle glycogen will not be restored. The available glucose in the blood will be spared for use by the brain and the muscle will be left wanting for a carbohydrate energy supply. If glucose levels fall below the levels necessary for proper brain function, other sources of glucose exist as emergency stores. Protein in the body can be degraded to its amino acid building blocks, which can then be converted to pyruvic acid in the liver. Two pyruvic acids can be combined to form one glucose molecule which enters the blood. The primary source of protein for this process is the breakdown of muscle tissue; thus, glucose may be generated at the expense of protein. This reaction also occurs when the body is in starvation or when the individual consumes low levels of calories.
Fats, are composed of glycerol, and three fatty acids, and can also be used to provide glucose. The glycerol can be converted to glucose; the fatty acids cannot. The fatty acids must either be used aerobically or they become ketones (a chemical compound including acetone) which are toxic in the blood at high levels. With starvation or fasting, low carbohydrate diets, prolonged exercise, or uncontrolled diabetes mellitus, ketone body formation accelerates. Adipose (fat) tissue releases large quantities of fatty acids due to an imbalance between triglyceride (fat) formation and lipolysis (breakdown of fats) caused by low blood insulin concentration. In all of these conditions, carbohydrate content in the body is low, and thus carbohydrate utilization and blood insulin concentration are low. Depending upon fat for the generation of glucose is not desirable. It is essential that an adequate amount of carbohydrate needs to be ingested for successful participation in any activity.
Muscle glycogen is the initial source of glucose for aerobic energy; however, the duration of the activity often outlasts the supplies stored within the muscle cell and other substrates (fats and protein) must be used.
During lower intensity aerobic activities, fats are the preferred substrate for utilization. Fats are stored within the muscle cells and fat storage cells (adipocytes); they also circulate in the blood. Blood-borne fat is available for muscle use as fatty acids and fat is easily liberated from the adipocytes to the circulation for muscle use. The use of fatty acids in aerobic metabolism results in far greater ATP production than the use of carbohydrates. Even when fats are the predominant energy source, carbohydrates must still be available. Carbohydrates are necessary to continuously prime the breakdown of fatty acids to provide cellular fuel in the form of ATP. Thus, fats are never the sole energy source. Even if there is an abundance of fat available, but no carbohydrates to prime their breakdown -- aerobic degradation of fat will stop. Other inhibitors of fat utilization are high insulin levels, such as those found after a large intake of simple sugars, and high lactic acid levels (resulting from overloading muscle cells with too much work).
There is a direct relationship between nutritional concerns for energy production and specific characteristics of human muscle. Normal human muscle is a mosaic of fiber types which can be viewed as having properties along a spectrum from highly aerobic, to highly anaerobic. However, it is usually viewed in simpler relationships, and three major groups of fibers (I, IIa, IIb) are classified according to their histochemical, physiological, anatomical, or biochemical properties.
Type I (slow twitch [ST], slow twitch oxidative [SO], red, dark) fibers are those that possess maximum capabilities for aerobic energy production. These are the fibers stressed in endurance activities such as running, cycling and swimming. All of the biochemical parameters that support aerobic metabolism are at high levels in these fibers; one will find elevated levels of carbohydrate (glycogen, glucose) and fat (neutral lipids, triglycerides), which are the substrates for aerobic energy production. It has been reported that individuals participating for long periods of time in endurance activities such as marathons and triathlons, have higher than normal percentages of type I fibers. The type I fiber, however, does not have the capabililty to hypertrophy (enlarge in size) to a great degree, even if it is overloaded due to the need to get oxygen to the centre of the fiber. Thus, the individuals who stress endurance activities stress the type I fiber and will find, with time, that the muscles will not enlarge greatly and that body fat stores tend to diminish. There is also a need to keep glycogen stores increased through diet. This is why "carb-loading" and diets high in carbohydrate intake has become an important part to athletic training in many endurance activities -- to maintain balanced levels of energy production and expenditure.
Type II fibers (fast twitch, white, light) have two major subgroups: fast-twitch-oxidative-glycolytic (IIa, FTa, FOG) and fast-twitch-glycolytic (IIb, FTb, FG). These fibers possess maximum capabilities for anaerobic energy delivery; however the IIa also has a high capacity for aerobic energy production, and, therefore, it is sometimes referred to as the "super fiber" in terms of performance. These are the fibers stressed in resistance activities such as weightlifting and bodybuilding. All of the biochemical parameters that support anaerobic metabolism are at high levels in these fibers. Consequently, high amounts of glycogen are found in the IIa fiber.
Unlike the Type I fibers, it has not been shown that individuals who participate in resistance activities over long periods of time have greater than normal percentages of Type II fibers. Rather, transitions between the percentages of IIa and the IIb have been shown to occur with exercise along with changes in their size, thereby either enhancing the aerobic (IIa) or anaerobic (IIb) capacity of the muscle as a whole. The Type II fiber will significantly hypertrophy with use and is responsible for the great increases in muscle mass seen in strength conditioning and bodybuilding. There is a consideration for speed of movement, as slower movements show more rapid increases in power, thereby increasing muscle tension and hypertrophy to a greater degree than fast movements. Thus, individuals who stress the use of Type II fibers will find that their muscles will enlarge and that they have a need for carbohydrate in their diet in order to produce energy through anaerobic glycolysis (breakdown of sugars without the use of oxygen).
Recreational athletes should understand energy requirements and cellular consequences of their activities and should choose the nutritional regimens which support this choice. However, regardless of the choice of activity and diet, health should be the major concern. Recreational athletes comprise a large portion of the millions of individuals who wish to "lose weight," "modify or diminish their fat stores," or "tone up to look better" through the use of the hundreds of thousands of commercialized weight-loss programs available to the public. Considering the fact that there are over 50 000 published diets alone, it cannot help but be noticed that if one of them worked successfully and healthfully over the long term, there would not be a need for the other 49 999. Recommendations of the American College of Sports Medicine (ACSM) addresss the more important aspects of what we loosely term "physical fitness" to include: the development and maintenance of cardiorespiratory fitness, muscular strength, endurance, quality of life in terms of health (disease-free years), agility, mental and emotional fitness, etc. For more detailed information from the ACSM, please visit the ACSM web-site.