Journal of Strength and Conditioning Research, 2002, 16(1), 50–57 q 2002 National Strength & Conditioning Association
The Acute 1-Week Effects of the Zone Diet on Body Composition, Blood Lipid Levels, and Performance in Recreational Endurance Athletes MARK JARVIS,1 LARS MCNAUGHTON,2 ALAN SEDDON,1 DYLAN THOMPSON2
Department of Life Sciences, Kingston University, Kingston KT1 2EE, England; 2Department of Sport and Exercise Science, University of Bath, Bath BA2 7AY, England. 1
ABSTRACT The aim of this study was to examine the effects of a 7-day Zone diet compared with a normal diet on maximal oxygen uptake (V˙O2max), running time to exhaustion during endurance performance, and body composition. Eight men, with the following physical characteristics (mean 6 SE), participated in this study: age, 26.1 6 1.9 years; height, 178 6 1.7 cm; mass, 70.7 6 2.1 kg; and V˙O2max, 54.6 6 3.1 ml·kg21·min21. All subjects undertook pretesting for V˙O2max, time to exhaustion (80% V˙O2max), and body composition (Biostat 1500) before following either the normal diet or the Zone diet for 7 days. These performance trials were performed before and after the dietary period. There was a significant (p , 0.05) decrease in total energy consumption from a mean of 2,314 6 334 kcal on a pretest diet to 1,994 6 438 kcal on the Zone diet. Subjects showed a significant reduction (p , 0.02) in body mass from 70.7 6 2.1 kg to 69.8 6 2.1 kg. In the 80% V˙O2max time to exhaustion trial, there was a significant reduction (p , 0.05) in time to exhaustion from 37.68 6 8.6 minutes for the pretest diet to 34.11 6 7.01 minutes for the Zone diet. In conclusion, the claim of the authors of the Zone diet that performance time and V˙O2max can be improved was not shown in this 1-week research trial. We would suggest that this is not a nutritional strategy that athletes should use until further work has been conducted.
Key Words: nutrition, protein, carbohydrate, energy, body fat, lean body mass Reference Data: Jarvis, M., L. McNaughton, A. Seddon, and D. Thompson. The acute 1-week effects of the Zone diet on body composition, blood lipid levels, and performance in recreational endurance athletes. J. Strength Cond. Res. 16(1):50–57. 2002.
arbohydrate is generally considered to be the most important fuel for athletic performance. A highcarbohydrate diet can produce increased pre-exercise muscle glycogen levels (4–6), which increase time to 50
exhaustion (1, 4, 10–12, 17) and long endurance work for several days (9). Most athletes should consume a diet containing 60–70% (or 7–12 g·kg21 body mass·d21) of total energy from carbohydrate to fuel training and maintain muscle and liver glycogen levels (10, 11). These carbohydrates should be predominantly complex carbohydrates because of their slower release into the blood stream and high fiber and vitamin content (4–7, 27–30, 41, 42). Recent research has also suggested that the absolute amount of carbohydrate is important, and athletes should consume in excess of 500 g·d21 (40). A new diet has recently emerged. Known as the Zone diet (33, 34), it offers radically different advice from the conventional wisdom. The claims made by the authors of the diet are dramatic and include losing excess body fat, reducing hunger between meals, reducing the risk of chronic disease, and improving athletic performance (33, 34). The contribution of each macronutrient to total energy consumption in the Zone diet is always the same. Initially, the amount of protein consumed per day is calculated based on activity levels and lean body mass (LBM), and the consumption of carbohydrate, protein, and fat is approximately 40:30:30, which is contrary to the recommended ratio of 55:15:30 (3). A recent comprehensive review of the diet (8) can be read for further information. Although the protein consumption prescribed by the Zone diet in a typical 70-kg man is higher than with a normal diet, the most significant difference between the diets is the low-energy value of the Zone diet as a result of the reduced carbohydrate and fat intakes. The claim that the Zone diet provides enough energy to maintain LBM in those at an ideal percentage of body fat would seem contradictory, since the energy deficit would ultimately result in loss of both LBM and total body mass. Although Sears (33, 34) suggests that elite athletes may increase their energy
Diet and Exercise Performance 51
