|
|
||||||||
ORIGINAL RESEARCH COMMUNICATION |
1 From the School of Medicine and Pharmacology (KLC, VB, LJB, and IBP) and the School of Human Movement and Exercise Science (ARM), University of Western Australia, Western Australian Institute for Medical Research and HeartSearch WA
2 Supported by the National Heart Foundation of Australia, the National Health and Medical Research Council of Australia, and the Royal Perth Medical Research Foundation. 3 Reprints not available. Address correspondence to KL Cox, University of Western Australia, School of Medicine and Pharmacology, Royal Perth Hospital, PO Box X2213, GPO Perth, Western Australia, 6847. E-mail: kaycox{at}cyllene.uwa.edu.au.
| ABSTRACT |
|---|
|
|
|---|
Objective: The objective was to evaluate the independent and additive effects of 16 wk of energy restriction and exercise on glucose and insulin concentrations.
Design: Sixty nonsmoking, overweight, sedentary men aged 2050 y were randomly assigned to either maintain or restrict their energy intake (41866279 kJ/d). Within each of these arms, the subjects were further randomly assigned to either a light-intensity (control) or a vigorous-intensity exercise program for 30 min 3 times/wk.
Results: Fifty-one subjects completed the study. Maximal oxygen uptake increased (
24%; P < 0.001) with vigorous but not with light exercise. Significant weight loss was observed with energy restriction (
: 10.12 kg; 95% CI: 8.02, 12.22 kg; P < 0.001) but not with exercise. Vigorous exercise reduced fasting glucose and glucose and insulin areas under the curve (AUCs) by 13% (P = 0.01) and 20% (P = 0.02), respectively. Exercise effects were independent of weight change. Energy restriction resulted in a 40% reduction in the insulin AUC (P = 0.01). Vigorous exercise and energy restriction were additive in reducing the insulin AUC.
Conclusions: Energy restriction and vigorous exercise independently and additively reduce glucose and insulin concentrations in response to an oral-glucose-tolerance test. Both of these lifestyle interventions provide a potent strategy that should be an integral part of any program to reduce the risk of impaired glucose tolerance, insulin resistance, and diabetes in overweight and sedentary persons.
Key Words: Energy restriction exercise glucose insulin overweight men
| INTRODUCTION |
|---|
|
|
|---|
Few well-controlled randomized studies have assessed the independent and additive effects of energy restriction and vigorous exercise training on glucose and insulin concentrations in persons without diabetes. We took the opportunity to investigate, in a controlled randomized intervention trial, the effects of energy restriction and vigorous exercise on blood pressure and body composition in free-living, sedentary, obese men (14, 15). We now report the effects of 16 wk of energy restriction alone, exercise alone, and energy restriction combined with exercise on glucose and insulin metabolism.
| SUBJECTS AND METHODS |
|---|
|
|
|---|
Study design
A 4-wk run-in period included familiarization with the fitness test, diet records, and other intervention procedures. During a 2-wk baseline period at the end of the run-in, assessments were made of fitness, usual dietary intake, and body weight; blood samples were collected; and an oral-glucose-tolerance test was completed. The subjects were then randomly assigned to 1 of 4 groups in a two-factor factorial parallel design study of 16 wk duration. Two groups were asked to maintain their usual dietary habits, whereas the subjects in the other 2 groups were given an individually tailored program that aimed to reduce their total energy intake by 41866279 kJ/d (10001500 kcal/d); the diet provided protein, fat, and carbohydrates at 15%, 30%, and 55% of energy, respectively. Within each of these 2 dietary arms, the subjects were further allocated to a light-intensity exercise control group or a vigorous-intensity exercise group for three 0.5-h sessions/wk. The light exercise group was used to minimize cointervention bias, a factor not usually accounted for in exercise training studies. The light exercise protocol consisted of a series of slow flexibility exercises once per week and stationary cycling (against zero resistance; Monark Crescent AB, Varberg, Sweden) twice a week. Every second week, subjects substituted one cycling session for a session where they walked slowly, at a rate of 
2 km in 30 min. The purpose of the session was to provide variety and to maximize compliance. To further encourage compliance in both groups, the sessions were offered at several different times during the day; make-up sessions were provided at the end of the week for those who missed their scheduled session. The vigorous exercise program consisted of stationary cycling only. Subjects cycled for 30 min at 6070% of their maximum workload, which was determined from their fitness assessment. This exercise protocol and level of intensity was selected because previous reports had shown a training effect on insulin sensitivity (7). Furthermore, we had previously shown this protocol to be effective in improving fitness and to be well tolerated (16). All exercise programs included a 5-min warm-up before the exercise session and a 5-min cool down after the exercise session and were supervised by a trained exercise leader.
