Carbohydrates

Carbohydrates provide 4 calories per gram and are the body’s preferred energy source [1, 2, 3, 4]. Carbs are ideally suited to provide fuel for the body’s many metabolic functions [4]. Carbs are broken down into simple sugar (glucose), which is easily assimilated and used in the body [2]. This glucose is absorbed into the bloodstream and powers cell energy to fuel activity (sitting, walking, eating, digesting, thinking, and the like) [2]. In fact, the brain runs exclusively on glucose, and if carb consumption is inadequate, the individual will likely experience brain fog or have difficulty concentrating [1, 2, 3, 4]. Carbs play other important roles, including satiety, providing flavor and sweet taste to foods, and serving as signaling molecules for essential biological reactions in the body [4]. In addition, fiber, which is an important type of carb, improves digestive health and blood cholesterol levels [4]. In fact, healthy carbohydrates chosen in appropriate amounts and at appropriate times can benefit athletic performance, weight management, and optimal health [4].

When carbs are readily available, the body does not need to break down protein for fuel [1, 4]. This protein sparing allows protein to be used to build muscle and other important body tissues and structures rather than for energy [1, 4]. Furthermore, carbs can be used for energy during endurance and resistance exercise and are required to efficiently break down fat [1, 2]. Carbs consumed in the diet that are not immediately used for energy are stored as glycogen [1]. Glycogen is stored in the liver (about 90 grams) and muscle cells (about 150 grams); and it can be broken down into single glucose molecules to provide a rapid source of energy [1, 4, 6]. The amount of stored glycogen can be increased fivefold with physical training and carb loading [1, 4]. Because glycogen contains many water molecules, it is large and bulky, which makes it unsuitable for long-term energy storage [1]. Therefore, if a person continues to consume more carbs than the body can use or store, the body will convert the excess glucose into fat for long-term storage [1, 3, 4].

Simple and Complex Carbohydrates

Carbohydrates are built from chains of monosaccharides that bind together to form larger carbohydrate compounds called disaccharides, oligosaccharides, and polysaccharides [3, 4]. Simple carbs include monosaccharides and disaccharides, and complex carbs include oligosaccharides and polysaccharides [4].

Monosaccharides. There are three monosaccharides found in nature that can be absorbed and used by humans: glucose, fructose, and galactose [3, 4]. Glucose is the predominant monosaccharide in nature and the basic building block of most other carbs [4]. Fructose is the sweetest of the monosaccharides and is found in varying levels in different types of fruits [4]. Galactose is most often bound to glucose to form the disaccharide lactose, the principal sugar found in milk. After being consumed, all carbs are eventually digested to monosaccharides and absorbed into the bloodstream [4]. The cells in the body use the monosaccharide form of glucose for energy. Fructose and galactose must be converted to glucose to be used by the cells for energy [4]. Monosaccharides are rarely found free in nature; they are usually found joined together as disaccharides, oligosaccharides, and polysaccharides [3].

Disaccharides. There are three disaccharides found in food, which are lactose, sucrose, and maltose [4]. Lactose, which is a glucose molecule and galactose molecule bound together, is found in dairy products such as milk, yogurt, and ice cream [4]. Sucrose (table sugar) is formed by glucose and fructose, and maltose (malt sugar) is two glucose molecules bound together [4]. Maltose, which is generated when starchy foods are digested, is the least abundant disaccharide in the food supply [4]. Most caloric sweeteners are disaccharides [4].

Oligosaccharides. An oligosaccharide is a chain of approximately three to ten simple sugars (i.e., monosaccharides) [4]. Fructo-oligosaccharides, a category of oligosaccharides found naturally in some fruits and vegetables and commercially produced as a reduced-calorie sweetener, are mostly indigestible [4]. These oligosaccharides help relieve constipation, improve triglyceride levels, and decrease production of foul-smelling digestive byproducts [4].

Polysaccharides. Polysaccharides can consist of hundreds of monosaccharides bound together. There are three categories: starch, fiber, and glycogen [4]. Plants (like different grains and vegetables) make starch, which is an energy source for the plant and provides carbs for animals that consume the plant [4]. There are two types of starch: amylose and amylopectin [4]. Amylose is a small, linear molecule of tightly packed glucose molecules that is mostly resistant to digestion [4]. Amylopectin is a larger, highly branched chain of glucose molecules that is easily digested [4]. Because starches are longer than disaccharides and oligosaccharides, they take longer to digest [4]. Still, humans are easily able to break down and digest starches with specific enzymes [4]. However, the rest of the plant, which is formed largely of the carbohydrate cellulose and other fibers, is indigestible because humans do not produce the necessary enzymes to break the glycosidic bonds found in these polysaccharides [4]. The third polysaccharide, glycogen, is produced by humans [4].

Glycemic Index and Load

Generally, carbohydrates are ranked based on their blood glucose response using the Glycemic Index (GI). Foods are divided into those that have a high glycemic index of 70+ (glucose, bread, potatoes, breakfast cereal, sports drinks), a moderate glycemic index of 56-69 (sucrose, soft drinks, oats, tropical fruits such as bananas and mangos), or a low glycemic index of 0-55 (fructose, milk, yogurt, lentils, pasta, nuts, cold climate fruits such as apples and oranges) [6]. High-GI foods break down quickly (causing a large glucose spike), and low-GI foods break down slower (causing a smaller glucose spike) [1, 3, 5]. Although valuable, the GI does not account for the caloric content of a food.

