An Assessment of Carbohydrate Intake in Collegiate Distance Runners

Transcription

1 International Journal of Sport Nutrition, 1995, 5, it 1995 Human Kinetics Publishers, inc. An Assessment of Carbohydrate Intake in Collegiate Distance Runners Jill A. Tanaka, Hirofumi Tanaka, and William Landis To determine the extent to which well-trained endurance athletes practice the dietary recommendations for maximizing muscle glycogen resynthesis, collegiate cross-country runners (14 males and 10 females) kept 4-day dietary and activity records during a training period and a competitive period in the regular cross-country season. The mean running mileages for men and women were 16.0 ± 1.0 and 10.7 ± 0.6 km/day during the training period and 14.6 ± 0.8 and 8.7 ± 0.5 km/day during the competitive period, respectively. Males reported adequate energy intake in both phases, whereas females fell short of the RDA. However, the percentage of calories from carbohydrate was found to be inadequate (<60%) for male runners. Although female runners derived 65-67% of calories from carbohydrate, the daily amount of carbohydrate taken was insufficient (<10 g/kg body weight). Carbohydrate was ingested immediately postexercise approximately 50% of the time or less, with even far less taken in suggested quantities (-1 g carbohydrate/kg body weight). There were no significant differences in dietary trends between training and competitive phases. The results suggest that these endurance athletes were not practicing the recommended feeding regimen for optimal muscle glycogen restoration. Key Words: exercise, nutrition, dietary carbohydrate, muscle glycogen It is well established that there is a significant relationship between muscle glycogen content and endurance capacity (2). Since muscle glycogen appears to be the major fuel both in competition and in daily training, the resynthesis of muscle glycogen is of primary importance for optimum endurance performance and training adaptations. A considerable amount of research (6, 13, 14) has been conducted in an attempt to design the optimum regimen for rapid repletion of muscle glycogen stores following strenuous exercise. It is recommended that, besides balancing daily energy intake with energy expenditure, endurance athletes who train exhaustively on successive days should consume a diet containing 60 to 70% of calories from carbohydrate or 10 g of carbohydrate/kg body weight/ day (5, 6, 21). In addition to the carbohydrate consumption in the daily diet, a J.A. Tanaka and W. Landis are with the Department of Home Economics, Ball State University, Muncie, IN H. Tanaka is with the Exercise Science Unit, University of Tennessee Knoxville, Knoxville, TN Direct correspondence to H. Tanaka. 206

2 Carbohydrate Intake in Runners / -207 carbohydrate supplement of approximately 1 g/kg body weight should be consumed immediately after a glycogen-depleting exercise bout, especially during intensive training periods (13, 14). The continuation of supplementation every 2 hr will maintain a maximal rate of storage up to 6 hr after exercise (13, 14). Only when this regimen is carefully followed can muscle glycogen be resynthesized to at least preexercise levels within 24 hr of completion of strenuous exercise (13). A study by Costill et al. (4) suggested that endurance athletes experienced chronic muscle fatigue primarily because of depleted muscle glycogen, as a result of insufficient carbohydrate intake to match the energy demand of repeated days of intense training. This is in line with findings of Kirwan et al. (17), who reported a higher perceived exertion and lower running economy when endurance athletes consumed a moderate-carbohydrate diet. Thus, one can suspect that if an endurance athlete consumes an optimum amount of carbohydrate within the proper time frame, the risk of overtraining may be minimized due to maximized glycogen repletion. " Several studies (10, 24) have shown that few of today's endurance athletes consume adequate dietary carbohydrate. Likewise, it is suspected that most endurance athletes do not adhere to recommendations for maximizing muscle glycogen repletion immediately following exercise. Since no research has demonstrated whether endurance athletes practice the recommended carbohydrate feeding regimen, the primary purpose of this study was to compare collegiate cross-country runners' carbohydrate intakes with the dietary recommendations made to ensure maintenance of muscle glycogen levels. A unique aspect of this study was the special reference to carbohydrate intake immediately postexercise. Subjects Methods Intercollegiate male (n = 14) and female (n = 10) cross-country runners on a men's varsity team and a women's varsity team (NCAA Division I) served as subjects for this study. Prior to participation, the procedure was explained verbally and in writing, and informed consents were obtained in accordance with the procedures of the Institutional Review Board for Human Subjects. All the data were collected during the regular competitive cross-country season. The subject characteristics are shown in Table 1. Dietary Information Food intake records were used to collect dietary data for the regular cross-country season during a training period and during a competitive period, including an important racing day. A 4-day food record was chosen because 4 to 6 days are required to accurately quantify true average group macronutrient intake data (1). Moreover, I weekend day was included in the 4 days to obtain valid information on a group level (1). Less than a week before the first recording period, a registered dietitian gave each team standardized instructions on the procedures for completing the food intake records. The athletes were instructed to record the exact times of

