Carbohydrates

Lecture Macronutrients Patricia Davidson DCN,RDN,CDE, LDN, FAND Session Objectives Understand the importance of carbohydrates as the "master fuel” as it relates to the other macronutrients Discuss the different classifications of carbohydrates and how they differ from artificial sweeteners. Explain the different functions that carbohydrates serve in the body as it relates to exercise. Discuss the various sources of dietary carbohydrates. Define glycemic index and glycemic load and explain how they can be applied in sports nutrition. Explain how carbohydrates are utilized during exercise. Discuss the type, quantity, and timing of carbohydrate consumption before exercise or competition. Outline I. What's the big deal about carbohydrates? A. They are an important dietary source of energy. B. They are particularly important to athletes, because they are the only macronutrient that can be metabolized anaerobically. C. They are important for the recovery of carbohydrate stores used during exercise. D. Carbohydrates are so important to athletes that they are considered the "master fuel" of the body. II. How are carbohydrates classified? A. Simple carbohydrates 1. Monosaccharides are single-molecule sugars (e.g., glucose, fructose). 2. Disaccharides are two sugar molecules linked together (e.g., sucrose). B. Complex carbohydrates 1. Oligosaccharides are short chains of 3 to 10 linked sugar molecules (e.g., maltodextrins). 2. Polysaccharides are chains of 11 or more linked sugars (e.g., starch, fiber, glycogen). 3. Dietary fiber is, for the most part, indigestible. It exists in two basic forms: soluble and insoluble. Fiber can also be classified as dietary or functional fiber. III. What functions do carbohydrates serve in the body? A. They are an important energy source for the body, particularly during high-intensity exercise. B. The adequate intake of carbohydrates helps to spare muscle tissue. C. Carbohydrates are the primary energy source for the nervous system. IV. How can carbohydrates affect overall health? A. Dietary fiber, although not digestible, can add bulk to intestinal contents and speed up transit time, thus preventing constipation and possibly hemorrhoids and colon cancer. B. Soluble fiber appears to play a role in reducing risk for heart disease by decreasing blood cholesterol levels. C. Foods rich in complex carbohydrates may indirectly help with weight loss or maintenance of a healthy weight, because such food sources are relatively low in calories and may replace high-calorie foods. D. Simple sugars, unlike complex carbohydrates, may have some negative health consequences. Diets high in simple sugars tend to be dense in calories and low in other nutrients and fiber. The end result is weight gain, an increased risk for dental caries, and loss of the health benefits provided by high-fiber foods. E. Carbohydrate needs can be determined based on a percentage of total calories. 1. The recommended range is 45 to 65% of the total daily calories. 2. The higher end of the recommended range is suggested for athletes. F. Carbohydrate intake will vary slightly for athletes depending on training volume, when the next competition is, or the stage of training (i.e., preseason, in season, postseason). For most athletes, carbohydrate needs will increase slightly as training volume increases or as a competition approaches. V. What are the various sources of dietary carbohydrates? A. Most of the foods in the grains group are excellent sources. The key is to choose whole-grain products that are nutrient dense. B. Fruits and vegetables are also good choices, because they also contain soluble and insoluble fiber, vitamin C, potassium, beta carotene, and other antioxidants. C. Milk and milk alternatives provide a good source of carbohydrates and protein. Most choices in this group also provide calcium as well as vitamin D. D. Bean, lentils, nuts, seeds, and soy products are good sources of carbohydrates in the meat and bean group. Animal meats do not contain carbohydrates. VI. What are the glycemic index and glycemic load and how can they be used in sports nutrition? A. The glycemic index (GI) indicates how much a certain food increases blood glucose levels when consumed in isolation. However, the GI of a food can be altered by the type of carbohydrate, its fiber content, its protein and fat content, its form (i.e., solid or liquid), the timing of its consumption relative to other meals/snacks, what other foods are being consumed at the same time, and the total amount of carbohydrate being consumed. B. Glycemic load not only considers the glycemic index of a food, but also its carbohydrate content. The usefulness of glycemic load beyond that of the glycemic index is controversial, because many believe that the glycemic index is a questionable way of classifying carbohydrate sources to begin with. C. The use of the GI of foods to help athletes improve sport performance appears to have some merit. Current information on the GI of foods can help athletes make decisions concerning the best