Carbohydrates are macronutrients composed of carbon, hydrogen, and oxygen atoms that can be divided into many categories based on their structure (i.e., monosaccharides, disaccharides, polysaccharides, and non-digestible polysaccharides) as well as their metabolic effects (e.g., digestibility, glycemic responses, etc.). The term “saccharide” describes a molecule that is a sugar or sweet and is often used as a synonym for “carbohydrate.” These nutrients are typically the most abundantly consumed components of our diets. Many foods from each of the five major food groups, as wells as sweets and foods specifically developed for athletes such as sports drinks, bars, and gels, can provide significant amounts of carbohydrate in the diet. Some examples of carbohydrate-rich foods and their carbohydrate contents in 100-gram servings are provided in Table. Although many of the foods on the list appear to be rather low in carbohydrates, most of those foods have a high or very high water content, which can be misleading as the values are expressed relative to the weights of the foods rather than as a percentage of energy content provided by carbohydrate, which is high for all of the foods listed.
Carbohydrates are often classified structurally as either simple sugars (typically monosaccharides and disaccharides) or complex carbohydrates (i.e., polysaccharides or starches). Monosaccharides are carbohydrates composed of a single monomeric unit, while disaccharides include two monosaccharides linked by a glycosidic bond. Mono- and disaccharides are often referred to collectively and sometimes individually as sugars or simple sugars, although the term “sugar” is often used to refer to carbohydrates in general or specifically to table sugar (sucrose) or blood sugar (glucose). The major dietary monosaccharides are glucose, fructose, and galactose, while sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose) compose the key disaccharides of the diet. Sugars are found in a variety of naturally occurring and processed foods. Glucose is a hexose found abundantly in various forms in an array of foods. Free glucose is found in many fruits, honey, corn syrup, sports drinks and numerous other foods. Glucose is also a component of most other carbohydrates including, but not limited to, starches, sucrose, lactose, and maltose.
Any foods rich in these nutrients ultimately provide the body with significant amounts of glucose. Furthermore, the other primary dietary monosaccharides fructose and galactose are ultimately converted to glucose or glucose derivatives after absorption. Because most carbohydrate can be converted into glucose eventually, it is an extremely important nutrient physiologically, as will be discussed. Glucose can be made available for the body from more than dietary carbohydrates, however. Many amino acids, glycerol, and pyruvic and lactic acids can be used to produce glucose. The monosaccharide fructose is also a hexose. It is primarily found in food in either its simple form or bound to glucose as part of the disaccharide sucrose.
Any foods rich in these nutrients ultimately provide the body with significant amounts of glucose. Furthermore, the other primary dietary monosaccharides fructose and galactose are ultimately converted to glucose or glucose derivatives after absorption. Because most carbohydrate can be converted into glucose eventually, it is an extremely important nutrient physiologically, as will be discussed. Glucose can be made available for the body from more than dietary carbohydrates, however. Many amino acids, glycerol, and pyruvic and lactic acids can be used to produce glucose. The monosaccharide fructose is also a hexose. It is primarily found in food in either its simple form or bound to glucose as part of the disaccharide sucrose.
Foods rich in this monosaccharide as the hexose monomer include many fruits and honey, although much of the fructose in the diet is consumed in processed foods such as soft drinks, sports drinks, baked goods, etc., that are sweetened with high fructose corn syrup. Because one half of the sucrose molecule is fructose, foods rich in sucrose also provide much of the fructose in the diet. Another major hexose consumed in the diet is galactose, which is found predominately in food as part of the disaccharide lactose.
Other foods containing galactose include peas, lentils, some legumes, organ meats, cereals, and some fruits and vegetables. Some sports foods contain galactose as well, which will be discussed in more detail later. Other monosaccharides, including pentoses such as ribose and xylose, are found in the diet in small quantities as well. Ribose, commonly consumed as a part of nucleic acids, is produced metabolically from glucose through the hexose monophosphate shunt. Three major disaccharides, sucrose, lactose and maltose, are also found in most individuals’ diets. Sucrose is present at high levels in sugar cane and sugar beets, from which it is extracted to produce table sugar. It is also found in lower amounts in many fruits as well as vegetables and grains.