3 208 / Tanaka, Tanaka, and Landis Table 1 Subject Characteristics Males (n = 14) Females (n = 10) Variable Al SE M SE Age (years) Height (cm) Weight (kg) exercise and intake of any food or drink. The importance of accurately recording each food or beverage immediately after consumption was emphasized. Runners were asked to record supplements in a separate area on the food records. They were also instructed to follow their customary eating habits during recording days. At the end of the 4-day period for both occasions (training and competitive), the records were returned to a registered dietitian for review. When any information on the diet record was in question, clarification was immediately sought from the athlete via telephone. The dietary records were analyzed for nutrient content using Nutritionist IV (N-Squared Computing, Salem, OR). Carbohydrates consumed immediately postexercise were classified as glucose, fructose, sucrose, or other sugar (e.g., lactose or starch), based on the primary ingredient in each food/beverage. Only food and beverage sources of nutrients were included in the dietary analyses. When a food item was not available in the program, the food was broken down into its components. In cases where a subject recorded with uncertainty a food item eaten in the university's cafeteria, verification of portion sizes and ingredients was obtained via telephone from the food service managers. Moreover, to ensure consistency, all instructions and coding work were conducted by one investigator. During the 4-day recording periods, each athlete reported his or her daily running mileage along with the time of exercise, which was recorded on the same daily dietary intake records. Since the women performed deep water running once a week, their running mileage was assessed based on a mean of the days where land running was the sole mode of exercise. Statistical Analysis Standard descriptive statistics were used to calculate means and standard errors (SE). Test data were analyzed with a two-way ANOVA with repeated measures. When a significant F ratio was obtained, a post hoc test using Newman-Keuls procedure was used to further identify the difference among mean values. The level of significance was set at p <.05 in all comparisons. Results The mean running mileages for men were 16.0 ± 1.0 km/day in the training period and 14.6 ± 0.8 km/day in the competitive period. Most runners performed two or more bouts of running per day. The mean running mileages for women

4 Carbohydrate Intake in Runners / 209 were 10.7 ± 0.6 km/day in the training period and 8.7 ± 0.5 km/day in the competitive period. Daily energy-providing nutrient intakes are shown in Table 2. A significantly (p <.05) lower percentage of total calories was derived from carbohydrates among the men compared to the women in both phases. Unlike the females, male runners fell short of recommendations for carbohydrate intake (60-70%). Although female runners derived 65-67% of calories from carbohydrate, the daily amount of carbohydrate ingested was substantially lower than the recommended value (>10 g/kg/day) (19). When the carbohydrate intake was expressed in a fixed daily amount, male runners were closer to the recommendation than female runners (8 vs. 6 g/kg/day). Both males and females ingested significantly more simple carbohydrate during the training phase than during the competitive phase. Trends for postexercise carbohydrate ingestion are displayed in Table 3. Whereas females ingested carbohydrate approximately half of the time immediately postexercise, the males ingested carbohydrate less than 40% of the time during both phases. While the men had only a slight variation in frequency between training and competitive phases, the women ingested carbohydrate more often during the competitive phase. When the amounts of carbohydrate taken by these athletes were compared to amounts recommended (>1 g carbohydrate/kg body weight) in the literature (13), less than a quarter of the time were the recommendations met during the training phase. Table 2 Mean (± SE) Daily Nutrient Intake of Male and Female Distance Runners Nutrient M SE Males M SE M SE Females M SE Energy (kcal)* 3, , , , (kcal/kg)* Total CHO (g)* (g/kg BW)* (% kcal)* Complex CHO (g)* Simple CHO (g)*# Protein (g)* (% kcal) Fat (g)* (% kcal)* Alcohol (g) Fiber (g)** *Males significantly different (p <.05) from females. # phase significantly different (p <.05) from competitive phase. **Males significantly different (p <.05) from females in training phase.

