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Effects of caffeine ingestion on endurance racing in heat and humidity

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Effects of caffeine ingestion on endurance racing in heat and humidity
  Eur J Appl Physiol (1996) 73:358-363 9 Springer-Verlag 1996 Barry S. Cohen 9 Arnold G. Nelson 9 Michael C. Prevost Gerald D. Thompson 9 Brian D. Marx 9 G. Stephen Morris Effects of caffeine ingestion on endurance racing in heat and humidity Accepted: 17 November 1995 Abstract A hot and humid environment can be detri- mental to race performance. Caffeine, on the other hand, has been shown to be an ergogenic aid for im- proving endurance performance. To examine the influ- ence of caffeine ingestion on race performance during high heat stress, seven endurance trained competitive road racers aged between 23 and 51 years (five men, two women) performed three maximal effort 21-km road races outdoors in hot and humid conditions. The caffeine dose, randomly assigned in a double-blind fashion, consisted of either 0, 5, or 9mg'kg -~ body mass. During each run, the subjects were allowed to drink water ad libitum at each 5-km point. Blood sam- ples were obtained immediately before and after each run and analysed for changes in concentrations of Na § K § glucose, lactate, and hematocrit. Pre and postrun data were also collected for body mass and tympanic membrane temperature. Race times were not signifi- cantly different among the races or caffeine doses, with the average times within 1.1% of each other. In addi- tion, none of the other variables measured varied signif- icantly among the races or caffeine doses. In summary, caffeine intake did not affect race performance. There- fore it was concluded from our study that caffeine is not of ergogenic benefit in endurance races during high heat stress. Key words Heat stress 9 Dehydration 9 Endurance performance B.S. Cohen " A.G. Nelson (l~) ' M.C. Prevost - G.D. Thompson - G.S. Morris Department of Kinesiology, Louisiana State University, 112 Long Field House, Baton Rouge, LA 70803, USA B.D. Marx Department of Experimental Statistics, Louisiana State University, Baton Rouge, LA 70803, USA Introduction It has been found that the performance of prolonged endurance activities can be compromised by conditions of high heat and humidity (Sawka and Wenger 1988; Wyndham 1973). A hot environment reduces convec- tion, conduction, and radiation capacities, thus limiting the body's ability to remove metabolically generated heat. A humid environment intensifies the effects of heat by reducing the capacity for evaporative cooling, the main body mechanism of heat loss. Heat and hu- midity combined with exercise limit performance by increasing the amount of metabolic heat, and by forc- ing the cardiovascular system to work simultaneously to support both metabolic and thermoregulatory de- mands for blood flow. It has been shown that if the heat and humidity are sufficiently intense (wet bulb globe temperature, WBGT > 23~ exercise performance may greatly decrease because of the increased need to devote an escalating amount of the cardiac output to the transportation of internal core heat rather than oxygen and nutrients (Sawka and Wenger 1988; Wyndham 1973). Exercising in hot and humid conditions also induces an increase in sweat production. Increased sweating places additional stress on the cardiovascular system if it leads to dehydration (i.e. the individual does not replenish fluid by drinking). It has been found that dehydration can further negatively affect exercise per- formance by decreasing muscle endurance, maximal aerobic power, and physical work capacity (see review by Sawka and Pandolf 1990). Finally, exercise in high heat and humidity has been shown to cause an in- creased reliance upon anaerobic metabolism and carbohydrate stores (Sawka and Wenger 1988; Young 1990). This type of alteration in metabolism has been shown to lead to a more rapid loss of carbohydrate stores which can also decrease endurance exercise per- formance (Conlee 1987).  