Carbohydrates are the fuel with which the body gains energy. Carbohydrates are the most prominent example of a substance that has a wide name recognition, but a lesser understanding of their actual role in human energy production.
Foods are generally classified for nutritional purposes into three groups: carbohydrates, proteins, and fats. For the purposes of measuring how much fuel is involved in energy production, the calorie is the unit of measurement used.
Nutritionally, there are simple carbohydrates (found in foods such as granulated table sugar or fruits) and complex carbohydrates (those present in typically more densely constructed foods such as rice, pastas, whole grains, and many kinds of vegetables). Complex carbohydrates are valuable both as energy and as a mineral source. Protein is the material required by the body to build muscle, as well as to repair and maintain all bodily tissues. Excess protein consumption places stress upon the kidneys, creating potential deficiencies of the mineral calcium. Proteins are present in meat of most types, fish, soy, and dairy products.
Fats are essential to a healthy diet, as they are the source of fatty acids, which are crucial to the absorption by the body of fat-soluble vitamins such as vitamins A, D, and E. Fats also assist with the body' insulation and proper cell function.
As a general nutritional guideline, approximately from 60-65% of a healthy adult's caloric intake should be derived from carbohydrates; proteins should constitute 12-15%; and fat sources should be less than 30% of a properly balanced diet.
Carbohydrates are the substances that will produce the essential fuel for the demands of human movement. Carbohydrates are simple sugars, composed of carbon, hydrogen, and oxygen atoms present in a ratio of 1:2:1. These sugars, once extracted from digested foods, are water-soluble compounds that are the fundamental energy source for many forms of organic life. In the single sugar form, carbohydrates are monosaccharides, of which glucose and fructose are the best known. The polysaccharides, also known as starches, are converted upon ingestion by the human body for storage into glycogen; as glycogen, the sugars can be converted for later use as a fuel source. The primary storage locations of glycogen are the skeletal muscles and the liver.
While glycogen has a molecular structure similar to the starches found in certain green plants, there are few foods that contain glycogen; potato starches are closest in structure, and accordingly, potatoes have enjoyed a timeless reputation as a useful energy source for athletes. The complex carbohydrate starches are created in plants through the process of photosynthesis, whereby sunlight reacts with carbon and hydrogen atoms to create complex molecules. Plant products such as breads, pasta, cereals, beans, fruits, and vegetables will all possess varying amounts of carbohydrates.
Many diets and other nutritional references make mention of "good carbs" and "bad carbs." These descriptions are not a reflection on the chemistry of the particular carbohydrate being ingested as carbohydrates have a well-defined molecular structure. Good carbohydrates are generally those derived from whole, primarily unprocessed foods such as grains and vegetables. Consuming the requisite carbohydrates from these types of foods provides the added nutritional benefits of fiber, which assists in the good digestion of all foods in the human intestines, as well as providing vitamins essential to many metabolic processes. The so-called bad carbohydrates are those ingested through sugared, processed foods and snack foods, which have no nutritional value other than as a mediocre energy source. Excess carbohydrates, those that cannot be processed for immediate use in the bloodstream, or stored in the muscles or liver as glycogen, will be stored by the body as fat.
Carbohydrates enter the body as foods in a variety of forms; the processing, conversion, and storage of carbohydrates as usable energy begins in the mouth. Hydrolysis is the process by which water and heat will break down a substance; this mechanism is present in saliva and it continues with the fluids of the small intestine. There, the complex starches are reduced to simple glucose. The glucose passes through the wall of the small intestine where it is stored in the liver as glycogen. As much as 10% of the total weight of the liver can be stored glycogen; twice as much glycogen is stored in the muscles throughout the entire body. The liver serves an additional, regulatory purpose with respect to how much glucose is entering the bloodstream at any time.
Seventy-five percent of the glucose stored in the body will typically be directed to the functions of the brain, with the balance used for the purpose of red blood cell production and skeletal muscle and heart muscle activity.
The function of carbohydrates both as simple sugars as well as stored glycogen is determined largely by which of the body's energy systems is operational during athletic activity. The anaerobic energy system is the body's method for fueling itself in shorter, more intense types of activity, in which the presence of oxygen in the muscle cells is not required to produce energy. The anaerobic system has two aspects: the anaerobic alactic system and the anaerobic lactic system. The aerobic system is the energy system predominately used to fuel activity that occurs over a longer period.
