Glycemic Load Food Guide Pyramid for Athletic Performance
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Glycemic Load Food Guide Pyramid for Athletic Performance Kristen M. Beavers, MPH, RD1 and Brian Leutholtz, PhD2 1 Exercise and Sport Nutrition Laboratory; 2Department of Health, Human Performance, and Recreation, Baylor University, Waco, Texas SUMMARY classification of CHO, as it provides desirable for athletes during sustained relevant information for the prevention competition. THE CLASSIFICATION OF CARBO- and treatment of chronic disease. Since Unfortunately, classifying CHO as HYDRATES (CHO) BASED ON then, the use of the GI to promote simple or complex is flawed, confusing, GLYCEMIC INDEX (GI), ALTHOUGH weight loss, improve chronic health and inaccurate; especially when these CLINICALLY USEFUL, MAY LACK conditions, and fuel athletic perfor- terms are used as surrogates for GI. To PRACTICAL UTILITY IN SPORT AS mance has been studied by numerous begin, most foods are a combination of THE AMOUNT OF CHO INGESTED researchers in a variety of settings simple and complex CHO, and so this ALSO AFFECTS CIRCULATING (3,5,17,21,25). strict dichotomy only works for a select BLOOD SUGAR. IN ATTEMPT TO Other ways to categorize CHO have few food items. Furthermore, to label CAPTURE THE ENTIRE GLUCOSE been promoted by the food industry; simple CHO as unhealthy and com- RAISING POTENTIAL OF DIETARY sometimes referring to their products plex CHO as healthy is simply not CARBOHYDRATES, THE accurate. Contrast the simple sugars CONCEPT OF GLYCEMIC LOAD as low in ‘‘sugars’’ or ‘‘impact carbs.’’ These terms can be defined as CHO found in fruits to the complex CHO (GL) HAS BEEN INTRODUCED TO found in foods such as pizza, fries, and that are high on the GI, and have INCORPORATE BOTH THE potato chips. Despite their simple a great impact on blood glucose levels QUALITY AND THE QUANTITY OF sugar content, fruits are considered by causing a large release of insulin. CHO CONSUMED. THIS REVIEW healthy foods as they provide a good Other times CHO are reported as DISCUSSES CURRENT RESEARCH source of select vitamins, minerals, ‘‘simple’’ versus ‘‘complex.’’ Simple IN THE AREA OF GI AND GL, WITH and fiber. Conversely, pizza, fries, and CHO are mono-, di-, or oligo-saccha- EMPHASIS PLACED ON THE ROLE potato chips are less nutritious and rides, often times referred to as the OF GL IN GUIDING ATHLETE’S higher in fat than other ‘‘simple sugar’’ ‘‘bad carbs.’’ These CHO are lumped DIETARY FOOD CHOICES PRE, foods (such as fruit), despite their into a broad category reported to cause DURING, AND POST EXERCISE. ‘‘complex CHO’’ label. Using this large, rapid changes in blood glucose classification system Americans have levels, followed by a greater fall in been falsely educated to correlate INTRODUCTION blood glucose. The end result is simple sugars with negative health n the 1970s, the term glycemic a temporary over production of insulin outcomes, and complex CHO with I index (GI) was first introduced to the nutrition community as a way to classify carbohydrates (CHO) based that can result in hypoglycemia. Com- plex CHO, or the ‘‘good carbs,’’ are polysaccharides or starches and are positive health outcomes. Clearly, a better system is needed to classify and discuss CHO. on their physiologic effect on blood usually touted as containing significant amounts of other nutrients, including We propose classifying CHO into glucose levels. Prior to this, CHO were dietary fiber, making them more nutri- ‘‘refined’’ (processed foods) and ‘‘un- (and still are), categorized based on refined’’ (natural foods) categories. chemical structure. Most notably, ent dense than simple CHO. Since the Refining is a process that can apply when stratified by chemical composi- digestion and absorption of complex tion, CHO can be thought of as sugars, CHO foods are slower, they produce KEY WORDS: starches, and fibers. In clinical settings, more stable or sustained blood glucose the GI has proven to be a more useful and insulin responses. Theoretically, glycemic load; glycemic index; athletic nutritional concept than the chemical these responses would be more performance; nutrition; food pyramid 10 VOLUME 30 | NUMBER 3 | JUNE 2008 Copyright Ó National Strength and Conditioning Association