5 210 / Tanaka, Tanaka, and Landis As can be seen in Table 4, Combo (liquid and solid carbohydrate taken together, other than sources already used), such as cold cereal with milk, represented well over half of the carbohydrate forms consumed by the males immediately postexercise. Solids were the least ingested form of carbohydrate by males during both phases. In contrast, females preferentially consumed a solid form of carbohydrate. Carbohydrates with primary ingredients other glucose, fructose, and sucrose, including sweetened cereals and milk, doughnuts, and waffles, were the types selected most often by the men immediately postexercise. Similarly, other types of carbohydrate, mainly dry cereals and breads, were ingested by the women most often. While the male runners' next favorites were a relatively equal distribution of glucose and sucrose sources, such as bananas and sports drinks, the women's second choices were glucose and fructose, usually via sports bars and sports drinks. The men chose fructose the least often, and the women sucrose. Discussion The endurance athlete must balance daily energy intake with expenditure, as negative caloric balance can compromise successful muscle glycogen resynthesis. The male runners' mean energy intake was kcal/kg/day, which is very similar to the RDA of 58 kcal/kg/day for exceptionally active men. In contrast, kcal/kg/day, the female runners' mean energy intake, corresponds to the RDA for women whose activity is light to moderate. The inadequacy of caloric intake is in agreement with previous studies (7, 19) that have reported a negative caloric balance among female long-distance runners. This has been attributed to increased metabolic efficiency through aerobic exercise. Thompson et al. (23) reported a significantly lower 24-hr energy expenditure in endurance athletes with low energy intake than in athletes with adequate energy intake Table 3 Trends of Postexercise Carbohydrate Ingestion of Male and Female Distance Runners Males Females CHO intake immediately postexercise % of time CHO taken _ % of time recommended amounts of CHO were met' CHO intake at 2-hr intervals % of time CHO taken % of time recommended amounts of CHO were met' The recommended amount of CHO refers to?_ 1 g carbohydrate/kg body weight (14).

6 Carbohydrate Intake in Runners / 211 Table 4 Relative Percentages of the Forms and Types of Carbohydrate Ingested Immediately Postexercise in Distance Runners Males Females CHO forms Solid Liquid Combo CHO types Glucose Fructose Sucrose Other Note. Percentages include times in which the recommended amount of CHO 1 g carbohydrate/kg body weight) (15) was not consumed as well. Combo represents both a solid and a liquid taken together, while Other represents a primary sugar besides those listed, such as lactose or starch. matched for lean body mass, body weight, and activity levels. However, Horton et al. (11), using trained female cyclists and lean controls, determined energy balance between energy expenditure measured with whole room indirect calorimetry and energy intake determined by a controlled feeding regimen. They found no significant difference in the energetic efficiency between the two groups. This is in line with findings of Edwards et al. (8), who compared dietary intake to energy expenditure using doubly labeled water. These results suggested that low caloric intake in female endurance athletes may have been the result of underreporting by the subjects. The prevalence of underreporting in the present study is difficult to determine since underreporting appears to be subconscious in nature. We cannot exclude the possibility that some female runners underreported their dietary intakes since underreporting is quite common in this weightconscious population (8). Along with adequate energy intake, the percentage of calories from carbohydrate should be approximately 60-70% to maintain muscle glycogen day to day (6, 21). The male runners in this study failed to ingest the recommended amount despite their adequate caloric intake. Unfortunately, when very high carbohydrate diets are practiced, the amount of food that should be ingested tends to exceed that required for satiety (5). Given the absolute amount of calories the male runners ingested, it seems likely that a feeling of fullness was responsible for their inadequate carbohydrate intake. Ellsworth et al. (9) reported that in a group of cross-country skiers, cafeteria eating at a training camp led to decreased carbohydrate intake because the food offered did not reflect personal choice. Interestingly, these skiers' overall intakes consisted of significantly greater amounts of carbohydrate when they ate on their