Serum glucose and insulin
After the subjects had fasted overnight, blood samples were collected for the measurement of glycated hemoglobin. Blood samples for the measurement of glucose and insulin were collected with the needle indwelling for 20 min while the subjects rested supine. Subjects were then given a 200-mL drink of Glucotol (Orion Laboratories, Perth, Australia) that contained 75 g glucose. Additional blood samples were drawn 30, 60, 90, and 120 min after the drink for the measurement of serum glucose and insulin. An attempt was made to test the subjects at the same time of the day before and after intervention. Blood was sampled before and after intervention with
24 h between the last exercise session and venipuncture.
Serum glucose from the oral-glucose-tolerance test was assayed on a Beckman Glucose analyzer 2 (Beckman Instruments, Fullerton, CA) with the use of reagents from Boehringer Mannheim (Mannheim, Germany). The CV (assay precision) was 2.6% at a concentration of 4.7 mmol/L and was 3.2% at a concentration of 15.4 mmol/L. Serum insulin was measured by radioimmunoassay (17), and the CV was 9% in the range of 15154 mU/L. Glycated hemoglobin was measured from red blood cells in blood collected into EDTA-coated tubes by using the colorimetric method (18). The assay had a within batch CV of 2.8% at 7% glycated hemoglobin and of 3.2% at 13% glycated hemoglobin. The between batch CV was 2.6% at 7% glycated hemoglobin and 4% at 13.6% glycated hemoglobin.
Diet records
Three-day diet records were completed at baseline and at the end of intervention. The same dietitian monitored dietary intake throughout the study. The subjects were given careful written and verbal instructions on how to keep detailed and accurate records of food weight and volume on a 3-d diet record every fortnight. The subjects were given a target weight loss of
78 kg over the 16-wk study. Weight loss was achieved by substituting low-fat foods for high-fat foods, increasing fruit and vegetable consumption, and substituting complex for refined carbohydrates. All subjects were asked to maintain their consumption of tea, coffee, salt, and alcohol and were advised to make no major changes to their sodium intake. We previously used this dietary regimen and found the target weight loss to be achievable and the dietary changes to be manageable and well tolerated by the participants (19). Those subjects allocated to continue their usual energy intake were also seen fortnightly by the dietitian, who determined (from both the 3-d diet records and through an interview) that no substantial alterations to their usual energy intakes were being made. The maintenance of usual dietary patterns was reinforced at each visit, and the subjects were offered a weight-loss program on completion of the study. The diet records were coded by using the NUTTAB89 database (20).
Alcohol intake was monitored each fortnight and recorded in 7-d retrospective diaries, which were subsequently coded by using standard industry tables of alcohol beverage content. The diary data were used to estimate alcohol consumption (in mL ethanol/wk) before and after intervention. In addition,
-glutamyltranspeptidase was measured as a potential objective biomarker of any change in alcohol intake (21).