to simple or complex CHO. Quite or lactose, the ratio of amylopectin emphasis placed on exploiting the literally, it is the act of removing fiber, and amylase in starch, starch-protein principles of GI and GL to maximize nutrients, usually water, and other or starch-fat interactions, and the endogenous CHO stores, thereby min- items contained within the food in its presence of anti-nutrients such as imizing the potential ergolytic effects natural state. The act of refining foods phytates and lectins (5,26). In 1981, of CHO depletion (11). In the sections typically concentrates sugars and re- Jenkins et al. produced table showing that follow, current research in this sults in food items with a heightened GI response to 62 commonly eaten area will be summarized, with empha- GI. Unrefined CHO are usually foods (15), and based on this research, sis placed on the role of GL in guiding healthy, unadulterated, foods such as the concept of stratifying CHO by their athlete’s dietary food choices pre-, fruits, vegetables, and whole grains, GI ranking was born; with a GI score during, and post-exercise. which are lower in the GI, and result of 55 or less constituting a low GI food, in a more flattened blood glucose 56–69 a medium GI food, and 70 or PRE-EXERCISE response (Table 1). higher a high GI food. Previous literature has shown that It has been argued that the GI lacks ingestion of a CHO-rich meal (~200– GLYCEMIC INDEX VERSUS GLYCEMIC LOAD practical utility as the amount of CHO 300 g) within four hours of exercise By definition, GI is a ranking of foods ingested also affects circulating blood improves endurance or performance based on their actual postprandial sugar. Thus, to capture the entire during prolonged moderate intensity glucose-raising potential of dietary exercise (7,19,20,27) presumably due to blood glucose response compared to CHO, the concept of glycemic load increases muscle glycogen (10). How- a reference food, either glucose or (GL) was introduced to simultaneously ever, recent literature suggests that white bread. GI is calculated by incorporate both the quality and the when CHO is ingested during exercise measuring the incremental area under quantity of CHO consumed. In es- in amounts presently recommended by the blood glucose curve following sence, GL is a function of the GI of sports nutrition guidelines, pre-exercise ingestion of a test food providing a certain food multiplied by the CHO intake has little effect on metab- 50 g of CHO, compared with the area number of grams of CHO of that food olism or on subsequent performance under the curve following an equal in a single serving. We believe the GL during prolonged cycling (4). CHO intake from the reference food, with all tests being conducted after an represents a more relative and accurate It has also been speculated that the overnight fast. effect a food has on blood glucose. negative outcomes of CHO ingestion GL ¼ GI/100 3 g of CHO in a given 30 to 60 minutes prior to exercise (i.e., Blood glucose area increase muscle glycogen utilization after test food amount of food ð13Þ GI5 3 100 and decreased time to fatigue) may be Blood glucose area Proponents of the GI/GL based diets avoided by consuming a low GI food after reference food argue that manipulating dietary pre- (24). Moreover, low GI foods may be Simply put, GI reflects the rate of scription to include low GI/GL foods favored one hour prior to exercise digestion and absorption of a CHO- decreases the occurrence of hypergly- because the slow digestion of these rich food. Numerous factors have been cemia and hyperinsulinemia, both of foods results in the availability of fuels identified that influenced the digestion which can lead to insulin resistance near the end of exercise (25). and absorption rate of CHO rich foods, and further