intake through increased fat consumption (40–45%), energy intake may still fail to match energy expenditure in this population. Previous work (1, 4–7, 10–12, 17) suggests that, contrary to the claims of improved performance with the Zone diet, the Zone diet may reduce time to exhaustion and impair performance. The rationale for not consuming carbohydrates alone in the Zone diet is to avoid large insulin secretions and subsequent decreases in plasma glucose levels. However, it is known that catecholamine release during exercise suppresses insulin secretion and allows the consumption of simple sugars without the threat of lowering plasma glucose levels. This strategy has been shown to be effective in improving time to exhaustion by 15–20 minutes (13). The Zone diet stipulates that as much as possible carbohydrates should have a low glycemic index. Also, it claims that not more than 25% of total carbohydrates should come from high–glycemic index foods. Optimal muscle glycogen synthesis rates can be achieved by consuming 1 g·kg21 of carbohydrates immediately after exercise and at 2-hour intervals thereafter (35). By consuming low–glycemic index carbohydrates combined with protein and fat, the rate of gastric emptying will decrease, which will thus slow down the replenishment of muscle and liver glycogen stores. Despite all of the existing studies that support the endurance benefits of a high-carbohydrate diet (11–13, 15, 16, 22, 26), it is claimed that the Zone diet will increase time to exhaustion (33, 34). This is based on the theory that the Zone diet’s aim of reduced insulin levels will promote free fatty acid (FFA) mobilization and therefore delay glycogen depletion (19, 21). The diet also claims to enhance athletic performance by inducing vasodilation through its hormonal balance and thereby improving oxygen transfer, which may benefit athletic performance by increasing V˙O2max (2, 31, 33). Sears and Lawren (34) and Sears (33) claim that all of the benefits of the Zone diet will be seen within 7–10 days of commencing the diet. In light of the claims made on behalf of this new dietary regimen, the major aim of this study was to examine the effects of the Zone diet on athletic performance. Specifically, this study compared the effects ˙ O2max, of a 7-day application of the Zone diet on V body composition, and time to exhaustion during endurance performance compared with 7 days of a normal diet.
Methods Approach to the Problem and Experimental Design We investigated the Zone diet as a continuation from the normal diet that athletes would typically use rather than a double-blind, crossover design, because this would occur in a real-life setting. Measurements were
made in a number of areas pertinent to the claims of the Zone diet’s authors (33, 34). Subjects Eight male subjects who were recreational endurance athletes participated in this study on a voluntary basis. Their physical characteristics (mean 6 SE) were as follows: age, 26.1 6 1.9 years; height, 178 6 1.7 cm; body mass, 70.7 6 2.1 kg; and V˙O2max, 54.6 6 3.1 ml·kg21·min21. All subjects trained for more than 4 d·wk21 for more than 60 min·d21 and had been doing so for a minimum of 6 years. Before participation, the subjects were informed verbally and in writing as to the nature and possible risks and all signed informed consent forms. The study was approved by the departmental ethics committee. Subjects were given no information as to the nature of the diet to be consumed with respect to its purported benefits and neither were they told this was the Zone diet. Subjects were asked to continue with their normal training routines and were questioned regarding their routines before and after the testing period. We should add that at this time the Zone diet was not receiving much attention from the press in the United Kingdom. We also think it is unlikely that any of these recreational athletes would have been familiar with this particular diet, since, on questioning, none of the subjects had previously used a specific performance diet (except carbohydrate loading), such as the Zone diet, to improve their athletic performance. Pre-experimental Protocol Before the initial laboratory testing, subjects were asked to complete a 7-day food diary, listing every item of food and drink consumed as accurately as possible. This was then analyzed on a computer using Diet 1 computer software (14) to assess the macronutrient contribution in comparison to the Zone diet. This process was also completed after testing to determine whether differences in the normal diet existed. Subjects were also asked to complete a personal diary, giving details of current training status and any dietary restrictions, such as food allergies or vegetarian. Experimental Protocol All tests were carried out in the physiology laboratory of Kingston University. The study required each subject to report to the laboratory on 2 separate occasions 7 days apart. On each occasion, the same set of tests as described herein were performed at the same time of day. Subjects were asked not to train up to 24 hours before testing or to eat up to 4 hours before testing, but they were asked to hydrate sufficiently before testing with plain tap water. Body Composition With footwear and excessive clothing removed, subjects initially had their height and mass recorded.