359 Notwithstanding the deleterious effects of heat and humidity, athletic events are frequently performed in these adverse conditions. Since the environmental con- ditions cannot be changed and a reduction in metabolic output would probably prevent success, the most sen- sible mechanism to maintain optimal performance would appear to be the prevention of dehydration by constantly replenishing lost fluids. Many distance run- ners, however, are reluctant to drink during a race. Fluid ingestion sometimes leads to gastric distress, and stopping or slowing down to drink can hinder perfor- mance. For instance, at a race pace of 270 m'min -1, stopping for 1 s would give other competitors as much as 4.5 m advantage. Since dehydration impedes performance and runners are reluctant to stop and drink, it has been questioned whether the use of an ergogenic aid during exercise in heat and humidity could offset the negative effect of dehydration. One substance which has been shown to be an erogenic aid to endurance performance is caf- feine. The ergogenic benefit of caffeine has been de- bated for years, but, in a recent review. Dodd et al. (1993) have claimed that the majority of available re- search points to caffeine being beneficial. The success of caffeine in aiding performance is best illustrated by Graham and Spriet (1991) who have seen average in- creases in running endurance performance of 44% fol- lowing ingestion of a caffeine dose of 9 mg'kg- 1 body mass. Caffeine consumption has been shown to in- crease blood catecholamine concentrations and con- currently stimulates free fatty acid (FFA) mobilization from the adipose tissue (Conlee 1991; Dodd et al. 1993; Jacobsen and Kulling 1989). It has been thought that this increased release of FFA results in muscle glycogen sparing. It has been suggested that reduction in the utilization of muscle glycogen may account for an en- hancement of performance, since the sparing of muscle glycogen delays the onset of exhaustion during pro- longed exercise (Conlee 1987). Therefore, it has been questioned whether the use of caffeine will positively affect racing performance in the heat and humidity. There has been much work inves- tigating the effects of different doses of caffeine on endurance performance, defined as time to exhaustion, in normal thermal laboratory conditions (i.e. 22~ (see references cited by Conlee 1991; Dodd et al. 1993; Jacobsen and Kulling 1989). Information concerning the relationship between caffeine, exercise, and heat stress, however, is limited. Falk et al. (1990) have found that caffeine did not influence performance when exer- cising to exhaustion at 70%-75% of maximal oxygen uptake (incline walking with a 22-kg backpack) under laboratory conditions (25~ 50% relative humidity) which would be classified as moderate heat stress (American College of Sports Medicine 1984). The re- sults of this study, however, are difficult to translate to race performance in the heat since the activity did not mimic a race setting. Speed was held constant, and the convective, evaporative, and radiant heat transfer properties unique to an outdoor environment were not taken into account. Gordon et al. (1982) have used an outdoor environ- ment to investigate caffeine and running in the heat. Their results are also difficult to interpret because the subjects did not serve as their own controls and they did not report exercise intensity. Thus, the effects of caffeine ingestion on actual performance of a high intensity endurance race during high heat stress (WBGT > 23~ remain largely unknown. Hence, this study was designed to examine the effects of two doses of caffeine upon the performance of a 21-km simulated race performed under high risk heat stress conditions. Methods Subjects Seven experienced and competitive distance runners/road racers (five men, two women) were recruited from local running clubs for the study. The subjects' mean physical characteristics and race performances are given in Table 1. The subjects' average training program consisted of having run a minimum of 64 kin.week-1 for the previous 6 months with the last 3 months of training being performed in hot and humid weather. Therefore, the subjects were categorized as being well trained and heat acclimatized. All the subjects were free of any health problems that would contra-indicate their involvement in this study. The two women were experiencing no menstrual abnormalities, and they had never noticed a relation- ship between their menstrual cycles and race performance. Daily caffeine consumption, determined by questionnaire, ranged between 0 mg and 300 mg, and none of the subjects used caffeine as an ergogenic aid to their racing. The research was approved by the Institutional Review Board of Louisiana State University, and each subject signed an informed consent document prior to participation in the study. Experimental field trial The subjects ran a series of three 21-km outdoor road races on the same course. The subjects arrived at the test site at 0600 hours, following an overnight fast which had begun at 2100 hours after which time only water had been ingested. They were also instructed to follow their normal diets throughout the experiment and to abstain from caffeine, alcohol, and vigorous exercise during the 24 h preceding each run. The subjects swallowed capsules (with an d libitum volume of water) of anhydrous caffeine or placebo (baking flour), which they had received the previous day, 1 h prior to the