There is an apparent discrepancy in the literature regarding the effects of dietary fats on endurance performance in man. Several older studies(2,4,8,21) clearly demonstrated a significantly lower endurance performance after short-term adaptation (3-14 d) to fat-rich diets when compared with carbohydrate-rich diets. However, more recent studies also in man have demonstrated an increased endurance performance after short-term (7-14 d) adaptation to fat-rich diet versus carbohydrate-rich diet (14,18). There is only limited evidence in humans as to how endurance performance is affected by adaptation to a high fat diet of longer duration. Phinney et al.(20) demonstrated that five well trained bicyclists that continued normal training could maintain endurance performance after 4 wk adaptation to an eucaloric, ketogenic diet when compared with a normal, balanced diet. Recently, it was demonstrated that endurance performance was lower after 7 wk adaptation to training and a high fat diet compared with training and a high carbohydrate diet (9). The time course for development of this detrimental effect of a fat diet during training on endurance performance in man is not known.
Therefore, in the present study the aim was to investigate the interaction between training and a fat-rich or a carbohydrate-rich diet on endurance performance in healthy untrained males for 2 and 4 wk.
Subjects. Fifteen healthy and untrained male subjects participated in the study. Subject characteristics are displayed inTable 1. The subjects were fully informed of the nature, stresses, and possible risks associated with the study before they volunteered. The study was approved by the Copenhagen Ethics Committee and complied with the guidelines in the ACSM policy statement regarding the use of human subjects and informed consent.
Protocol. The subjects were randomly assigned into two groups, of which one group consumed a fat-rich diet (T-FAT, N = 8) and one group consumed a carbohydrate-rich diet (T-CHO, N = 7) for 4 wk. In the same period, both groups followed a supervised training program, and maximal oxygen uptake was determined on a Krogh bicycle ergometer before and after 4 wk intervention. Initially, after 2 and 4 wk, an endurance test to exhaustion was performed on a Krogh bicycle ergometer. Before and after the 4 wk, a muscle biopsy was obtained from the vastus lateralis muscle before the endurance exercise test.
Diets. Initially habitual daily energy intake and diet composition were determined in each subject using a 4-d dietary record (three weekdays and one weekend day). All food and fluid intake was carefully weighed and registered. In addition, individual energy intake was calculated using the World Health Organization's equation for calculation of energy needs(23). The latter was done to control for underreporting of food intake due to the effect of keeping dietary records.
The energy composition of the experimental diets was 65 energy percent (E%) carbohydrate, 15 E% protein, 20 E% fat on the carbohydrate-rich diet (CHO), and 21 E% carbohydrate, 17 E% protein, and 62 E% fat on the fat-rich diet(FAT). The composition of the two diets was designed to be markedly different in fat and carbohydrate content and as similar as possible in protein content. Eucaloric 7-d cycle menus were designed at the subject's individual energy levels to keep the day-to-day variation as low as possible. The energy cost of training was calculated individually and added to the subject's daily energy intake on training days. Dietary intake was controlled meticulously throughout the dietary period. All food and beverages were prepared accurate to the gram. Parts of the diets were prepared in the metabolic kitchen and delivered to the subjects. The remaining parts were prepared by the subjects, and all meals were consumed at home. All food intake was registered and so were all omissions from the prescribed food intake. The subjects weighed themselves every morning, and the individual energy level was adjusted according to any change in body weight of more than 1 kg. Diets similar to those used in the present study have been used in our former studies, and chemical analyses of the food composition of the diets have been described earlier(9).
Training. An identical, supervised training program was followed by both dietary groups. Training was performed on bicycle ergometers four times a week. Each training session was initiated with a 10-min warm up (60% of ˙VO2max) and ended with a 10-min active recovery (50%˙VO2max). In the remaining 40-55 min of exercise, subjects alternated between short and long intervals, exercising at 60-85% of˙VO2max interspersed with short breaks (exercise at 50%˙VO2max). Training intensity was adjusted to changes in maximal oxygen uptake measured during the training period. At every training session, heart rate, and thus training intensity, was monitored with a heart rate recorder. In addition, pulmonary oxygen uptake was measured frequently to provide further control of the training intensity.