52 Jarvis, McNaughton, Seddon, and Thompson
Body fat percentage was then measured with bioelectrical impedance analysis, using a Bodystat 1500 analyzer. In our laboratory, this analyzer has a technical error of measurement of 3.3% (31). Maximum Oxygen Uptake ˙ O2max was measured using an incremental The V treadmill protocol. The subjects, all of whom had prior experience with treadmill running, began with a 5minute warm-up at low speeds, which were recorded for repetition. This allowed the subjects a physiological warm-up and a familiarization of the treadmill. The warm-up was followed by light stretching after which the subjects were fitted with a Polar Vantage (Polar Electro Oy, Kempele, Finland) heart rate monitor and an expired gas collection mask. Expired gases were analyzed breath by breath using a Jaeger Oxycon Alpha (Eric Jaeger, Hoechberg, Germany) system. The gas analyzers and pneumotachograph were calibrated before and after testing according to manufacturer’s specifications. The analyzers were calibrated with a gas on known concentration (BOC Alpha Standard Gases, Sheffield, England; oxygen, 16%; carbon dioxide, 4%; balance, nitrogen), and a Hans Rudolph 3-L syringe (Hans Rudolph, Kansas City, KS) was used to calibrate the pneumotachograph. The treadmill test began with an initial 0% grade at a velocity of 12 k·h21. After 2 minutes and at every 1 minute thereafter, the velocity of the treadmill was increased by 1 k·h21. If the subjects were still running at 17 k·h21, then the speed was kept constant and the incline was increased by 2% each minute. The test criteria for attainment of ˙ O2max was either voluntary exhaustion, heart rate V within 5 b·min21 of age-related maximal heart rate (220 2 age), or an increase in work rate with less than a 200 ml·min21 increase in V˙O2. It should be noted that the time display and heart rate data were hidden from the subjects during the trial to avoid any motivational bias. Time to Exhaustion ˙ O2max test, subjects To provide recovery from the V were given 1 hour of rest during which they were permitted to drink water but no food or carbohydrate drinks. Before commencing the time to exhaustion trial, subjects performed the same warm-up and stretch routine as used in the V˙O2max test. Once again, the runners were connected to the online gas analysis system and fitted with the heart rate monitor. The treadmill was set at a velocity that was estimated to elicit an intensity of approximately 80% V˙O2max based on ˙ O2max trial. After an initial 5 minutes data from the V of running, the treadmill speed and incline remained ˙ O2 drifted outside the constant unless the subject’s V acceptable range of 62 ml·kg21 from 80% V˙O2max. In this case, the treadmill velocity was adjusted accordingly and the changes recorded. The test continued until the subject reached voluntary exhaustion.
The expired gas analysis data were recorded constantly throughout the test, and heart rates were noted at the end of every minute to the point of exhaustion. The time, distance, and heart rate data displays on the treadmill were masked from the subjects. Seven-Day Zone Diet Between each trial, the subjects were required to follow the Zone diet for 7 days. Based on activity levels and LBM predicted by bioelectrical impedance analysis, each subject’s nutritional requirements on the Zone diet were calculated using Zone Manager, a software package designed to assist readers in planning the Zone diet. The subjects were each given a Zone menu, which consisted of 24 different meals and snacks that are appropriate to the principles of the Zone diet (33). The specific quantities of each ingredient were calculated for each individual to ensure that the caloric value and contribution of each macronutrient to total energy consumption were appropriate. Information on some of the more crucial points of the diet was also given to the subjects. It was made clear to the subjects that carbohydrate should not be consumed alone and that no more than 5 hours should pass without eating (except during sleep). Following completion of the 7day period, subjects returned to the laboratory to perform the same tests in an identical fashion. Blood Analysis Before each test session and after the run to exhaustion test, subjects had 5 ml of blood removed aseptically from the antecubital vein. Blood was centrifuged and then stored (2808 C) for future analysis. Blood was analyzed for FFAs (Wako), total protein, and triglycerides (Johnson and Johnson Ektachem DTII System, New York, NY). Statistical Analyses Statistical analysis of the data was carried out using a 1-tailed Student’s t-test with statistical significance accepted on a 1-way test at the p # 0.05 level. A 2-way analysis of variance (ANOVA) with repeated measures was used to analyze the blood data, macronutrient energy contributions to both diets, and the heart rate values during time to exhaustion trial. Fisher’s post hoc tests were used to determine significant differences between the means when the ANOVA was significant. The reliability of the normal (pre-Zone and post-Zone) diet was reported by Pearson correlation coefficient, and Statview (38) was used to compute all analyses. In all cases, values are expressed as the mean 6 SE.