Several sports foods are sweetened with sucrose. Lactose is a disaccharide produced from the monosaccharides glucose and galactose and is the primary sugar found in milk. Maltose is found in malted milk products as well as sweet potatoes, pears, and in lower amounts in other fruits, vegetables, grain products, and honey. “Complex carbohydrate” usually describes digestible polysaccharides made up of many glucose monomers.
Digestible polysaccharides are found in a variety of plant foods and are often richest in foods such as grains and grain-based foods (e.g., pasta, breads, cereals, etc.), potatoes, beans, and peas, etc. Non-digestible polysaccharides (i.e., fiber) are also made of saccharide units (not always glucose), but are resistant to digestion by human enzymes. The most prevalent examples of complex carbohdyrates in the human diet are amylose and amylopectin, both of which are considered starches. Amylopectin comprises a higher percentage (typically 70–80%) of the starch in foods containing complex carbohydrate. Glucose polymers or maltodextrins are also typically considered complex carbohydrates. These molecules are relatively short chains of glucose units linked by glycosidic bonds in a manner similar to amylose and are produced by partial hydrolysis of starch molecules. Many nutritionists have used the term “complex carbohydrate” simply to refer to foods that are rich in starches. These foods typically provide many nutrients other than the glucose that composes the starches and have long been considered to provide a nutritional advantage over simple sugars.
Historically, this advantage has been used as a basis for suggesting that the diets of exercisers and athletes be rich in complex carbohydrates. While many foods rich in complex carbohydrates are also rich in other nutrients, this is not always the case. Foods rich in complex carbohydrate as well as fiber (e.g., whole grain breads, etc.) are prime examples of nutrient-rich foods that should form the basis of a healthy diet. Fiber is a group of non-digestible polysaccharides found in plant foods as well as lignin, which is primarily associated with the structural components of plants. Although fibers are not digestible by enzymes of the human intestinal tract, colonic bacteria possess the ability to partially ferment some fiber, not lignin, thereby producing short-chain fatty acids that can be absorbed by the colonic epithelium and provide some energy to the body. Therefore, while the term non-digestible is accurate regarding human digestion, some fiber digestion does occur within the human body through the assistance of our colonic microflora. Fibers are usually classified as those that are soluble in water and those that are insoluble in water. Water-soluble fibers that are common in the diet include pectins, gums, mucilages, algal polysaccharides, beta-glucans, pysllium, resistant starches, and inulin.
Water-insoluble fibers include cellulose, some hemicelluloses, and lignin. The potential impact of fiber consumption on exercise performance has not been directly studied; however, for meals consumed prior to or during competition, most sports dietitians recommend foods relatively low in fiber to avoid gastrointestinal discomfort, which may negatively impact performance. Most practitioners also recommend that the typical diet of an athlete contain similar amounts of fiber to those recommended to the general population (usually approximately 20–35 grams per day). Research does not necessarily support the notion that consumption of moderate amounts of fiber will adversely affect performance during competition. In fact, as described in the section on glycemic index, consumption of some fiber-containing foods has produced a lower glycemic response with pre-exercise feedings, which translated to improvements in performance. Other studies have not demonstrated such performance improvements but also do not report decreases in performance. Until solid research regarding alternative recommendations is available, the amount of fiber that an athlete should consume before competition depends on how well the individual can tolerate foods that contain fiber and must be determined on a case-by-case basis. The Acceptable Macronutrient Distribution Range established by the Institute of Medicine when determining the Dietary Reference Intakes is 45–65% of total energy intake.
Research on dietary intake of various athletes suggests that most are consuming carbohydrates at rates that are within this range. For example, one group of researchers demonstrated that the average intake of U.S. collegiate cyclists during training was 58 ±8% of energy.1 Similar data were obtained for Australian nationallevel triathletes and runners who respectively consumed 60 ±8% and 52 ±5% of energy from carbohydrate and U.S. swimmers and divers who consumed 65 ±7% of energy from carbohydrate at season’s end.3 At the extreme, elite male Kenyan runners have been demonstrated to eat as much as 607 ±57 grams of carbohydrate per day during training, accounting for approximately 77% of energy intake on average.