chronic disease. Applica- including degree of food processing tions from this area of research were DURING EXERCISE and cooking, the presence of fructose then extended to the athlete; with Athletes undertaking prolonged exer- cise are advised to consume CHO Table 1 during the event to enhance perfor- Refined versus unrefined foods mance, most often as diluted liquids. Refined Foods Unrefined Foods In general, ;1 g CHOmin21 is advised, since this appears to be the Cakes and cookies Brown rice maximal rate of oxidation of ingested Potato chips Oat bran CHO (5). Although it would make sense that CHO consumed during Candy bars Oatmeal exercise should be easily digested and Jellies and jams Whole grain pasta absorbed to provide a rapid supply of energy (8), the effect of the GI and GL Fruit beverages Wheat bread of CHO rich foods/drinks during Sodas Fresh fruits exercise has not yet been systemati- cally studied. That being said, many Salad dressings with added sugars Fresh vegetables athletes choose CHO sources, Strength and Conditioning Journal | www.nsca-lift.org 11
Glycemic Load Food Guide Pyramid for Athletic Performance including specially manufactured immediately post exercise (23). In prolonged exercise. During this study, sports drinks and bars, which would a recent study by Levenhagen et al., eight trained male athletes participated be classified as moderate to high in GI ten subjects were used in a crossover in a crossover design composed of two or GL (13). Moreover, low GI/GL design to examine the protein dynam- trials separated by at least seven days. foods tend to cause more stomach ics of early vs. late macronutrient post- In both trials, subsequent to an over- distress and are avoided during exer- exercise supplementation (18). The night fast, subjects completed a 90 cise. Studies need to be undertaken to same oral supplement (10 g protein, 8 minute run at 70% VO2max, with meals determine if low GI/GL CHO are g CHO, 3 g fat) was administered for provided thirty minutes and two hours advantageous over high GI/GL CHO both trials, either immediately or three following cessation of exercise. Follow- before and during exercise. hours post-exercise. Markers of protein ing the ingestion of the second meal, synthesis were measured, and results serum insulin concentration was higher POST EXERCISE showed that post-exercise ingestion of in the high-GI trial as compared to the Previous literature has shown that the a combined CHO and protein supple- low GI trial. Results suggest that a low highest rate of muscle glycogen storage ment does enhance protein accretion. GI diet may be more beneficial for occurs during the first hour after Authors speculate that this finding is continued utilization of fat during the exercise (14,16). Moreover, the most due largely to the presence of circulat- later recovery period (22). important dietary factor in glycogen ing insulin, which increases in blood recovery during this immediate post- immediately after CHO ingestion, and DISCUSSION exercise period is the amount of CHO is critical in the regulation of protein In 2004, Burke and colleagues put forth that is consumed, with an intake of synthesis (2) and proteolysis (12). revised guidelines for the intake of ;7–10 g kg21 body mass providing The GI of the CHO consumed later in CHO in the everyday or training diets maximal daily glycogen storage (9). the post-exercise period, has also be of athletes (6). These recommenda- These findings inherently lend them- show to be important because of their tions focus on the practical application selves to high GL foods, regardless of influence on substrate oxidation. In of substrate utilization research. As an GI rank. Additionally, high GI CHO 2005, Stevenson et al. conducted extension to this work, we feel that consumed together with protein have a study to examine the metabolic recommendations for the intake of been shown to enhance exercise- responses to high GI or low GI meals CHO in terms of GL would prove induced muscle formation when given consumed during recovery from a useful tool for the athlete or Figure 1. The Glycemic Load Food Guide Pyramid for Athletic Performance. 12 VOLUME 30 | NUMBER 3 | JUNE 2008