Physical fitness assessment
Fitness was assessed from a multistage exercise test conducted with an electrically braked cycle ergometer (Siemans-Elema AB; Medicinsk Teknik, Solna, Sweden); the test began at zero workload and increased by 20 W every minute with a pedal rate of 60 rpm until the subjects reached volitional fatigue. Oxygen consumption was measured throughout the assessment and was recorded each minute. Maximum oxygen consumption (
O2max) was the maximum minute value with a respiratory gas exchange ratio of 1.12 or the plateau of oxygen consumption. Inspired volume was measured with a Morgan ventilation monitor (PK Morgan Ltd, Chatham Kent, United Kingdom), expired oxygen with an Applied Electrochemistry S-3A oxygen analyzer (Ametek Thermox Instrument Division Pittsburgh PA), and expired carbon dioxide with a Datex CD-101 carbon dioxide analyzer (Datex Instrument DY, Helsinki). An online computer controlled the metabolic data. Heart rate (HR) was taken over the last 15 s of each minute with a Cardiofax ECG-6511 (Nihon Konden Corp, Tokyo). Fitness was reassessed at the end of the study between 48 and 72 h after the last exercise session.
Body weight
Body weight was measured (to the nearest 1/100th of a kilogram) with electronic balance scales (August Sauter West Germany, FSE Scientific, Perth Australia) while the subjects were wearing light clothing and no shoes. Body composition was assessed by using the underwater weighing technique, and body fat distribution was determined from the waist-to-hip ratio (WHR) (15).
Statistical analysis
SPSS statistical software (release 10.1.0; SPSS Inc, Chicago) was used to analyze the data. A two-factor analysis of variance model with interaction was used to compare the changes in glucose, insulin, glycated hemoglobin, and dietary components. To assess the effect of exercise independent of weight loss on serum glucose and insulin concentrations, the analysis of variance was repeated with the change in weight entered as a covariate. Glucose and insulin values from the glucose tolerance test were calculated by the integrated area under the curve (AUC) method (22). Within-group comparisons of differences between baseline and postintervention measurements were made by using the Bonferroni t test. One-factor analysis of variance and chi-square tests were used to test for any between-group differences in baseline characteristics. Pearsons correlation coefficients were used to determine relations between variables. It was calculated a posteriori that the study had 80% power at
= 0.05 to detect energy restriction or exercise main effects of 15% on glucose AUC and 37% on insulin AUC. Baseline results are expressed as means ± SDs, and all other results are expressed as means and 95% CIs.
| RESULTS |
|---|
|
|
|---|
|
|
O2max by 24% [0.56 L/min; 0.47, 0.65 L/min; n = 51; P < 0.001], but there was no significant change in fitness seen with light exercise. Within-group changes in body mass and body composition are shown in Table 1The WHR was significantly reduced with energy restriction by 0.03 (0.01, 0.04; n = 51; P < 0.001), and this change was independent of the change in body mass or body mass index. There was no significant effect of vigorous exercise on WHR. Significant changes in regional adiposity, as assessed by arm and calf skinfold thicknesses and girths, were seen with energy restriction only (data not shown) (15).
Diet components
Daily energy intake and all dietary components, except protein, were reduced in the group that reduced energy intake relative to the group who maintained their usual diet (Table 3
). There was no significant effect of vigorous exercise on intake or nutrients.
|
Alcohol consumption assessed from the 7-d retrospective diaries was significantly reduced with energy restriction. The difference relative to the usual energy intake group was 52.5 (93.7, 11.2) mL/wk (n = 51; P = 0.04). This amount is equivalent to the consumption of less than one standard drink per day, and no effect on the biochemical marker
-glutamyltranspeptidase was observed.
Serum glucose and insulin
Changes in fasting glucose and insulin from baseline to after the intervention in the 4 groups are shown in Table 2
. There was a significant main effect of vigorous exercise compared with that of light exercise on fasting glucose; concentrations in the vigorous exercise group were 0.30 (0.06, 0.54) mmol/L lower than those in the light exercise group at the end of the study (n = 51; P = 0.02). There was no significant main effect of vigorous exercise on fasting insulin (n = 40; P = 0.09). There was no significant effect of energy restriction on fasting glucose or insulin concentrations. Changes in fasting insulin concentrations were available for 40 subjects, and changes in insulin AUC were available for 37 subjects.