Endurance test. Subjects were asked to refrain from physical activity 24 h before the endurance tests and to report to the laboratory either by car or bus in the morning after a 10-h fast. After a 30-min rest, a needle biopsy was taken with suction from the vastus lateralis muscle under local anesthesia with lidocaine. Then, an endurance test to exhaustion was performed on a Krogh bicycle ergometer. Subjects commenced working at the selected workload at 80% of maximal oxygen uptake without warm up. During exercise, oxygen uptake was measured after 5, 15, and then every 15 min and at termination of exercise. Heart rate was recorded continuously. During exercise, subjects were allowed 200 mL of water every 15 min. Exhaustion was defined as the point where the pedaling frequency, 65 rpm, could no longer be maintained at the desired frequency, despite verbal encouragement. Verbal encouragement was only given by staff in the laboratory that had no knowledge of the groupings of the subjects. To avoid bias, subjects were blinded from their performance times throughout the experiment. The endurance test to exhaustion was repeated after 2 and 4 wk following the same protocol. The same absolute workload was maintained at all three tests.
Analyses. RER and pulmonary oxygen uptake was determined during exercise by collection of expired air in Douglas bags. The volume of air was measured in a Collins bell-spirometer (W.E. Collins, Braintree, MA), and the fractions of oxygen and carbon dioxide were determined with paramagnetic(Servomex) and infrared (Beckmann LB-2) systems, respectively. Two gas samples with known compositions were used to calibrate both systems regularly. Heart rate was recorded with a PE 3000 Sports Tester (Polar Electro, Finland).
Muscle biopsies were frozen within 5-10 s. Before freezing, a section of the samples was cut off, mounted in embedding medium, and frozen in isopentane cooled to its freezing point in liquid nitrogen. Both parts of the biopsy were stored at -80°C until further analysis. The embedded muscle section was used to analyze fibertype composition. Serial transverse muscle sections were stained for myofibrillar ATPase to identify fiber types(3). Before biochemical analysis, muscle biopsy samples were freeze dried and dissected free of connective tissue, visible fat, and blood using a stereomicroscope. Muscle glycogen concentration was determined as glucose residues after hydrolysis of the muscle sample in 1 M HCl at 100°C for 2 h (16). Intramuscular triacylglycerol was determined as described by Kiens and Richter(13).
Statistics. Differences due to diet and time were tested with a two way variance analysis (model I) using diet and time as fixed factors. Wherever ANOVA revealed significant effects, a Student-Newman-Keul test was used to discern differences between groups. Statistical significance was set at P < 0.05. Analyses were performed by using the SigmaStat Statistical Analysis System version 1.3 (Jandel Corp., GmBh Germany). Data are presented as means ± SE.
During the 4 wk, all subjects were trained four times a week, except for two subjects who both missed one training session because of illness. Before the experiment, antrophometric characteristics, maximal oxygen uptake(˙VO2max), and muscle fiber-type composition were similar in the two groups, except that the mean age, despite randomization, was significantly higher in T-FAT than in T-CHO (Table 1). After 4 wk, body weight, body mass index (BMI), and fiber-type composition were unchanged in both groups (Table 1). After 4 wk, ˙VO2max was similarly and significantly increased by 9.2% in both T-FAT and T-CHO.