professionals working with athletes. may change in the non-fasted state is of 4. Burke, LM, Claassen, A, Hawley, JA, and We have begun this process by merg- critical importance to the athlete. Noakes, TD. Carbohydrate intake during prolonged cycling minimizes effect of ing one of the most iconic representa- Finally, research and development in glycemic index of pre-exercise meal. J Appl tions of nutrition education with such the food science arena needs to focus Physiol 85: 2220–2226, 1998. recommendations, with the end prod- on palatable low glycemic foods spe- 5. Burke, LM, Collier, GR, and Hargreaves, M. uct being The Glycemic Load Food cifically engineered for the athlete. Glycemic index–a new tool in sport nutrition? Guide Pyramid for Athletes (Figure 1). Previous studies assessing the effect Int J Sport Nutr 8: 401–415, 1998. This tool is designed to be used in the of low glycemic foods on athletic 6. Burke, LM, Kiens, B, and Ivy, JL. lay literature to help athletes and performance have historically chosen Carbohydrates and fat for training and professionals working with athletes lentils – often consumed in very large recovery. J Sports Sci 22: 15–30, 2004. understand how to apply the concept portions - which may not be a practical 7. Chryssanthopoulos, C, Williams, C, of GL to optimal athletic performance. choice for the athlete for myriad Nowitz, A, Kotsiopoulou, C, and Vleck, V. Because numerous methodological reasons. Although low GI/GL pre- The effect of a high carbohydrate meal on problems exist in the definition of GI competition meals have been shown to endurance running capacity. Int J Sport and GL, including poor standardiza- prolong endurance during strenuous Nutr Exerc Metab 12: 157–171, 2002. tion, poor reproducibility, and high exercise, if accompanied by gastric 8. Ciok, J and Dolna, A. The role of glycemic variability, (1) it was paramount to use distress, the athlete is unlikely to follow index concept in carbohydrate metabolism. a single, reliable measure of GI/GL on such recommendations.j Przegl Lek 63: 287–291, 2006. which to base our recommendations. 9. Costill, DL, Sherman, WM, Fink, WJ, In 2002, Foster-Powell et al. published Maresh, C, Witten, M, and Miller, JM. The Kristen M. role of dietary carbohydrates in muscle an extensive table of GI and GL values Beavers is glycogen resynthesis after strenuous for over 750 different foods (13). This a PhD student running. Am J Clin Nutr 34: table compiles all relevant data pub- in Exercise, 1831–1836,1981. lished between 1981 and 2001, and for Nutrition, and 10. Coyle, EF. Substrate utilization during multiple listings of a food, means and Preventative exercise in active people. Am J Clin Nutr standard deviations have been calcu- Health at Baylor University in the 61(4 Suppl): 968S–979S, 1995. lated. It is this table upon which our Department of Health, Human 11. Coyle, EF, Coggan, AR, Hemmert, MK, and classification of GL is based. Performance, and Recreation. Ivy, JL. Muscle glycogen utilization during FUTURE DIRECTION prolonged strenuous exercise when fed Future research addressing the role of Brian C. carbohydrate. J Appl Physiol 61: GL as a strategy for exercise enhance- 165–172, 1986. Leutholtz is ment needs to focus of how it may Professor of 12. Flakoll, PJ, Kulaylat, M, Frexes-Steed, M, enhance athletic performance. Too Hourani, H, Brown, LL, Hill, JO, and Recreation at Abumrad, NN. Amino acids augment often we assess improvements in Baylor University insulin’s suppression of whole body athletic performance with enhanced in Waco, proteolysis. Am J Physiol 257(6 Pt 1): strength or endurance capacity, and Texas. E839–47, 1989. this may not translate into enhance- 13. Foster-Powell, K, Holt, SH, and Brand- ment of performance. While such Miller, JC. International table of glycemic studies prove beneficial, studies de- index and glycemic load values: 2002. signed to capture athletic performance Am J Clin Nutr 76: 5–56, 2002. outcomes are needed. 