run (0530 hours). The dose was either 0, 5, or 9 1 body mass administered random- ly in a double-blind fashion. All of the runs took place during the middle of the summer in Baton Rouge, Louisiana (30.3~ 91.1~ Each run started at approximately 0630 hours. The runs were performed at this time for two reasons. Firstly, this was the approx- imate time of day at which the majority of the subjects trained. Secondly, it was the time of day at which the climatic conditions did not exceed the maximally recommended heat stress (American Col- lege of Sports Medicine 1984). Climatic conditions were measured prior to and after each run. The WGBT ranged between 24~ and 26~ at the start, and from 25~ to 28~ by the end of the 21-kin run. During each race the wind was negligible, and the sky was clear of clouds. Thus, all races were performed under high risk heat stress (American College of Sports  360 Table 1 Physical characteristics of subjects Characteristic Value 1 5 mean SD Age (years) 33.29 9.18 Height (m) 1.73 0.06 Training distance (kin.week-1) 82.13 24.69 Years racing 14.29 7.99 Best marathon time (min) 172.56 22.32 Medicine 1984). Each race was separated by a 2-week period during which the subjects maintained their normal training regime. For each race, the subjects were encouraged to perform their best, and run at a pace as close to race conditions as possible. To mimic a typical race as closely as possible, the subjects were encouraged to perform their usual prerace rituals and dietary habits and during the race to ingest water in their usual pattern. Also, in an attempt to ensure maximal effort in each trial, the subjects were informed that monetary incentives would be provided based upon the lowest cumulative individual and total team finishing times. The subjects were unaware of which team they were in until the final trial ended. Blood samples, body mass, and tympanic membrane temperature (Try) were collected immediately prior to and immediately after each run (Try and body mass 1-2 rain post, blood samples 2 7 min post). In addition, ad ibitum water intake and rating of perceived exertion (RPE, Borg 20 point scale; Borg 1982) were recorded at the 5-km (water intake only), 10-km, 15-kin, and 21-km (RPE only) intervals. The pre-and postrun blood samples were analysed for hematocrit, and concentrations of glucose, lactate, and electrolytes using auto- mated analysers. Changes in plasma volume were calculated from the pre- and posthematocrit values following the procedure of van Beaumont (1972). A postrun (5-7 min post) urine sample was ob- tained, and analysed for urine specific gravity. Statistical procedures The data were analysed using a repeated measures analysis of covariance routine. For each variable, the significance of the change, postrun minus prerun, was compared combining both sexes in one group. Outdoor temperature and humidity were used as covariates to remove any variability across the three caffeine treatments. The 0.05 level of significance was used. Results Upon comparison of the three different drug treat- ments, ingestion of caffeine did not significantly effect the performance time of any of the 21-km runs (P > 0.05; Fig. 1). In addition, the performance time was not significantly different between the two doses of caffeine. The time for the 0 mg'kg-1 dose was on aver- age 0.8% slower than the time for the 5 mg'kg -1, and was on average 0.1% faster than the time for the 9 mg'kg-1 dose. Also, the time for the 5 mg'kg-1 dose averaged 1.1% faster than the time for the 9 dose. Similar to the run times, the RPE at the 10-kin, 15-km, and 21-km marks were also nonsignificant (Fig. 2). Finally, it should be noted that while the caf- feine doses were administered in a double-blind and random fashion, each racer was able to identify 84 r m E 63 E I-- 42 r "1 21 0 0 mg 5 mg 9 mg affeine Dose Fig. 1 Race times for the 21-kin races for each of three caffeine doses. Values are mean and S.D. (n = 7) ILl a, 20 15 10 Caffeine Dose [ I 0 mg [~5mg I9 mg Z 10 km 15 km 21 km Distance into Race Fig. 2 Rate of perceived exertion (RPE) at the 10-km, 15-kin, and 21-kin points for each 21-km race for each caffeine dose. Values are mean and S.D. (n = 7) correctly that they had ingested caffeine when they were on the 9 1 dose. Also, the ingestion of caffeine did not have a signifi- cantly greater effect upon the parameters used to mark the level of heat stress (i.e. body mass loss, hematocrit, plasma volume, urine specific gravity, water intake, Try . AS shown in Table 2, neither prerace nor postrace body masses differed among treatments. Percentage body mass changes (adjusted for water intake) were mean 4.2% (SD 0.8) for 0 mg'kg-1, 3.9% (SD 0.8) for  Table 2 Physiological responses to various doses of caffeine. Try Tympuric