The Glycemic Load (GL) considers both the GI and caloric content in one serving of a food [4, 5]. GI is based on a reference carb amount of 50 grams, whereas GL accounts for portion size (GL = GI x grams of carbs / 100). Notably, a food can have a high glycemic index but low glycemic load [4]. For example, while carrots have a high glycemic index, to actually eat 50 grams of carrot, a person would need to eat 4 cups of the vegetable [4]. Because the typical serving size is approximately one-half cup, the glycemic load is small [4]. Also, minimally processed carb-containing foods that are also moderate to high in fat or protein, fiber, and other nutrients may have a high glycemic index but a low glycemic load [4].

A growing body of research supports eating a diet that is mostly of lower glycemic load [4]. A lower glycemic load diet may offer health benefits including weight control, decreased risk of diabetes and heart disease, as well as reduced morbidity in individuals with chronic diseases including diabetes and heart disease [4]. Furthermore, foods with a low glycemic load are commonly nutrient dense, meaning they provide more nutrients per calorie [4]. For example, 16 ounces of soda have about the same amount of carbs as two medium-sized apples, though the glycemic load of the soda is much higher [4]. The two apples provide more vitamins, minerals, and fiber compared to the soda, making the apples more nutrient dense [4].

Fiber

Fiber is a diverse group of carbohydrates that serve many important and beneficial roles in the human body. Fiber lowers the risk of developing various conditions, including heart disease, diabetes, diverticulitis, and constipation [7]. Fiber also plays a key role in weight management by regulating the body’s use of sugars, helping to keep hunger and blood sugar in check [4, 7]. Fiber is nondigestible, because humans lack the digestive enzymes required to break these types of carbs down, so they pass through the digestive system undigested [4, 7, 8, 9]. Though humans cannot digest fiber, there are different types and varieties of fiber that are crucial for optimal health. Fiber is formally classified into two main types of fiber called functional fiber and dietary fiber, which together comprise total fiber [4, 9]. Fiber is further classified based on its solubility (soluble vs. insoluble), viscosity (viscous vs. non-viscous), and fermentability (fermentable vs. non-fermentable) [9]. Soluble fibers dissolve in water, while insoluble fibers do not [4, 7, 8, 9]. Viscous fibers thicken in the presence of water, forming sticky, glue-like solutions [8]. Fermentable fiber is metabolized to form short-chain fatty acids, which are absorbed and metabolized to produce energy [8]. Fiber fermentation contributes up to 10 percent of daily energy intake [8].

Soluble, viscous/gel forming, readily fermented fiber. Fibers that are soluble, viscous, and fermentable have been shown to improve glycemic control and to lower blood cholesterol concentration [8]. This type of fiber slows gastric emptying (the passage of food from the stomach into the intestines) [4, 7]. Consequently, the delayed gastric emptying slows the release of sugar into the bloodstream, lowering glucose levels and blood cholesterol [4, 7]. However, this type of fiber’s water-holding capacity is lost when it is fermented in the colon, which results in no laxative effect [8]. Foods with soluble fiber include oatmeal, chia seeds, nuts, beans, lentils, apples, and blueberries [7].

Soluble, viscous/gel forming, non-fermented fiber. Fibers that are soluble, viscous, and non-fermentable can also improve glycemic control and lower blood cholesterol concentration [8]. Since they are resistant to fermentation, they retain their water-holding/gel-forming capacity in the large intestine [8]. As a result, they can exert a stool-normalizing effect, preventing constipation or softening hard stool as well as firming loose/liquid stool in diarrhea and fecal incontinence [8].

Soluble, non-viscous, readily fermented fiber. Although these fibers can dissolve in water, they cannot provide any health benefits associated with fiber viscosity [8]. They are fully fermented and thus do not exert a laxative effect [8].

Insoluble, poorly fermented fiber. These fibers do not dissolve in water, do not trap water, and are poorly fermented [8]. Large/coarse fiber particles can have a laxative effect [8]. They can irritate the large intestine mucosa and trigger the secretion of mucus and water, which increases the water content of stools [8]. They can help food move through the digestive system, promoting regularity and preventing constipation [7]. Small, insoluble fiber particles (e.g., finely ground wheat bran) have no laxative effect and can have a constipating effect by adding only to the dry stool mass [7]. Foods with insoluble fibers include whole wheat products (especially wheat bran), quinoa, brown rice, legumes, leafy greens like kale, almonds, walnuts, seeds, and fruits with edible skins like pears and apples [7].

Fiber Recommendation

• The Adequate Intake (AI) recommendations for total fiber intake, set by the Food and Nutrition Board of the Institute of Medicine, are approximately 14 grams for every 1,000 calories (kcal) consumed [4, 7, 8, 9].