14. Ivy, JL, Lee, MC, Brozinick Jr, JT, and Additionally, more research needs to be REFERENCES Reed MJ. Muscle glycogen storage conducted looking at how GI and GL 1. Arteaga Llona, A. The glycemic index. A after different amounts of carbohydrate values change when food is consumed current controversy. Nutr Hosp 21 Suppl 2: ingestion. J Appl Physiol 65: 2018– 53–59, 55–60, 2006. 2023,1988. in a non-fasted state. Recall that the GI is calculated by measuring the incre- 2. Biolo, G, Declan Fleming, RY, and Wolfe, 15. Jenkins, DJ, Wolever, TM, Taylor, RH, mental area under the blood glucose RR. Physiologic hyperinsulinemia Barker, H, Fielden, H, Baldwin, JM, stimulates protein synthesis and enhances Bowling, AC, Newman, HC, Jenkins, AL, curve following ingestion of a test food transport of selected amino acids in human and Goff, DV. Glycemic index of foods: providing 50 g of CHO, compared with skeletal muscle. J Clin Invest 95: 811– a physiological basis for carbohydrate the area under the curve following an 819, 1995. exchange. Am J Clin Nutr 34: 362–366, equal CHO intake from the reference 1981. 3. Brand-Miller, J, Hayne, S, Petocz, P, and food, with all tests being conducted after Colagiuri, S. Low-glycemic index diets in 16. Jentjens, R and Jeukendrup, A. an overnight fast. Seldom in practice will the management of diabetes: a meta- Determinants of post-exercise glycogen athletes fast prior to a competition, analysis of randomized controlled trials. synthesis during short-term recovery. let alone during and after. How GI Diabetes Care 26: 2261–2267, 2003. Sports Med 33: 117–144, 2003. Strength and Conditioning Journal | www.nsca-lift.org 13
Glycemic Load Food Guide Pyramid for Athletic Performance 17. Jimenez-Cruz, A, Gutierrez-Gonzalez, AN, 21. Sloth, B, Krog-Mikkelsen, I, Flint, A, Tetens, 24. Thomas, DE, Brotherhood, JR, and Brand, and Bacardi-Gascon, M. Low glycemic I, Björck, I, Vinoy, S, Elmståhl, H, Astrup, A, JC. Carbohydrate feeding before exercise: index lunch on satiety in overweight and Lang, V, and Raben, A. No difference in effect of glycemic index. Int J Sports Med obese people with type 2 diabetes. Nutr body weight decrease between a low- 12: 180–186, 1991. Hosp 20: 348–350, 2005. glycemic-index and a high-glycemic-index diet but reduced LDL cholesterol after 25. Thomas, DE, Brotherhood, JR, and Miller, 18. Levenhagen, DK, Gresham, JD, Carlson, 10-wk ad libitum intake of the low- JB. Plasma glucose levels after prolonged MG, Maron, DJ, Borel, MJ, and Flakoll, PJ. Postexercise nutrient intake timing in glycemic-index diet. Am J Clin Nutr 80: strenuous exercise correlate inversely with humans is critical to recovery of leg glucose 337–347, 2004. glycemic response to food consumed and protein homeostasis. Am J Physiol 22. Stevenson, E, Williams, C, and Biscoe, H. before exercise. Int J Sport Nutr 4: Endocrinol Metab 280: E982–993, 2001. The metabolic responses to high 361–373, 1994. 19. Schabort, EJ, Bosch, AN, Weltan, SM, and carbohydrate meals with different 26. Thorburn, AW, Brand, JC, and Truswell, Noakes, TD. The effect of a preexercise glycemic indices consumed during AS. The glycaemic index of foods. Med J meal on time to fatigue during prolonged recovery from prolonged strenuous Aust 144: 580–582, 1986. cycling exercise. Med Sci Sports Exerc 31: exercise. Int J Sport Nutr Exerc Metab 15: 464–471, 1999. 291–307, 2005. 27. Wright, DA, Sherman, WM, and Dernbach, AR. Carbohydrate feedings before, 20. Sherman, WM, Brodowicz, G, Wright, DA, 23. Suzuki, M. Glycemic carbohydrates consumed with amino acids or protein during, or in combination improve Allen, WK, Simonsen, J, and Dernbach, A. Effects of 4 h preexercise carbohydrate right after exercise enhance muscle cycling endurance performance. feedings on cycling performance. Med Sci formation. Nutr Rev 61(5 Pt 2): S88–94, J Appl Physiol 71: 1082–1088, Sports Exerc 21: 598–604, 1989. 2003. 1991. 14 VOLUME 30 | NUMBER 3 | JUNE 2008
You can also read