Results ˙ O2max test, all subjects met at least 2 of the 3 In the V ˙ O2max. Of criteria previously listed for attainment of V the 8 subjects, all voluntarily stopped the test. Three subjects achieved heart rates within 5 beats of their
Diet and Exercise Performance 53
Table 3. Mean (6SE) micronutrient and major mineral intake for the 2 diets during the 7 days of the diet.
Table 1. Analysis of pre-experimental nutrition.
Subject no. 1 2 3 4 5 6 7 8 Mean (SD)
57 53 49 44 55 54 65 52 54 (6.09)
16 28 20 30 21 27 12 24 22 (6.20)
27 19 31 26 24 19 23 24 24 (4.02)
Vitamin A (mg) Vitamin B1 (mg) Vitamin C (mg) Vitamin D (mg) Vitamin E (mg) Calcium (mg) Sodium (mg) Iron (mg)
1.8 (0.6) 2.3 (1.0) 140.0 (18.6) 0.04 (0.01) 12.6 (2.4) 1624.0 (186) 2768.0 (53) 9.7 (0.9)
1.4 (0.8) 1.7 (1.3) 127.0 (23.9) 0.03 (0.01) 11.8 (2.1) 1785.0 (131) 2831.0 (68) 10.2 (1.3)
from a mean of 2,314 6 334 kcal on a pretest diet to 1,994 6 438 kcal on the Zone diet. The mean daily energy values provided by each macronutrient were compared between the 2 diets using ANOVA, which showed a significant difference (p , 0.0001). The post hoc analysis indicated that there was a significant difference (p , 0.0001) between carbohydrate energy values with a mean of 1,243 6 226.2 kcal on a pretest diet compared with 722 6 157.7 kcal on the Zone diet. The Zone diet represented a significant (p , 0.04) increase in protein from 552 6 115.6 kcal on the pretest diet to 731 6 161.2 kcal on the Zone diet. Finally, no significant difference (p . 0.75) was found between fat values, with the pretest diets providing 519 6 160.3 kcal compared with 541 6 118.9 kcal on the Zone diet. The dietary analysis showed that the recommended dietary allowance of vitamins and minerals was met on both the normal and Zone diets. Although all of the subjects were able to follow the Zone diet as prescribed, 5 complained that they often experienced feelings of hunger at the end of a meal, whereas 1 found the quantities of food difficult to consume. In addition to this, 6 subjects found the combinations of foods in the meals ‘‘unnatural.’’
age-predicted maximum; of the other 5, 2 were within 7 beats, whereas 3 exceeded their age-predicted maximal heart rates by more than 5 beats. Five subjects had an increase in workload with less than a 200 ml·min21 increase in V˙O2. Dietary Analysis The results of the 7-day food diary dietary analysis from a written record show that all of the subjects had a pretest diet predominantly composed of carbohydrate; these figures are given in Table 1. There were no significant differences in mean daily energy intake (1,908 6 82 before the Zone diet vs. 1,866 6 93 after the Zone diet; p . 0.19), and we therefore assume that their reporting habits were reliable (r 5 0.95, p , 0.001). The quantities of each macronutrient consumed each day along with energy values of the Zone diet are compared with the subject’s pretest diets in Table 2. An analysis of the micronutrient content of the 2 diets indicated that these all fell within or exceeded the recommended dietary allowance (28). These can be seen in Table 3. These results show that the carbohydratefat-protein ratio changed from approximately 54:22:24 to 36:27:37 on the Zone diet. There was also a significant (p , 0.04) decrease in total energy consumption
Table 2. Macronutrient and energy value of pretest diet vs. the Zone diet. Carbohydrate (g) Subject no. 1 2 3 4 5 6 7 8 Mean (SD) weight
Total energy (kcal)
234 315 228 206 361 297 351 320
162 142 147 113 203 220 179 210
30 76 51 62 61 67 30 65
54 47 49 38 68 74 60 70
81 84 136 89 114 75 92 110
122 106 110 85 153 166 135 158
1722 2512 2436 1951 2734 2307 2276 2577
1877 1637 1700 1312 2356 2558 2079 2433
296 (53.85) 172 (37.54)
98 (20.46) 129 (28.53)
2314 (334.35) 1994 (437.71)
54 Jarvis, McNaughton, Seddon, and Thompson
Figure 1. Respiratory quotient values throughout the trial to exhaustion.