7 212 / Tanaka, Tanaka, and Landis own. The majority of the cross-country runners in the present study ate in the cafeteria, and it is possible that many of the athletes may have consumed more carbohydrates had more sources been available. Many studies have reported inadequate carbohydrate intake among female distance runners (7, 19). Surprisingly, the female runners in the present study ingested more than 65% of calories from carbohydrate in both phases. However, when carbohydrate intake was expressed in a fixed daily amount, their intake of 6.0 and 6.1 g/kg body weight/day was substantially lower than the amount recommended (<10 g/kg/day) in the literature (21). These results indicate that although the female distance runners in the present study chose carbohydrate as a primary energy source, they failed to meet the caloric requirements of this macronutrient, resulting from the lower total energy intake. Recently, Lamb et al. (18) reported no advantage of an 80% carbohydrate diet over 43% for maintaining strenuous swim training. The authors argued that a 43% carbohydrate diet provided a sufficient amount of carbohydrate to fully replenish muscle glycogen since the overall energy intake was high (18). These results suggest that the dietary recommendation for carbohydrate intake should be expressed as a daily amount of carbohydrate relative to body weight rather than as a percentage of energy from carbohydrate (21). Overall, no significant differences were found in carbohydrate intake between training and competitive phases. Both male and female cross-country runners failed to improve their diets in the competitive phase, when performance is highly regarded. The present finding is in line with other studies (15, 22) in which no significant differences were found in the carbohydrate intakes of endurance athletes between phases. Yet these investigators noted that calories from carbohydrate rose from an inadequate quantity to a desirable level, and their athletes were able to increase muscle glycogen resynthesis in a prerace period by increasing bread and fruit consumption. While bread consumption rose in both genders from the training period to the competitive period in the present investigation, the difference did not significantly affect carbohydrate adequacy. Carbohydrate intake immediately after exercise is important since the rate of muscle glycogen resynthesis is enhanced during this time (14). In order for endurance athletes to fully replenish muscle glycogen stores on a daily basis, they not only must consume adequate calories consisting of 60-70% carbohydrate but also must follow a specific regimen in ingesting carbohydrates immediately after exercise. Ingesting carbohydrate in the quantity of approximately 1 g/kg body weight is suggested within the first 30 min following a bout of intense exercise (14). Slightly less than 40% of males and slightly more than 50% of females in the present study ingested carbohydrate immediately postexercise. In addition, the percentage of times carbohydrate was taken immediately in the recommended amounts was only about 20-25%. This trend was observed despite the fact that the athletic training rooms provide sports drinks for runners after practice. An overall grazing pattern was evident in the distance runners, as has been demonstrated in other studies (9, 12, 16). This nontraditional pattern of eating appears to allow athletes to meet their high energy demands without causing unwanted gastrointestinal distress. It seems that this frequent eating pattern would resemble the regimen for maximizing muscle glycogen resynthesis following exercise. Kirsch and von Ameln (16) reported that runners' and cyclists' average

8 Carbohydrate Intake in Runners / 213 frequency of intake was 8-10 times per day; the runners specifically showed a very low eating frequency the hour after training. Since heavy exercise tends to depress appetite immediately following the exercise bout, most endurance athletes would be less inclined to consume carbohydrate during a time critical for complete muscle glycogen repletion. These results suggest that coaches and trainers should encourage athletes to consume carbohydrate after intense exercise, along with providing these carbohydrate sources. While some studies (4, 5, 17) have reported beneficial effects of highcarbohydrate diets, other studies (18, 20, 21) do not support the hypothesis that reductions in dietary carbohydrate stores impair training adaptations or endurance performance. For example, Sherman et al. (20) recently reported that a highcarbohydrate diet had no significant benefit on training capability or exercise performance although it maintained muscle glycogen levels over 7 consecutive days of training. However, since dietary carbohydrate helps to maintain carbohydrate stores in the body and a high-carbohydrate diet does not impair endurance performance, endurance athletes should be advised to follow nutritional practices for maximizing muscle glycogen repletion, especially during intense training (20, 21). This investigation described dietary practices for endurance athletes. A unique aspect of this study was the examination of additional needs and strategies in respect to muscle glycogen repletion. Although it is uncertain whether these athletes were experiencing difficulties in maintaining normal exercise intensity or were experiencing a gradual deterioration in performance, it is evident that they were not following recommendations to maximize muscle glycogen stores. The results suggest that endurance athletes need to be educated on how to follow nutritional guidelines. Second, consistent support and encouragement by registered dietitians, coaches, and trainers are of great importance if athletes are to adhere to dietary practices on a regular basis. As these factors are implemented, following the dietary recommendations for reducing the risk of chronic fatigue, and thus promoting the change for optimal performance, may become a trend among endurance athletes. References 1. Basiotis, P.O.,. S.O. Welsh, F.J. Cronin, J.L. Kelsay, and W. Mertz. Number of days of food intake records required to estimate individual and group nutrient intakes with defined confidence. J. Nutr. 117: , Bergstrom, J., L. Hemiansen, E. Hultman, and B. Saltin. Diet, muscle glycogen, and physical performance. Acta Physiol. Scand. 71: , Blair, S.N., N.M. Ellsworth, W.L. Haskell, M.P. Stern, J.N. Farquhar, and P.D. Wood. Comparison of nutrient intake in middle-aged men and women runners and controls. Med. Sci. Sports Exerc. 13: , Costill, D.L., M.G. Flynn, J.P. Kirwan, J.A. Houmard, J.B. Mitchell, R. Thomas, and S.H. Park. Effects of repeated days of intensified training on muscle glycogen and swimming performance. Med. Sci. Sports Exerc. 20(3): , Costill, D.L., and J.M. Miller. Nutrition for endurance sport: Carbohydrate and fluid balance. Int. J. Sports Med. 1:2-14, Costill, D.L., W.M. Sherman, W.J. Fink, C. Maresh, M. Witten, and J.M. Miller. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am. J. Clin. Nutr. 34: , 1981.

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