Diet intake. The habitual dietary fat and protein intake was similar in the T-CHO and T-FAT groups. In the T-CHO group, the habitual carbohydrate intake was higher and the energy intake lower than in the T-FAT group (P < 0.05) (Table 2). The experimental diet was significantly different from the habitual dietary intake in both groups, with the only exception being the energy intake, which remained unchanged in T-FAT (Table 2). There was a marked change in the daily carbohydrate and fat intake. T-FAT consumed 95% more fat and 52% less carbohydrate compared with the habitual diet. T-CHO consumed 14% less fat and 39% more carbohydrate compared with the habitual diet. However, there was also a small but significant difference in protein intake during the experimental period between the two groups (Table 2), where T-CHO and T-FAT consumed 1.6 and 1.8 g of protein per kg-1 of body weight per d, respectively. Overall, the subjects adhered very well to the dietary diaries during the 4 wk, and the experimental intake was therefore according to the protocol and significantly different between T-CHO and T-FAT(Table 2). Body weights were maintained throughout the 4 wk of dietary adaptation (Table 1), which indicates that the prescribed dietary intake was in fact eucaloric.
Endurance time. At the initial endurance test, time to exhaustion was similar in both groups averaging 29.5 ± 4.3 min and 31.7 ± 4.3 min in T-FAT and T-CHO, respectively (Fig. 1). At the initial test, subjects exercised at 79.5 ± 1.5% and 80.2 ± 2.2% of maximal oxygen uptake in T-FAT and T-CHO, respectively. After 4 wk of dietary adaptation, this load significantly decreased to 72.1 ± 1.7% and 72.6 ± 1.1% of maximal oxygen uptake in T-FAT and T-CHO, respectively. There was no difference in relative exercise intensity between T-CHO and T-FAT at any of the three endurance tests. After 2 wk of exercise, time to exhaustion was increased by 62% in T-FAT (NS) and by 87% in T-CHO(P < 0.05). After 4 wk on the experimental diets, time to exhaustion was significantly increased by 166% to 78.5 ± 8.2 min and by 150% to 79.3 ± 15.1 min in T-FAT and T-CHO, respectively. There was, however, no significant difference in exercise time until exhaustion between T-FAT and T-CHO at any time. During exercise, heart rate was similar at the initial test (178 ± 3 and 173 ± 3) after 2 wk (169 ± 2 and 164 ± 2) and after 4 wk (170 ± 2 and 166 ± 2) in T-FAT and T-CHO, respectively.
RER during exercise. At the initial endurance test, there were no significant differences in RER during exercise between T-CHO and T-FAT(Fig. 2). After 2 and 4 wk of adaptation to diet and training, there was a main effect of diet (P < 0.05). After 4 wk of adaptation to the fat-rich diet, RER values during exercise were significantly lower compared with RER values during exercise after the carbohydrate-rich diet (Fig. 2). After 2 and 4 wk of adaptation to fat-rich diet, RER values were lower than at the initial test, whereas after 2 and 4 wk on the carbohydrate-rich diet, RER values were unchanged (Fig. 2). The RER values did not decrease significantly during exercise at any of the three endurance tests.
Muscle substrates. Before the initial endurance test, muscle glycogen content was similar in T-FAT and T-CHO (Table 3). After 4 wk, muscle glycogen concentrations at rest were increased by 34% in T-CHO compared with the initial endurance test (P < 0.05) and significantly higher than in T-FAT (Table 3). In T-FAT, muscle glycogen content was unchanged after 4 wk of dietary adaptation. Muscle triacylglycerol content was similar in T-FAT and T-CHO before the initial endurance test (Table 3). After 4 wk, adaptation muscle triacylglycerol content was significantly increased in T-FAT and significantly higher than in T-CHO. These data are consistent with our previous studies in which a fat-rich diet was consumed(12). In T-CHO, muscle triacylglycerol content was unchanged after the 4 wk of dietary adaptation.