Performance Trials ˙ O2max trials, there was a nonsigIn the incremental V nificant (p . 0.401) reduction in V˙O2max from 54.6 6 3.1 ml·kg·min21 on the pretest diet to 53.9 6 3.1 ml·kg·min21 on the Zone diet. End heart rate values showed no significant difference (p . 0.7), with a mean of 181 6 9 b·min21 on the pretest diet and 181 6 7 b·min21 on the Zone diet trials. There was no performance difference, since all subjects ended the test at the same stage in tests 1 and 2. ˙ O2max time to exhaustion trial, there In the 80% V was a significant reduction (p , 0.05) in time to exhaustion from 37.68 6 8.6 minutes on the pretest diet to 34.11 6 7.01 minutes on the Zone diet. During the time to exhaustion trial, respiratory exchange ratio (RER) was recorded every 30 seconds (Figure 1). These values were analyzed for up to 31 minutes of running (after this point fewer than 6 subjects continued). The ANOVA showed a nonsignificant increase (p , 0.06) in RER values from 0.933 6 0.029 on a normal diet compared with 0.946 6 0.026 on the Zone diet. There was no significant difference (p . 0.5) in average heart rate values from 165 6 3 b·min21 on the normal diet compared with 164 6 5 b·min21 on the Zone diet. The subjects ran at the same velocity during both trials, with a mean speed of 13.1 6 1.7 kph. This was ˙ O2 values of 42.99 6 6.69 ml·kg21·min21 found to elicit V in test 1 and 42.93 6 7.37 ml·kg21·min21 in test 2. There was no significant difference between these 2 sets of values (p . 0.85). These V˙O2 values represent 78.8 6 ˙ O2max in test 1 and ˙ O2max and 79.5 6 4.59% V 3.8% V test 2, respectively. Blood Analysis The 2 3 2 (normal/Zone diet vs. pretest/posttest) factorial ANOVA undertaken on the triglyceride data indicated a main effect of diet (p , 0.0001), no main effect of test (p . 0.18), and no diet-by-test interaction effect (p . 0.28). In the normal diet, the mean value was 1.6 6 0.13 mmol·L21, whereas in the Zone diet the level decreased significantly (p , 0.0001) to 0.5 6 0.04 mmol·L21. All values for triglycerides in all conditions were within the normal range for this test. When the
analysis for total protein was undertaken, the same results were found: a main effect of diet (p , 0.0001), no main effect of test (p . 0.27), and no interaction effect (p . 0.34). The 2-way ANOVA for FFAs indicated significant differences in the main effects of both diet (p , 0.0001) and test (p , 0.0001). There was, however, an interaction effect that was also significant (p , 0.0001). In the diet condition, the concentration rose from 0.4 6 0.1 mmol·L21 to 1.6 6 0.2 mmol·L21, whereas in the Zone diet it rose from 0.07 6 0.01 to 0.34 6 0.02 mmol·L21. Body Composition Several body composition changes were seen in the subjects between the normal and Zone diets. Subjects showed a significant reduction (p , 0.02) in body weight from 70.7 6 2.1 kg to 69.8 6 2.1 kg. This weight loss was partly a result of a general but nonsignificant trend (p , 0.1) showing reduction in body fat percentage from 15.3 6 3.1% to 14.7 6 3.3%. In addition to the loss of body fat, LBM showed a small but again nonsignificant (p , 0.06) decrease from 66.4 6 1.1 kg to 64.6 6 1.3 kg. The bioelectrical impedance analysis test used to measure these body composition changes showed that hydration levels rose from 57.8 6 0.05% to 58.1 6 0.05%; this difference was not significant (p . 0.3).