types of carbohydrates to consume before, during, and after training or competition. Further research will over time help to refine usage of GI in sport applications. VII. How are carbohydrates utilized during exercise? A. The crossover concept of carbohydrate and fat utilization indicates that carbohydrates become more important as an energy source as exercise intensity increases Carbohydrates become even more important than fats as exercise intensities approach 70% of aerobic capacity. B. Unfortunately, very little carbohydrate is stored in the body relative to fats. Roughly, only 400 to 600 grams are stored. This is the equivalent of about 1,600 to 2,400 calories. Furthermore, muscle does not share its stored glycogen and, as a result, only about 400 to 500 calories stored in the form of liver glycogen are actually directly available to help maintain blood glucose levels. C. Carbohydrates are a good source of rapid energy for several reasons: 1. Carbohydrates are stored in the muscle cells and thus are readily available at the very outset of exercise, unlike fats, which are stored in remote sites and must be delivered to the muscle via the bloodstream. 2. Carbohydrates are also capable of serving as an energy source for short periods of time without the need for oxygen. In other words, carbohydrates can be metabolized anaerobically, fats cannot. 3. When metabolized aerobically, carbohydrates require less oxygen than fats. Because oxygen delivery is often the rate-limiting step in aerobic metabolism, it is an advantage to metabolize carbohydrates over fats during high-intensity exercise when the need for oxygen is at a premium. D. There is little question that carbohydrate intake can positively affect sport performance. High-carbohydrate diets increase muscle glycogen levels and extend exercise time to fatigue. Also, carbohydrate intake during exercise can increase blood glucose levels and extend exercise time at a give workload. Clearly, adequate carbohydrate intake before, during, and after exercise is important during endurance sports. Although depletion of carbohydrate stores during strength/power sports is not a major concern, adequate carbohydrate intake is still important to prevent glycogen depletion over days of training and to help maintain blood glucose levels. Falling blood glucose levels can negatively impact mental attitudes, such as focus and motivation and ultimately lead to decreases in sport performance. VIII. What type, how much, and when should carbohydrates be consumed before exercise? A. It is critical to consume adequate amounts of carbohydrates in the days, as well as hours, leading up to an exercise session or competition. The practice of combining the tapering of training with carbohydrate loading has been shown to be an effective means of increasing muscle glycogen levels. Alternatively, if an athlete consumes 55 to 70% of total calories from carbohydrates daily, muscle glycogen levels can be "topped off" without the need for carbohydrate-loading practices. B. Carbohydrate-rich meals are important in the 24 hours leading up to an important training session or competition. 1. 4 to 24 hours prior to exercise, foods high in carbohydrates should compose the majority of each meal and snack, providing approximately 60 to 70% of total calories. Also, the foods should be "familiar foods" to minimize the chance of gastric or intestinal upset. 2. 0 to 4 hours prior to exercise, carbohydrate stores should already be at their peak. During the period, the focus shifts to foods and beverages that will digest easily and prevent the athlete from feeling hungry at the beginning of competition. Low GI foods relatively low in fiber are recommended in the 2 to 4 hours prior to competition or exercise. IX. What type, how much, and when should carbohydrates be consumed during exercise? A. Consuming carbohydrates during exercise has been shown to help delay fatigue in both short- and long-duration activities. B. Research has shown that glucose, sucrose, glucose polymers, and starches are appropriate sources of carbohydrates during exercise. C. Consuming carbohydrates in the form of sports beverages, gels, or energy bars at a rate of about 60 to 70 grams per hour appears to be appropriate during exercise. D. It is recommended that athletes begin consuming carbohydrates early during exercise. Also, carbohydrates should be consumed at a steady rate throughout the exercise period; repetitive small feedings are preferable to a single large feeding. X What type, how much, and when should carbohydrates be consumed after exercise? A. After exercise, it is critical to feed the muscle with carbohydrates to replenish the stores of muscle and liver glycogen used. Failure to do so can lead to low muscle-glycogen levels and poor sport performance. B. High GI foods and/or beverages are recommended. C. A carbohydrate intake of 1 gram per kilogram of bodyweight is recommended. D. Carbohydrate ingestion should begin as soon as possible, but within two hours from the end of training or competition.