Several sports foods are sweetened with sucrose. Lactose is a disaccharide produced from the monosaccharides glucose and galactose and is the primary sugar found in milk. Maltose is found in malted milk products as well as sweet potatoes, pears, and in lower amounts in other fruits, vegetables, grain products, and honey. “Complex carbohydrate” usually describes digestible polysaccharides made up of many glucose monomers.
Digestible polysaccharides are found in a variety of plant foods and are often richest in foods such as grains and grain-based foods (e.g., pasta, breads, cereals, etc.), potatoes, beans, and peas, etc. Non-digestible polysaccharides (i.e., fiber) are also made of saccharide units (not always glucose), but are resistant to digestion by human enzymes. The most prevalent examples of complex carbohdyrates in the human diet are amylose and amylopectin, both of which are considered starches. Amylopectin comprises a higher percentage (typically 70–80%) of the starch in foods containing complex carbohydrate. Glucose polymers or maltodextrins are also typically considered complex carbohydrates. These molecules are relatively short chains of glucose units linked by glycosidic bonds in a manner similar to amylose and are produced by partial hydrolysis of starch molecules. Many nutritionists have used the term “complex carbohydrate” simply to refer to foods that are rich in starches. These foods typically provide many nutrients other than the glucose that composes the starches and have long been considered to provide a nutritional advantage over simple sugars.
Historically, this advantage has been used as a basis for suggesting that the diets of exercisers and athletes be rich in complex carbohydrates. While many foods rich in complex carbohydrates are also rich in other nutrients, this is not always the case. Foods rich in complex carbohydrate as well as fiber (e.g., whole grain breads, etc.) are prime examples of nutrient-rich foods that should form the basis of a healthy diet. Fiber is a group of non-digestible polysaccharides found in plant foods as well as lignin, which is primarily associated with the structural components of plants. Although fibers are not digestible by enzymes of the human intestinal tract, colonic bacteria possess the ability to partially ferment some fiber, not lignin, thereby producing short-chain fatty acids that can be absorbed by the colonic epithelium and provide some energy to the body. Therefore, while the term non-digestible is accurate regarding human digestion, some fiber digestion does occur within the human body through the assistance of our colonic microflora. Fibers are usually classified as those that are soluble in water and those that are insoluble in water. Water-soluble fibers that are common in the diet include pectins, gums, mucilages, algal polysaccharides, beta-glucans, pysllium, resistant starches, and inulin.
Water-insoluble fibers include cellulose, some hemicelluloses, and lignin. The potential impact of fiber consumption on exercise performance has not been directly studied; however, for meals consumed prior to or during competition, most sports dietitians recommend foods relatively low in fiber to avoid gastrointestinal discomfort, which may negatively impact performance. Most practitioners also recommend that the typical diet of an athlete contain similar amounts of fiber to those recommended to the general population (usually approximately 20–35 grams per day). Research does not necessarily support the notion that consumption of moderate amounts of fiber will adversely affect performance during competition. In fact, as described in the section on glycemic index, consumption of some fiber-containing foods has produced a lower glycemic response with pre-exercise feedings, which translated to improvements in performance. Other studies have not demonstrated such performance improvements but also do not report decreases in performance. Until solid research regarding alternative recommendations is available, the amount of fiber that an athlete should consume before competition depends on how well the individual can tolerate foods that contain fiber and must be determined on a case-by-case basis. The Acceptable Macronutrient Distribution Range established by the Institute of Medicine when determining the Dietary Reference Intakes is 45–65% of total energy intake.
Research on dietary intake of various athletes suggests that most are consuming carbohydrates at rates that are within this range. For example, one group of researchers demonstrated that the average intake of U.S. collegiate cyclists during training was 58 ±8% of energy.1 Similar data were obtained for Australian nationallevel triathletes and runners who respectively consumed 60 ±8% and 52 ±5% of energy from carbohydrate and U.S. swimmers and divers who consumed 65 ±7% of energy from carbohydrate at season’s end.3 At the extreme, elite male Kenyan runners have been demonstrated to eat as much as 607 ±57 grams of carbohydrate per day during training, accounting for approximately 77% of energy intake on average. 