Discussion The aim of this study was to examine the short-term effects of the Zone diet on aspects of athletic perfor˙ O2max, and mance, including body composition, V ˙ O2max. The computer analtime to exhaustion at 80% V ysis of the subject’s 7-day food diary showed that, although they were not identical, the mean carbohydrate-fat-protein energy ratios consumed by the subjects (54:22:24) were quite similar to the previously recommended ratio (55:30:15) (3). The 54% energy contribution from carbohydrate consumption means that differences observed in testing between the Zone diet and the subjects’ normal diet can realistically be related to standard nutritional guidelines. Analysis of the energy contribution from each macronutrient on the Zone diet as prescribed by Zone Manager computer software (44) showed that the carbohydrate-fat-protein ratios were 36:27:37. This is interesting, since the composition of the diet differs slightly from the described ratio of 40:30:30. It has been reported (20) that a computer analysis of meals featured in Sears (33) showed lower carbohydrate (around 30–35%) and higher protein intakes than the intended ratio. One of the fundamental rules of the Zone diet is that carbohydrate, fat, and protein are always consumed in macronutrient ‘‘blocks’’ of the same ratio. Sears and Lawren (34) define these blocks by weight as protein 5 7 g, carbohydrate 5 9 g, and fat 5 1.5 g. Assuming energy values of carbohydrate 5
Diet and Exercise Performance 55
4.2 kcal, protein 5 5.65 kcal, and fat 5 9.2 kcal, as described by McArdle et al. (28), it can be demonstrated that this will not elicit the intended energy ratio of 40:30:30. The dietary analysis also showed that the Zone diet represented a significant decrease in energy intake of approximately 14% when compared with the pretest diet. Although this short-term trial only lasted for 7 days, it is likely that in the long term it will not only be a factor in fatigue but may also have serious implications on body composition. For longer periods, the effects of these low-energy values are likely to compound themselves, particularly during periods of high-intensity training, which may become very difficult to maintain. This decrease in weight also has ˙ O2max, since in relative terms implications for V ˙ O2max increases with decreases in body weight as V long as oxygen intake remains the same. In our trial, not only did the subjects lose weight, as one would expect, but the subjects also tended to show a decrease in V˙O2max. If this trend is shown to continue in the long term, it would have a significant detrimental effect on performance, since it is likely that maximal power output would also decrease. Studies with subjects who use the Zone diet for months are clearly necessary to determine the diet’s long-term effects. Although the Zone represents a large decrease in carbohydrate intake, in the meals tested, the focus on high-fiber, low–glycemic index sources ensured that sufficient micronutrients were still consumed. An individual intending to use the Zone diet should be made aware of the importance of such nutrient-rich sources, as has been suggested (43), so that any possible potential deficiencies in vitamin and minerals do not pose a threat to both health and performance. Provided that carbohydrate intake is based on predominantly nutrient-rich sources, the Zone diet should meet the athlete’s vitamin and mineral needs and supplementation is not required. It is claimed that the Zone diet may reduce hunger between meals through 2 mechanisms. First, carbohydrate sources should be low energy and high fiber, so that a larger volume of food is consumed for a given amount of carbohydrate intake. Second, by combining carbohydrate, protein, and fat in each meal, gastric emptying is slower than with a pure carbohydrate meal, providing a slow and steady release of energy into the blood stream. The concept of slower gastric emptying with the Zone diet has been previously supported (28); however, in our experiment this was found to be ineffective in reducing hunger, since 5 subjects complained of hunger, whereas only 1 found the amounts of food difficult to consume. Although slow gastric emptying may be advantageous for an individual on a weight reduction program, it is often a priority of the athlete to achieve maximal glycogen synthesis rates.
The results of the incremental treadmill trial showed that the Zone diet did not provide any signif˙ O2max when compared with icant improvement in V the subjects’ normal diet, and therefore no evidence was found for the claim that the Zone diet will im˙ O2max (33, 34). The mechanism for this claim prove V is an endocrine response that can be seen during exercise, which will result in vasodilation and in turn improve oxygen transfer to working muscles. Although there is disagreement among exercise physi˙ O2max, this theory ologists as to the ultimate limits to V ignores several important factors. It has been suggested that the limit is set by the capacity of the central cardiovascular system to transport oxygen to working muscles (2, 32, 39). An early study that highlights the weaknesses of the Zone theory was undertaken several years ago (37). After a period of leg-only exercise, arm exercise was combined to increase the vascular bed. This resulted in peripheral vascular constriction and therefore a reduced leg blood flow to maintain blood pressure. This demonstrates that the degree of vasodilation exhibited during exercise will ultimately be determined by the sympathetic nervous system and not nutritional factors. ˙ O2max, During a time to exhaustion trial at 80% V subjects showed a reduced time to exhaustion on the Zone diet, which is contrary