• For most adults (19-50 years), the recommended intake is 38 grams for men and 25 grams for women. This fiber goal is easily achievable by eating a wide variety of fruits, vegetables, whole grains, legumes, nuts, and seeds [4, 7, 8, 9].

Carbohydrates and Athletic Performance

Healthy carbohydrates chosen in appropriate amounts and at appropriate times can benefit athletic performance [4]. Some practitioners suggest that manipulating the GI of foods and meals may enhance carbohydrate availability and improve athletic performance [6]. For example, low GI carbohydrate-rich foods may be recommended before exercise to promote sustained carbohydrate availability [6]. Moderate to high GI carbohydrate-rich foods may be recommended during exercise to promote carbohydrate oxidation and after exercise to promote glycogen repletion [6]. Research suggests that a diet based on consumption of high-GI carbs promotes greater glycogen storage following strenuous exercise [1]. Overall, high-GI glucose-rich foods are good for refueling and athletic performance [1]. However, the total amount of carbs consumed is the most important consideration for replenishing glycogen stores following daily training sessions and competitive events [6]. While the GI may be useful in sports by helping to fine tune food choices, it should not be used exclusively to provide guidelines for carb and food intake before, during, and after exercise [6]. Other features of foods such as nutritional content, palatability, portability, cost, gastrointestinal comfort, and ease of preparation are also important [6]. Athletes should choose foods according to their nutritional goals and exercise situation [6].

A small snack before strenuous or prolonged exercise will help to optimize the training session. The food should be relatively high in carbohydrate to maximize blood glucose availability, relatively low in fat and fiber to minimize gastrointestinal distress and facilitate gastric emptying, moderate in protein, and well-tolerated by the individual [1]. During extended training sessions, exercisers should consume 30 to 60 grams of carbohydrate per hour of training to maintain blood glucose levels [1]. This is especially important for training sessions lasting longer than one hour; exercise in extreme heat, cold, or high altitude; and when the individual did not consume adequate amounts of food or drink prior to the training session. After exercise, individuals should focus on carbs and protein. Studies show that the best meals for post-workout refueling include an abundance of carbs accompanied by some protein (3:1 carb to protein ratio) [1, 4]. The carbs replenish the used-up energy that is normally stored as glycogen in the muscle and liver, and the protein helps to rebuild the muscles that were fatigued with exercise. The amount of refueling depends on the intensity and duration of the training session, but the exerciser should eat as soon after exercising as possible, preferably within 30 minutes. This is the time when the muscles are best able to replenish energy stores, enabling the body to prepare for the next workout [1].

Carbohydrate Recommendation

• Per the Acceptable Macronutrient Distribution Range (AMDR), which was developed by the Institute of Medicine, carbohydrates should account for 45-65 percent of total daily calories [4].

• Consume fewer than 10 percent of total calories from added sugars [1, 3, 4].

• The daily minimum for most healthy individuals is 130 grams per day [4]. This is a minimum requirement based on the amount of carbs needed by the brain daily [4]. Therefore, for energy maintenance the body needs more than 130 grams per day [4].

• Athletes need anywhere from 6 to 10 g/kg (3 to 5 g/lb.) of body weight per day depending on their total energy expenditure, type of exercise performed, gender, and environmental conditions, to maintain blood glucose levels during exercise and to replace muscle glycogen [1, 3, 4, 6].

• The American Dietetic Association (ADA) recommends a carb intake of 1.0 to 1.5 g/kg (0.5 to 0.7 g/lb.) of body weight in the first 30 minutes after exercise and then every 2 hours for 4-6 hours [1].

Sources

[1] Bryant, C. X., & Green, D. J. (2017). Ace Essentials of Exercise Science for Fitness Professionals. American Council on Exercise.

[2] Efferding, S., & McCune, D. (2021). The vertical diet. Victory Belt Publishing.

[3] Muth, N. D., & Tanaka, M. S. (2013). Ace Fitness Nutrition Manual. American Council on Exercise.

[4] Muth, N. D., & Zive, M. M. (2020). Sports nutrition for health professionals (2nd ed.). F.A. Davis.

[5] Zinczenko, D., & Spiker, T. (2010). The new ABS diet: The six-week plan to flatten your belly and firm up your body for life. Rodale Press.

[6] Bertucci D R Ferraresi C 2016 Strength Training: Methods, Health Benefits and DopingBertucci, D. R., & Ferraresi, C. (2016). Strength Training: Methods, Health Benefits and Doping. Nova Science Publishers. https://eds-s-ebscohost-com.ezproxy.umgc.edu/eds/ebookviewer/ebook/bmxlYmtfXzExMzQ0NTZfX0FO0?sid=b5e6955f-8aa1-4fff-906c-5bba28f3d5d2@redis&vid=1&format=EB&rid=1

[7] Fiber. The Nutrition Source. (2023, February 2). https://www.hsph.harvard.edu/nutritionsource/carbohydrates/fiber/

[8] Fiber. Linus Pauling Institute. (2023, September 8). https://lpi.oregonstate.edu/mic/other-nutrients/fiber#classification-systems

[9] Leech, J. (2017, June 4). Good fiber, bad fiber - how the different types affect you. Healthline. https://www.healthline.com/nutrition/different-types-of-fiber