temperature Variable Caffeine dose 0 -1 5 -1 9 -1 Pre Post Pre Post Pre Post Mass (kg) Mean 63.3 61.0 63.1 60.8 62.8 60.6 SD 7.7 7.5 7.9 7.5 7.5 7.2 Hematocrit (%) Mean 42.8 45.8 41.4 43.7 42.4 45.2 SD 3.7 6.3 2.7 4,2 4.5 4.9 Zty (~ Mean 35.9 36.8 36.2 36.9 36.0 36.8 SD 0.2 1.2 0,5 0.8 0.2 1.1 N V Q; D~ e- c- D 11 O I/1 Q_ -5 -15 -25 0 mg 5 mg 9 mg Caffeine Dose Fig. 3 Decreases in plasma volume during each 21-km race for each caffeine dose. Values are mean and S.D. (n = 7) 5 mgkg -1, and 3.9% (SD 0.9) for 9 mg'kg -1. Water intake varied greatly amongst individuals ranging from 0 ml during each of the three trials (the top three finishers) to 800 ml (one racer). There were, however, no significant differences in water intake among the three trials for any individual. Three of the subjects (one man two women) were unable to micturate following one of 361 the three races (a different race for each individual), and therefore urine specific gravity was analysed for only four subjects. Urine specific gravity [0mg'kg -1 caf- feine = 1.020 (SD 0.005); 5 mg'kg -1 caffeine = 1.013 (SD 0.007); 9mg'kg -1 caffeine = 1.017 (SD 0.009)1, however, for these four subjects was also not signifi- cantly different among the treatments. Hematocrit in- creased (Table 2) and plasma volume decreased (Fig. 3) in each race, but again there were no differences among the three treatments. Finally, there were also no differ- ences in Try changes (Table 2) among the three treatments. With respect to blood parameters, concentrations of the electrolytes sodium and potassium exhibited sim- ilar results (see Table 3), with again no significant differences appearing. Also, as shown in Table 3, glu- cose concentrations increased slightly with the moder- ate and high caffeine doses compared to the placebo, but these treatment differences were also nonsignifi- cant. Blood lactate concentrations were also nonsignifi- cant among all treatments. The postrace lactate con- centrations were mean 4.3 (SD 1.5)mmol'1-1 for 0 mg'kg-1, 4.6 (SD 2.2)mmol'l-1 for 5 mg'kg-1, and 4.9 (SD 1.6) mmol'l- 1 for 9 mg-kg- 1 Discussion Compared to exercising in thermoneutral conditions, exercising in hot and humid conditions substantially increases the physiological and metabolic demands on the body's systems. The cardiovascular system can be- come overextended due to a need both to provide oxygen to the working muscles and to dissipate heat by increasing peripheral blood flow (Sawka and Wenger 1988; Wyndham 1973). In addition, the body may be- come dehydrated by increasing sweat rates if the water lost through sweating is not replaced. The end result of this increased load on the cardiovascular system can manifest itself as a decrement in work performance. Research has shown that the deleterious effects of dehy- dration can be prevented by drinking fluids during the competition (Coyle and Montain 1992). Notwithstand- ing the obvious benefit, many runners are reluctant to drink during a race out of fear that their performance Table 3 Responses of blood parameters to various doses of caffeine Variable Caffeine dose 0 1 5 mg'kg 1 9 i Pre Post Pre Post Pre Post Na + mmol-1-1 Mean 138.6 142.1 138.5 140.8 137.7 141.6 SD 2.8 4.8 2.4 6.9 2.9 3,5 K + mmol.1-1 Mean 4.57 4.06 4.38 3.90 4.55 3.97 SD 0.39 0.46 0.40 0.26 0.52 0.60 Glucose (mmol-1-1) Mean 4.26 4.45 4.50 4.92 4.39 5.35 SD 0,56 1.08 0.68 0.60 0.58 7.24  362 will be impeded. Since conditions which increase the demands upon the body would be prime situations for the use of ergogenic aids, this study investigated whether caffeine provided any ergogenic benefit during a race under high heat stress conditions. The ergogenic property of caffeine has been at- tributed to its ability to increase the mobilization of FFA and spare or delay muscle glycogenolysis (Dodd et al. 1993), which is thought to enhance the perfor- mance of prolonged exercise. There appears to be a strong case for the efficacy of caffeine as an endur- ance-enhancing aid during exercise to exhaustion be- tween 60% and 85% maximal oxygen uptake when the duration approaches or exceeds 1 h (Dodd et al. 1993; Jacobsen and Kulling 1989). Moreover, Spriet et al. (1992) have shown that the glycogen sparing effect of a 9 mg'kg-1 caffeine dose on high intensity (80% max- imum oxygen consumption) long duration ( > 95 min) exercise is manifested during the first 15 min. These studies, however, were carried out under stringent la- boratory conditions which do not accurately reflect the outdoor thermal environment. The results of the present study showed that there were no significant differences among caffeine treat- ments of 0, 5, or 9 mg'kg- 1 on race performance. Not only were the race times unchanged, but the athletes perception of the difficulty of each performance, and the