to the claims of an increased time to exhaustion (33, 34). The proposed mechanism for this improvement is a reduction in exercising insulin levels, which will promote FFA mobilization and delay the onset of glycogen depletion. However, several researchers have shown that insulin secretion is suppressed during exercise at intensities above 45% V˙O2max through the release of catecholamines (9, 12, 16, 22). According to the Zone theory, lipid metabolism will play a greater role in energy contribution at a given exercise intensity when compared with a high-carbohydrate diet, and this will therefore result in reduced RERs. This was not the case in our work, since there was no significant difference in RER between the 2 diets and values remained very similar for both diets until the point of exhaustion drew near. At 80% ˙ O2max, a nonprotein RER of approximately 0.93–0.95 V represents approximately 77–84% carbohydrate and approximately 16–23% FFA as energy substrate. With the post-Zone FFA values, there is less energy available from FFA, and it is therefore not surprising that RER values changed toward the end of the test, because there is a greater reliance on carbohydrate as energy at this time. It has been shown that the use of fats in intermittent sports, such as soccer and ice hockey, is minimal, so a greater availability of FFAs is likely to be largely irrelevant (18, 37). It has also been demonstrated (27) that a high-protein, low-carbohydrate diet can result in metabolic acidosis, which may impair high-intensity
56 Jarvis, McNaughton, Seddon, and Thompson
exercise performance (23–25). Maughan and colleagues (27) found that feeding athletes a high-protein, low-carbohydrate diet resulted in a dramatic reduction in endurance capacity during cycling at maximal intensity for approximately 5 minutes. The results of the time to exhaustion trials have demonstrated that an athlete on the Zone diet is likely to reach voluntary exhaustion earlier than when consuming a high-carbohydrate diet. When exercising at ˙ O2max, carbohydrate feedings intensities of 60–80% V while exercising can postpone fatigue by 15–20 minutes and also produce an improved sprint capacity (13). At these intensities, there is no risk of a carbohydrate-induced rise in insulin levels because insulin secretion is suppressed by the release of catecholamines (10, 12). There is evidence to suggest that the debilitating effects of the Zone diet on endurance performance may be increased when undergoing repeated days of training. An early study (9) showed that athletes consuming a moderate-carbohydrate diet (45% daily energy expenditure) demonstrated progressive glycogen depletion during several days of training. It has been suggested (36) that muscle glycogen stores can be fully replenished within 24 hours by consuming 8–10 g·kg21 body weight of carbohydrate. If athletes use the Zone diet, it is unlikely that this would occur. It is suggested that although the subjects did show decreased body fat with the short-term use of the Zone diet, this was simply a result of the low-caloric value of the diet, which also caused a loss of LBM due to glyconeogenesis despite the high protein intake. It is suggested by Sears and Lawren (34) that in this situation the individual’s energy intake be increased by a greater consumption of monounsaturated fat. This is another major ambiguity, because eventually athletes will simply be consuming a high-fat, low-carbohydrate diet that can impair performance in both high-intensity and endurance exercise (19). To conclude, it is clear that during this short-term study using the Zone diet, no improvements in the elements of athletic performance under investigation were seen. No evidence was found to support the ˙ O2max or inclaims that the Zone diet will improve V crease the metabolism of FFAs during endurance exercise (32, 34). The diet’s claim of producing optimal body fat levels while maintaining LBM were found to be ineffective, because a significant loss of LBM was seen in the subjects. The Zone diet’s ability to affect body composition is simply a result of its low-energy values placing the body in a state of negative energy balance even in this short-term trial. A long-term trial of the Zone diet is warranted to further investigate these changes (33, 34). The approach we took to this particular investigation was to keep the problem as ‘‘real world’’ as possible, hence the use of a nonrandomized trial. Al-
though there are criticisms of the nonrandomized, noncrossover approach, we believe this to be the situation that best typifies the approach of athletes to diets in general. Typically, athletes would read about a new dietary regimen and would then put this in place after they have been on their previous normal diet.
Practical Applications We found none of the improvements claimed by the authors (33, 34) and would suggest that athletes carefully consider their dietary regimen with a qualified dietitian or nutritionist as part of their overall training strategy. There was a significant decrease in weight ˙ O2max, which and a trend toward a decrease in V would have implications for training and later performance in endurance effects, even in this short-term trial. We have to agree with the relatively damning comments of Cheuvront (8) in his recent comprehensive review of the diet (8), which states, ‘‘When it comes to improving performance through diet, athletes would be well advised to steer clear of the Zone.’’ However, a 7-day trial may not be sufficiently long to detect positive changes, and longer trials should be conducted to determine whether these changes do occur.
5. 6. 7. 8. 9.
10. 11. 12.
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