Changes in glucose and insulin AUCs over 2 h are shown in Table 2
for the 4 groups of subjects. Relative to light exercise, vigorous exercise had a significant effect on reducing the glucose AUC by 133.5 (59.8, 207.2) mmol · L1 · 120 min1 (n = 51; P = 0.01) and the insulin AUC by 3303 (677, 5929) mU · L1 · 120 min1 (n = 37; P = 0.02). This represented decreases of 13% and 20% for the glucose and insulin AUCs, respectively. An 8% decrease in the glucose AUC with energy restriction was nearly significant (P = 0.06). Five of the 7 subjects who had impaired glucose at baseline had plasma glucose concentrations in the normal range after the intervention. Energy restriction reduced the insulin AUC by 3925 (1299, 6551) mU · L1 · 120 min1 (n = 37; P = 0.01), or
40%. Energy restriction and vigorous exercise had additive effects on the insulin AUC.
The general linear model was repeated with the change in weight entered as a covariate. The reductions in fasting glucose and in the AUCs for glucose and insulin after vigorous exercise remained significant, whereas the reduction in the insulin AUC after energy restriction was no longer significant. When the analyses were repeated with the changes in diet composition also considered as covariates, the reductions in the AUCs for glucose and insulin remained significant. Glycated hemoglobin did not change significantly with energy restriction or vigorous exercise (Table 2
). There was no interaction of energy restriction or vigorous exercise on any of the variables. The change in fitness was not significantly correlated with the changes in any of the glucose or insulin variables.
| DISCUSSION |
|---|
|
|
|---|
The finding that exercise training resulted in lower insulin concentrations after a glucose challenge is consistent with the findings reported by others (4, 6, 7, 23); however, glucose concentrations were reported to be unchanged (1, 9, 10). Our findings that vigorous exercise independently reduced the insulin AUC by 20%, that energy restriction independently reduced the insulin AUC by 40%, and that both interventions had an additive effect are somewhat novel. We are aware of only one other study that had an appropriate design to evaluate the independent and additive effects of energy restriction and exercise training on glucose and insulin metabolism in obese nondiabetic persons (12). Our findings are consistent with those of a study that used a similar training regimen for 9 mo in older men (12). Other uncontrolled studies have reported additive effects of energy restriction and aerobic and resistance exercise (5), whereas others have not (13).
The role of exercise training in improving glucose tolerance is controversial, particularly in persons with normal glucose responses. In our study, only vigorous exercise reduced the glucose AUC, whereas Dengel et al (12) reported a reduction in the glucose AUC with energy restriction, exercise, and a combination of the 2 interventions. Although 12% of the men in our study had glucose responses to the glucose challenge at baseline that indicated impairment, the group overall had normal glucose tolerance. We showed a 13% improvement in glucose tolerance with exercise training. This finding is similar to that of Dengel et al (12). Our finding of an 8% reduction in the glucose AUC with energy restriction was of borderline significance, whereas similar reductions in other studies have been shown to be significant (10). We were unable to show an additive effect of energy restriction and exercise. The observation that glucose tolerance normalized in 5 of the 7 subjects with impaired glucose tolerance after the intervention was of interest; however, this small number of subjects did not allow for statistical analysis.
The effect of a change in diet composition and weight loss on glucose and insulin metabolism with energy restriction and exercise training is unclear. A low-fat, high-complex-carbohydrate diet has independent effects on glucose and insulin (24), and weight reduction has been shown to independently improve insulin sensitivity (4). In the current study the changes in the insulin AUC with energy restriction and exercise were independent of changes in diet composition and alcohol intake as was the effect of exercise on the glucose AUC. When the change in the glucose AUC was adjusted for the change in alcohol intake, energy restriction resulted in a reduction in the glucose AUC. Our observation that the changes in insulin and glucose with vigorous exercise were independent of any change in weight or body composition in obese men is consistent with the finding of some (25, 26) but not other (11) studies. The difference between our study and that of Dengel et al (12) was that in our exercise only group, the subjects were told to maintain their usual diet as was our control group, whereas in Dengel et als study the exercise only group was asked to increase their energy intake to maintain weight. This increase in energy intake, even if controlled for diet composition, may have resulted in different glucose and insulin responses compared with a protocol that maintained usual intake. The strength of our study is that we were able to compare the effects of vigorous exercise compared with control with both groups maintaining their usual diet. This may account for the differences in findings between our study and other studies.