The main finding of this study was that endurance performance until exhaustion was similarly and significantly increased after both 2 and 4 wk of training and adaptation to either a carbohydrate-rich or a fat-rich diet. This increase in endurance was observed despite a significantly lower muscle glycogen concentration before exercise after the fat-rich diet compared with the carbohydrate-rich diet. Other studies also demonstrated that endurance performance was maintained in man (20) and in rats(6), despite lower muscle glycogen content before the endurance exercise test after prolonged adaptation to a fat-rich diet than a carbohydrate-rich diet. These observations indicate that content of muscle glycogen before an endurance test does not seem to be closely correlated to submaximal performance time when prolonged adaptation to a fat-rich diet has been induced. The lack of a correlation after prolonged dietary adaptation was also demonstrated in the study by Helge et al. (9). However, after acute or a few days dietary manipulations, exercise time until exhaustion seems more closely related to initial muscle glycogen content(2,4). Finally, it should be noted that there was a small but significant difference in protein consumption during the 4 wk. It is unlikely that the extra protein consumed with the fat-rich diet could affect performance.
In the literature, there is a considerable variation in the effect of fat-rich diet on endurance performance in man. However, differences in experimental design, training status of the subjects, the way endurance performance has been measured, and the percentage of calories derived from fat in the fat-rich diet can explain some of these discrepancies. An example of this variation in design can be taken from the study by Muio et al.(18), where the fat-rich diet (used in a nonrandomized design) contained only 38 E% fat, and as such can hardly be characterized as a fat-rich diet.
In rats, most studies have shown an increased endurance performance after adaptation to a fat-rich diet versus a carbohydrate-rich diet(15,17,22). However, in those studies the fat-rich diet contained no carbohydrates and a very high proportion of fat. In a recent study in rats (unpublished observations), however, findings demonstrated that after 4 wk of adaptation to a fat-rich diet containing 15% of calories as carbohydrate, endurance performance was similarly enhanced compared with after a carbohydrate-rich diet. In man there is only one study where a eucaloric carbohydrate-free fat-rich diet was applied and endurance performance investigated. Phinney et al. (20) demonstrated that after 4 wk of adaptation to a basically carbohydrate-free fat-rich diet (eucaloric ketogenic diet), endurance performance was maintained in five trained subjects compared with a normal balanced diet. However, the big variability in performance time of the subjects makes the results difficult to interpret.
Training status, as indicated by maximal oxygen uptake of the subjects, is overall situated within a broad range in studies reporting increased (58-63 mL of O2·min-1·kg-1,(14,18)), similar (53-69 mL of O2·min-1·kg-1, (20 and present study)) or decreased (52-60 mL of O2·min-1·kg-1,(2,8,9,21)) endurance performance after adaptation to a fat-rich compared with a carbohydrate-rich diet. Thus, endurance performance measured after dietary adaptation does not seem to have training status as a major determinant.
However, the duration of the fat dietary period seems to be of importance for endurance performance in man. Studies have shown an increased(14,18), a similar (20 and present study), or a decreased (2,8,21) endurance time to exhaustion when on a fat-rich diet compared with a carbohydrate-rich diet in experimental periods lasting from 3 to 28 d. In the present study, however, four subjects, two from each dietary group, consented to an extra 14 d of training and dieting. After 6 wk, endurance time to exhaustion differed markedly between those on the fat-rich diet and those on the carbohydrate-rich diet, such that subjects on the carbohydrate-rich diet further increased their endurance performance by 53%, whereas there was no further increase for those on the fat-rich diet (unpublished observations). In line with these observations is the study by Helge et al. (9), where 7 wk of training and adaptation to a fat-rich diet revealed a 36% lower endurance performance to exhaustion (P < 0.05) compared with when a carbohydrate-rich diet was consumed (diets, training regimen, and endurance testing were similar to the present study.) Summarizing these findings, it seems that adaptation to a prolonged fat-rich diet up to 4 wk has no consistent impact on endurance performance, whereas beyond 4 wk, a fat-rich diet seems to have a detrimental effect on endurance performance.