physiological responses to the race were also not different. Therefore, it can be concluded that caffeine ingestion of 5 or 9 mg'kg-~ body mass by itself did not enhance the performance of the 21-km race performed during conditions of high heat stress. It should be pointed out, however, that the interpretation of these results is limited. As mentioned above, many runners do not practise physiologically prudent habits during a race in a hot and humid environment. Notably, they do not take advantage of available opportunities to maintain fluid homeostasis out of fear of compromising their performance. (Note that the top three finishers in this study did not drink any fluids during each race). Therefore, the results of this study referred to the use of caffeine as the sole modification to the individual's race practices, and assumed that a racer was not making additional modifications such as altering the usual fluid intake pattern. Nevertheless, this study did show that the ergogenic effects of caffeine can be inconsequential when exercise is performed under high risk heat stress. Assuming that the ergogenic benefit from caffeine ingestion relates to glycogen sparing, the lack of an ergogenic effect in this study suggested that glycogen sparing was not the limiting factor to race performance. Alternatively, the lack of a prerace meal may have influenced the results. In this study the races were begun 9.5 h postprandial, this decision being based on the racers' prerace dietary practices. Most of the athletes reported that normally they did not eat prior to an early morning run unless it was a marathon. For shorter races and their usual early morning training runs the athletes would occasionally eat a light "snack" of approximately 300 kcal (1260 k J). Since a few of the racers were hesitant to race with anything in their stomachs, were unwilling to get up before 0500 hours, and were experienced at performing while fasted, it was decided that all of the races would be 9.5 h postprandial. Nevertheless, it is possible that this fasted state could have induced an increase in FFA which masked the effects of the caffeine. We, however, do not believe that this was the case, because previous research has shown that a 5 mg'kg- 1 body mass dose of caffeine taken 12 h postprandial significantly increased serum FFA con- centrations (Essig et al. 1980). Finally, since the work of Spriet et al. (1992) has suggested that caffeine spares glycogen during the initial phases of exercise, it is possible that glycogen sparing did occur initially. This benefit of the spared glycogen was lost, however, be- cause as the race continued the increasing metabolic demands of the heat stress were sufficiently large to negate any benefit. Unfortunately without any muscle biopsy data, actual glycogen usage is not known, and thus the lack of ergogenic benefit cannot be linked to a failure of caffeine to induce glycogen sparing. Alternatively, as mentioned above, the mean per- centage body mass loss was approximately 4% for each trial. Assuming that this loss in mass can be attributed solely to dehydration, it is possible that the greater physiological demands and alterations placed on the cardiovascular system from dehydration were able to over-ride the ergogenic effects of caffeine, and thus were more of a limiting factor in the races. Craig and Cum- mings (1966) have reported a mass loss of 1.9% yielded a 22% decrease in endurance performance (inclined walking to exhaustion), and a 4.3% mass loss caused a 48% performance reduction. Similarly, Armstrong et al. (1985) have shown that a diuretic induced prerace dehydration of 2.1% significantly reduced running per- formance in a 10-km race. In addition, Coyle and Montain (1992) have suggested that even a 1% loss in body mass can adversely affect cycling performance. The athletes in the present study all experienced dehydration (based on mass loss and plasma volume changes) similar to that seen in the above studies. Hence, dehydration was of sufficient magnitude to in- fluence their performance, and this effect may have masked any possible benefit from glycogen sparing. This supposition that the ergogenic benefit of caffeine was blocked by dehydration, however, is also limited. We have no information concerning the performance capability or hydration level of the subjects following a race on the 21-km course under nonheat stress condi- tions. To obtain such information would have required an approximate 6-month interval between the races in the two conditions. This time interval would have been of sufficient length that any differences seen in race performance could have been attributed to physiolo- gical as well as environmental alterations. It should be
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