There is debate over whether the effects of exercise training on glucose and insulin metabolism are due to the acute effects of the exercise bout (27). It is unknown how long the acute effects of exercise last, but they may persist from 12 to 60 h after the last bout of exercise (28, 29). However, after a single bout of exercise no improvement in glucose tolerance was seen after 18 h (30). In the present study the mean time from the last exercise session until the oral-glucose-tolerance test was 31 h with a maximum of 72 h. Although unlikely, it is possible that the improved glucose tolerance and blunting of the insulin response to a glucose load may have been due in part to the last bout of exercise. This latter point may partially account for the difference in findings between our study and that of Ross et al (11), who failed to find an improvement in insulin sensitivity with exercise without weight loss when measurements were taken 4 d after the last exercise session.
Nevertheless, with respect to the effect of lifestyle changes on the prevention and management of impaired glucose metabolism, regular exercise training without weight loss may still bestow benefits to the individual. Even if the acute effect of exercise is transient, cumulatively repeated bouts may improve longer-term glucose tolerance and insulin sensitivity. Combined with energy restriction the effect may be of greater magnitude and perhaps longer lasting as seen by the additive effect in this study.
The practical application of these findings deserves comment. The results of the present study and of another (31) suggest that exercise must be regular and vigorous to improve glucose tolerance. A study in older men and women showed a 9% improvement in insulin responses with low-intensity exercise but a 23% improvement with higher intensity (23). A recent study reported improvements in insulin sensitivity with intensive dietary and exercise regimens but not with the current moderate activity recommendations (32). A limitation of our study was that the 16-wk intervention was relatively short term and was conducted in a controlled setting; the excellent adherence rates observed may not be maintainable in the long term or in a less highly supervised setting. Vigorous intensity exercise may not be desirable or acceptable to some groups such as the obese, the elderly, or the habitually inactive. However, even though our protocol involved vigorous exercise, the intensity level was at the lower end of the vigorous range. We also used a non-weight-bearing activity (stationary cycling), which alleviates some of the problems of the overweight and obese doing exercise. This may be the reason why the activity was well tolerated. The availability of stationary cycling in fitness centers and homes increases the practicability of such a protocol for use in the community. Notwithstanding this, moderate-intensity exercise is likely to be more conducive to long-term maintenance. Hence, well-controlled studies are needed to establish whether more-moderate-intensity exercise programs can confer the same metabolic improvements in predominantly normoglycemic subjects that we showed with vigorous exercise. Some studies have shown that moderate activity and walking with dietary changes can reduce the incidence of diabetes in subjects with impaired glucose tolerance (33, 34); others have found that, compared with a more intensive exercise and diet program, moderate changes in diet and exercise were not effective in improving insulin sensitivity (32). Although moderate activity may be more appealing from a public health perspective, these studies used a daily exercise regimen. Such an expectation in previously sedentary persons may also be unrealistic for long-term maintenance.
We showed that weight loss through energy restriction significantly improved the insulin response to a glucose challenge in this group of previously sedentary, middle-aged obese men. Vigorous exercise in the absence of weight loss had an independent effect on both glucose and insulin responses to an oral-glucose-tolerance test. The combination of energy restriction and vigorous exercise resulted in additive effects on the insulin response to an oral-glucose-tolerance test. The magnitude of the improvements in glucose and insulin we observed is similar to that observed in other studies (12, 23). These improvements are of clinical significance because strategies that involve the use of pharmacologic agents such as metformin report improvements in insulin sensitivity of
25% (35). Another potential limitation of our study was that the low number of insulin samples may have lessened the magnitude of the change in the insulin AUC. However, because this change was significant with both interventions, our conclusions remain unchanged. Furthermore, even though the vigorous exercise program attenuated the loss of lean body mass with energy restriction, such a loss has undesirable effects on glucose metabolism. This may have lessened the effect of the exercise on the glucose and insulin responses. Both of these lifestyle interventions have independent and sometimes additive influences on other diabetes risk factors and they are a potent strategy that should be an integral part of any program to reduce the risk of impaired glucose tolerance, insulin sensitivity, and diabetes in overweight and sedentary persons.