One possible mechanism behind the detrimental effect of a fat-rich diet consumed over a longer period could be an increasing sympathetic activity with time. In the present study, there were no differences in heart rate during exercise after 4 wk when a fat-rich or a carbohydrate-rich diet was consumed. However, after 7 wk of adaptation, a significantly higher heart rate and norepinephrine concentration during exercise was observed on a fat-rich diet than on a carbohydrate-rich diet (9). In the same study, heart rate and norepinephrine concentration remained higher when the subjects after 7 wk switched the fat diet to a carbohydrate-rich diet for 1 wk(9). Thus, the higher heart rate and norepinephrine concentration after a longer term adaptation to a fat-rich diet were not reversed after 1 wk on a carbohydrate-rich diet, suggesting a more sustained type of adaptation. Based on these findings, we speculate that the effect of a fat-rich diet over time is associated with an increase in sympathetic activity during exercise. However, how such an increase in sympathetic nervous activity impairs endurance performance is unclear.
Another possible explanation for the effect of fat diet on performance is related to dietary induced changes in phospholipid fatty acid membrane composition. Else and Hulbert (7) proposed the“leaky membrane hypothesis,” which basically suggests that a higher degree of unsaturation of membrane phospholipid fatty acids leads to a more fluid membrane and consequently a larger leakage of ions necessitating the use of more energy to maintain homeostasis. Studies have documented that the dietary fatty acid composition affects the membrane phospholipid fatty acid composition in muscle (1,19). In the present study as in our previous study (9) the fat-rich diet had a significantly higher content of unsaturated fatty acids compared with the carbohydrate-rich diet. It is reasonable to speculate that the fat diet over time induces a progressively increasing degree of unsaturation of the muscle cell membrane, thus requiring a higher energy expenditure to maintain ion homeostasis. It is possible that an increase in energy expenditure occurring over time due to larger ion leakage can partly explain the detrimental effect of long-term adaptation to a fat-rich diet.
Because the absolute volume and intensity of training is increased progressively during training to maintain the training stimulus, an additional explanation for the detrimental interaction between fat diet and training after 7 wk could be that a low carbohydrate intake (fat-rich diet) is sufficient to support low to moderate levels of activity but is not sufficient to fully support higher levels of activity attained after 4 wk of training.
The fraction of fat oxidized of the total oxidation during exercise at the same absolute workload is known to be increased after adaptation to training(5,10,11). In this study RER was unchanged during exercise after 2 and 4 wk of adaptation to training and a carbohydrate-rich diet, whereas the RER during exercise after training and fat-rich diet was significantly decreased. The underlying explanation for this maintained carbohydrate utilization is probably either a counteracting of the normally observed training effect induced by the 30-40% increase in carbohydrate intake compared with habitual intake or merely an effect of a high carbohydrate flux due to a sufficient supply of carbohydrates throughout the training program. In our previous study (9), we also demonstrated a significant decrease in RER during exercise after 7 wk of adaptation to training and fat-rich diet, whereas RER during exercise remained unchanged compared with initial values after a carbohydrate-rich diet. Thus, the present study supports our previous findings that consuming a carbohydrate-rich diet while performing regular training does not induce an increased fat oxidation during exercise. Moreover, this diet-induced difference in substrate utilization during exercise was demonstrated already after 2 wk. This is in line with earlier studies in which substrate utilization was affected already by 3-5 days of dietary intervention(2,4,8).
In conclusion, the present study demonstrated that endurance performance was similarly increased over a 4-wk training period when consuming a fat-rich or a carbohydrate-rich diet. From this and previous work(9), we suggest that the fat dietary period is of critical importance concerning physical performance. Moreover, we suggest that choice of fuel during exercise is strongly influenced by diet independent of training status.
Erik A. Richter is thanked for performing the muscle biopsies. The skilled technical assistance of Irene Bech Nielsen, Elene Skytte, and Vivian Ainsworth-Zink is acknowledged.
The study was supported by grants from the Danish Research Academy (J.nr. 77-7711), Team Danmark, the Danish Sports Research Council (J.nr. 94-1-09), and the Danish National Research Foundation Grant 504-14.
Address for correspondence: Jørn Wulff Helge, Copenhagen Muscle Research Centre, August Krogh Institute, DK-2100 Copenhagen Ø, Denmark. E-mail: email@example.com.
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