Adenosine triphosphate (ATP) is the fuel either used or created by each of the energy systems. It is the source of ATP that distinguishes one system from another. The anaerobic alactic system is the process employed by the body for very fast, intense physical activity that lasts no longer than 15 seconds. All muscles have a small amount of ATP contained within them, which recharges in relatively short periods; in this alactic system, the ATP is a form of instant energy to the muscle.
In the anaerobic lactic system, muscle glycogen (the stored complex sugars) break down into simple glucose, which produces ATP and provides the muscle with energy. The creation of ATP is a slower process than the simple access to ATP in the cell as in the alactic system. For as long as there is muscle glycogen present, ATP energy will be produced; the usual duration of the muscle glycogen/ATP process is from 60 to 90 seconds. As this conversion to ATP energy occurs outside of the muscle cell, oxygen is not required to facilitate this metabolism. However, the chemical byproduct of the conversion of glucose to ATP is lactate, or lactic acid, which will hinder athletic performance due to its cramping effect on working muscles. As an athlete becomes more efficient, the lactate is recycled through the heart and liver and recycled into usable fuel.
In the aerobic system, ATP is produced from glucose in the working muscles cells, a process using oxygen transported by way of the erythrocytes, or red blood cells. The process of the production of ATP in the aerobic system is longer than that of the anaerobic lactic, but the energy produced is for longer duration, less intense forms of muscle activity. The aerobic process of ATP does not create any waste products; the use of oxygen requires increased heart capacity to bring more oxygen-rich blood to working muscles.
Fatty acids (produced by fats obtained through food) and amino acids (derived from protein) are stored in lean muscle tissue within the body. These sources of ATP, which are not as efficient as the glycogen/glucose system, work in a complementary fashion by delaying the depletion of glycogen energy reserves. ATP generated from muscle or liver glycogen is at least twice as productive in the satisfaction of the body's energy requirements as the ATP production from fatty acids.
Different types of exercise place differing demands upon the energy systems over time, and the corresponding rate by which glycogen is depleted. As a general proposition, the longer the period of exercise, and the greater the ongoing demand upon the reserves of stored energy, the greater the proportion of energy that will be derived from the fat/fatty acid component of energy production. As an example, when the athlete is exercising for 30 minutes, more than 60% of energy will be produced through either muscle glycogen or glucose released from storage in the liver. At the other end of the exercise spectrum, when the athlete has worked for 240 minutes, the fatty acid mechanism for the production of ATP energy will be in excess of 60%; muscle glycogen stores account for less than a 10% contribution.
There is an interrelationship between the utilization of carbohydrate stores and the function of each energy system. In an event such as a cross-country ski race or a 31-mi (50 km) cycling race, the burst of desired energy to break from the starting line will be fueled by the anaerobic alactic system, using the readily available ATP reserve. As the race progresses, the athlete will draw energy from the aerobic system; a steep hill or sprint finish will engage the anaerobic lactic process. All three mechanisms are available at any time, with a system being predominate as opposed to exclusive.
The carbohydrate demands of specific sports are also a consideration in training. An adult distance runner training at a seven-minute mile pace will burn approximately 920 calories per hour. A cyclist with similar characteristics training at a speed of 16 mph (26 km/h) will expect to consume 680 calories. A byproduct of the energy consumption by the body during exercise is the production of lactate; when oxygen depletion occurs in the burning of converted glycogen into ATP, lactate is a byproduct, which contributes to inefficiency and a sluggish performance.
The commitment of an athlete to the restoration of glycogen stores within the body through proper carbohydrate intake after training or competition is of critical importance to long-term athletic success. The processes by which the body can reabsorb carbohydrates take place immediately after exercise. During training or competition, complex carbohydrate sources that can be easily consumed are energy bars and gels; however, products that contain significant amounts of simple sugars such as fructose and glucose should be avoided, as they cause a sugar spike that does not aid in carbohydrate processing into useful glycogen stored fuels.
Carbohydrates are essential to the healthy functioning of the human body for athletes and sedentary persons alike. It is in this context that the so-called "low-carb" diets, such as the popular Atkins diet, must be understood. As a general proposition, while the low-carbohydrate diets may produce weight loss in sedentary, overweight persons, it is difficult to imagine a healthy athlete with significant energy demands being able to maintain training levels with reduced carbohydrate diets. Conversely, the growth of long-distance running, and the demands of that sport in terms of carbohydrate loading as a pre-competition dietary strategy, has prompted significant research into the mechanics of precisely how the body utilizes the carbohydrates it ingests.