| ACKNOWLEDGMENTS |
|---|
KLC was responsible for the data collection and analysis, the interpretation of the results, the writing of the manuscript, and the concept, design, and formulation of the study hypothesis. VB contributed to the analysis and interpretation of the results and edited the manuscript. ARM contributed to the study design and protocol and edited the manuscript. LJB contributed to the study design, formulation of the hypothesis, and interpretation of the results and edited the manuscript. IBP contributed to the study design and the formulation of the hypothesis, the data collection, the interpretation of the results, and the writing and editing of the manuscript. None of the authors had any conflict of interest with the organization supporting this research.
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
J. P. Kirwan, T. P. J. Solomon, D. M. Wojta, M. A. Staten, and J. O. Holloszy Effects of 7 days of exercise training on insulin sensitivity and responsiveness in type 2 diabetes mellitus Am J Physiol Endocrinol Metab, July 1, 2009; 297(1): E151 - E156. [Abstract] [Full Text] [PDF] |
||||
![]() |
U. Ekelund, P. W. Franks, S. Sharp, S. Brage, and N. J. Wareham Increase in Physical Activity Energy Expenditure Is Associated With Reduced Metabolic Risk Independent of Change in Fatness and Fitness Diabetes Care, August 1, 2007; 30(8): 2101 - 2106. [Abstract] [Full Text] [PDF] |
||||
![]() |
L. M. Redman, L. K. Heilbronn, C. K. Martin, A. Alfonso, S. R. Smith, E. Ravussin, and for the Pennington CALERIE Team Effect of Calorie Restriction with or without Exercise on Body Composition and Fat Distribution J. Clin. Endocrinol. Metab., March 1, 2007; 92(3): 865 - 872. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. Rosenbaum, C. Nonas, R. Weil, M. Horlick, I. Fennoy, I. Vargas, P. Kringas, and and The El Camino Diabetes Prevention Group School-Based Intervention Acutely Improves Insulin Sensitivity and Decreases Inflammatory Markers and Body Fatness in Junior High School Students J. Clin. Endocrinol. Metab., February 1, 2007; 92(2): 504 - 508. [Abstract] [Full Text] [PDF] |
||||
![]() |
E. P Weiss, S. B Racette, D. T Villareal, L. Fontana, K. Steger-May, K. B Schechtman, S. Klein, J. O Holloszy, and and the Washington University School of Medicine C Improvements in glucose tolerance and insulin action induced by increasing energy expenditure or decreasing energy intake: a randomized controlled trial. Am. J. Clinical Nutrition, November 1, 2006; 84(5): 1033 - 1042. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. E. Black, E. Mitchell, P. S. Freedson, S. R. Chipkin, and B. Braun Improved insulin action following short-term exercise training: role of energy and carbohydrate balance J Appl Physiol, December 1, 2005; 99(6): 2285 - 2293. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. E. Cassidy, L. F. Bielak, Y. Zhou, P. F. Sheedy II, S. T. Turner, J. F. Breen, P. A. Araoz, I. J. Kullo, X. Lin, and P. A. Peyser Progression of Subclinical Coronary Atherosclerosis: Does Obesity Make a Difference? Circulation, April 19, 2005; 111(15): 1877 - 1882. [Abstract] [Full Text] [PDF] |
||||
![]() |
L. A. Adams, P. Angulo, and K. D. Lindor Nonalcoholic fatty liver disease Can. Med. Assoc. J., March 29, 2005; 172(7): 899 - 905. [Abstract] [Full Text] [PDF] |
||||
![]() |
T. J Wilkin and L. D Voss Metabolic syndrome: maladaptation to a modern world J R Soc Med, November 1, 2004; 97(11): 511 - 520. [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |