CURRENT OPINION Sports Med 2000 Mar; 29 (3): 147-151 0112-1642/00/0003-0147/$20.00/0 Adis International Limited. All rights reserved. Physical Activity and the Prevention of Type 2 Diabetes Mellitus How Much for How Long? Andrea Kriska Department of Epidemiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA Abstract From observational studies to clinical trials in a variety of populations and age groups, evidence is mounting in support of the hypothesis that physical activity plays a significant role in the prevention of type 2 diabetes mellitus. Based on the current findings, it is likely that physical activity can reduce the risk of diabetes mellitus development. What is less clear is how much physical activity is necessary, and for how long. Obviously, we are more likely to see the anticipated physiological changes if we maximise the dose. Yet, although maximal is better from a physiological point of view, we in the trenches have also recognised the fact that a sedentary individual will most likely not continue to undergo a high intensity activity exercise regimen. In contrast, evidence is mounting regarding long term compliance to moderate levels of activity, which appear to be easier to adopt in one s lifestyle and are less likely to result in injury. More importantly, there appear to be beneficial changes in insulin sensitivity and glucose tolerance in the sedentary individual who incorporates moderate levels of activity such as walking into the their lifestyle, although the onset of beneficial metabolic changes appear to occur much more slowly and less dramatically than what occurs with a high intensity regimen. Even if activity is shown to be beneficial, we are faced with the challenge of reaching the sedentary individuals who would most likely benefit from an increase in physical activity in the first place. This task is a difficult one because of the difficulty in quantifying the sum total of an entire day s worth of movement, rather than that of a few relatively higher intensity leisure activities. What is needed is a measure or combination of measures of physical activity that are simple to use, relatively inexpensive, and adequately capture the subtle changes in physical activity through the day that we are encouraging. It is unlikely that the protective nature of a physically active lifestyle in preventing diabetes mellitus will have a lasting impact once a switch to a sedentary way of life is made. Therefore, from a public health viewpoint, long term commitments to increased activity are required. This is the ultimate challenge.
148 Kriska From observational studies to clinical trials in a variety of populations and age groups, the epidemiological evidence is mounting in support of the hypothesis that physical activity plays a significant role in the prevention of type 2 diabetes mellitus. [1] At present, the most convincing of all of the epidemiological evidence are 2 intervention studies demonstrating a decrease in diabetes mellitus development at follow-up in adult Swedish men and Chinese men and women with impaired glucose intolerance at baseline. [2,3] The later study was a group-randomised clinical trial in which one of the intervention arms was exercise alone. This arm of the trial fared as well as the diet and/or the diet plus exercise arm in preventing diabetes mellitus. [3] Interestingly, this decrease in diabetes mellitus development occurred without a significant change in body mass index and was evident in both initially lean and overweight participants. The epidemiological evidence is well supported by exercise training studies that have provided the physiological basis for this relationship between physical activity and decreased risk of diabetes mellitus. Although the actual mechanisms have not been completely identified, it appears that physical activity may reduce the risk for type 2 diabetes mellitus both directly through improvements in insulin sensitivity as well as indirectly through beneficial changes in body mass and composition. [4] Based upon the current findings, it is clear that physical activity can reduce the risk of diabetes mellitus development. What is less clear, is how much exercise/physical activity is necessary and for how long? In other words, what exercise prescription should we advocate? 1. How Much is Enough? Technically, the answer to that question is an easy one: we don t know. It is not feasible to think that we will ever come up with a minimum daily requirement that is ideal for everyone, for the simple reason that each individual comes to the table with an entirely different set of variables. Bouchard [5] and others have shown that the response to exercise varies widely among individuals, regardless of the outcome variable. Obviously, we are more likely to see the anticipated physiological changes if we maximise the dose. As expected, physical training studies have demonstrated that high intensity exercises are more likely to bring about the desired metabolic changes in insulin sensitivity and glucose tolerance than lower intensity activities. [6] More is also better if caloric expenditure and bodyweight management are the outcome of interest. 2. And For How Long? A substantial part of the improvements in glucose tolerance and insulin resistance due to exercise are believed to be the result of the cumulative effect of a frequent lowering of the blood glucose levels and decreasing insulin resistance with each specific bout of exercise. [7] In fact, it appears that a large portion of the effect of exercise in decreasing insulin resistance is short lived, lasting for a few days, whereas the blood glucose lowering effect of activity may not even last that long. [8] Likewise, the bodyweight maintenance benefit of physical activity requires a long term commitment to physical activity. In other words, it is unlikely that the protective nature of a physically active lifestyle in preventing diabetes mellitus will have a lasting impact once a change is made to a sedentary way of life. Therefore, from a public health viewpoint, long term commitments to increased activity are needed. Yet, if this is the case, why would physical activity levels in the past, as long ago as the late teen years, be shown to be significantly associated with the presence or absence of diabetes mellitus in middle-aged adults? [9] It is likely that the individuals who reported more activity as young adults remained more physically active as middle-aged adults, and that it is the relatively more recent activity levels that are providing the protection. Recall bias may also be a possibility, such that the currently more active individuals may be overestimating their past activity levels. 3. Will They Do It? Although maximal exercise is better from a phys-
Physical Activity and Diabetes Mellitus 149 iological point of view, we in the trenches have also recognised the fact that a sedentary individual will most likely not continue to undergo a high intensity activity exercise regimen. [10] In contrast, evidence is mounting regarding compliance to moderate levels of activity which appear to be easier to include in one s lifestyle and are relatively less likely to result in injury. [11] We recently published a 10-year follow-up study of a previous 3-year clinical trial of walking in postmenopausal women. Not only were the women who were originally randomised to the walking intervention group more active at the end of the trial (based upon questionnaires and activity monitors), but they maintained higher physical activity levels compared with the control women 10 years after the closure of the clinical trial. [12] This is one of the most important findings of this study. If the beneficial lifestyle change that we are able to assist our participants in achieving as part of the intervention efforts ends when the study ends, are we, as health professionals, really helping the participants? In order for us to impact on diabetes mellitus through increasing levels of physical activity in a population, these increases in physical activity levels need to last beyond our intervention efforts. 4. Will Moderate Level Activity Help? Assuming that moderate intensity activities such as walking are more likely to be incorporated into one s lifestyle, what do we know about the health benefits of such activity? In other words, is it enough? There appear to be overall health benefits and reduced risk of cardiovascular disease from lower intensity activities. [10] In fact, in the postmenopausal women study mentioned in section 3, there was a striking difference noted between the 2 randomised groups in the 10-year incidence of reported heart disease, with a reduction in risk for the walking intervention women compared with the control women (2 vs 12%). [12] But what about diabetes mellitus? Can we anticipate any beneficial changes in insulin sensitivity and glucose tolerance in sedentary individuals who incorporate moderate levels of activity such as walking into their lifestyle? The answer appears to be yes, but not as quickly. According to training studies, the onset of the metabolic changes appear to occur much more slowly and less dramatically for lower intensity activities, but follow in the same general direction as high intensity activities. [13] Moderate level aerobic and resistance training studies that have not significantly resulted in increases in V. O 2max have still shown improvements in insulin sensitivity due to the exercise training. In fact, in the clinical trial of diabetes mellitus prevention in China, [3] the exercise intervention group, in which walking was the primary physical activity performed, did have a significant decrease in diabetes mellitus development over the course of the 6-year trial. Whether the participants in the exercise arm of this study will maintain these increased activity levels and exhibit a decreased incidence of diabetes mellitus for years beyond the end of the clinical trial is the lingering question. In the US, a randomised, multicentre clinical trial of type 2 diabetes mellitus prevention is currently underway. The National Institutes of Health Diabetes Prevention Program incorporates a combination of physical activity and dietary modification to comprise the lifestyle intervention arm of the study as one of the possible treatments. [14] With 3200 individuals participating in this study, half of which have been recruited from US minority subgroups, this trial should provide valuable information regarding the issue of diabetes mellitus prevention as well as the feasibility of keeping individuals exercising and dieting for 3 to 6 years. If proven to be successful, it will be important to see if these individuals maintain their higher levels of physical activity years after the trial ends. 5. Future Activity Promotion: Let s Keep It Simple With the understanding that physical activity may help to prevent or delay the development of type 2 diabetes mellitus, how do we take this message to the public to maximise the adoption and future maintenance of a physically active lifestyle. Based upon what we know to date, [1] what we are trying
150 Kriska to achieve is a permanent increase in physical activity levels gained by incorporating adequate physical activity into one s daily routine. The fact that there are endless and potentially ever-changing ways one can accomplish this increase in lifestyle activity maximises the feasibility that change will occur, but makes the monitoring of the progress extremely difficult. So how do we monitor the progress of such subtle activity changes? Since the outcome measure of interest is the sum total of an entire day s worth of movement rather than the recall of a few relatively higher intensity leisure activities, how do we identify sedentary individuals that would benefit from an increase in physical activity in the first place? What we need is a measure of physical activity that is simple to use, relatively inexpensive and adequately captures the subtle changes in physical activity throughout the day that we are encouraging. When assessing physical activity levels by questionnaire, lower intensity activities like walking tend to be less reproducible than higher intensity activities such as many organised sports. [15] Therefore, the activity questionnaire is likely not the best way to quantify these lower intensity, variable frequency lifestyle activities. A more feasible approach may be a simple, inexpensive, objective measure of activity such as an activity monitor or a pedometer. Step monitors are now being successfully used to estimate levels of movement expressed as daily steps taken throughout the day and have been shown to be related to simultaneous changes in insulin sensitivity. [16] However, activity monitors also have their own set of limitations such as the inability of capturing cycling, swimming and upper body movement. Likewise, monitors certainly are not as practical as physical activity questionnaires in assessing activity in populations studies. It is likely that a combination of the 2 methods of activity assessment would work best. Individuals who have scored low on the activity questionnaire could be given a step monitor to further evaluate their physical activity levels. Although promising, much more workneedstobedoneinthisarea. 6. Conclusion It is likely that we can significantly decrease the development of diabetes mellitus through physical activity intervention, particularly in conjunction with a reasonable diet. We anxiously await the results of the Diabetes Prevention Program in the US [14] to see if it confirms the previous 2 clinical trials in Sweden and China. [2,3] Even if activity is shown to be beneficial, we are faced with the challenge of reaching the sedentary individuals who would most likely benefit from such intervention. The exercise prescription that we are advocating has recently changed to one that is easier to swallow and more likely to be taken. Whether or not patients will keep refilling this prescription into the future, and whether these smaller doses will result in substantial decreases in the development of diabetes mellitus, will be the focus of the next several years of activity research. Acknowledgements The author is indebted to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) for continued funding in this area (currently funded by grant RO1 DK43394-06A1). The helpful comments of my colleague and friend, Dr William Knowler, and my former and present doctoral students, Dr Mark Pereira, Shannon FitzGerald and Jennifer Brach are greatly appreciated. References 1. Physical activity and health: a report of the Surgeon General. US Department of Health and Human Services. Centers for Disease Control and Prevention. National Center for Chronic Disease Prevention and Health Promotion. President s Council on Physical Fitness and Sports, 1996 2. Eriksson KF, Lindgärde F. Prevention of type 2 (non-insulindependent) diabetes mellitus by diet and physical exercise. Diabetologia 1991; 34: 891-8 3. Pan X, Li G, Hu Y, et al. Effects of diet and exercise in preventing NIDDM in people with impaired glucose tolerance: the Da Qing IGT and diabetes study. Diabetes Care 1997; 20: 537-44 4. Ivy JL, Zderic TW, Fogt DL. The prevention and treatment of non-insulin-dependent diabetes mellitus. Exerc Sports Sci Rev 1999; 27: 1-35 5. Bouchard C. Individual differences in the response to regular exercise. Int J Obes 1995; 19: S5-8 6. Holloszy JO, Schultz J, Kusnierkiewicz J, et al. Effects of exercise on glucose tolerance and insulin resistance. Acta Med Scand 1986; Suppl. 711: 55-65 7. Schneider SH, Amorosa LF, Khachadurian AK, et al. Studies on the mechanism of improved glucose control during regular exercise in type 2 diabetes. Diabetologia 1984; 26: 355-60
Physical Activity and Diabetes Mellitus 151 8. Koivisto VA, Yki-Jarvinen H, DeFronzo RA. Physical training and insulin sensitivity. Diabetes Metab Rev 1986; 1: 445-81 9. Kriska A, LaPorte R, Pettitt D, et al. The association of physical activity with obesity, fat distribution and glucose intolerance in Pima Indians. Diabetologia 1993; 36: 863-9 10. Pate RR, Pratt M, Blair SN, et al. Physical activity and public health: recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA 1995; 273: 402-7 11. Pollock ML, Carroll JF, Graves JE, et al. Injuries and adherence to walk/jog and resistance training programs in the elderly. Med Sci Sports Exerc 1991; 23: 1194-200 12. Pereira MA, Kriska AM, Day RD, et al. A randomized walking trial in postmenopausal women: effects on physical activity and health 10 years later. Arch Intern Med 1998; 158: 1695-701 13. Björntorp P, Krotkiewski M. Exercise treatment in diabetes mellitus. Acta Med Scand 1985; 217: 3-7 14. Diabetes Prevention Program Research Group. The diabetes prevention program: design and methods for a clinical trial in the prevention of type 2 diabetes. Diabetes Care 1999; 22: 623-34 15. Kriska A, Knowler W, LaPorte R, et al. Development of a questionnaire to examine relationship of physical activity and diabetes in Pima Indians. Diabetes Care 1990; 13: 401-11 16. Yamanouchi K, Ozawa N, Shinozaki T, et al. Daily walking combined with diet therapy is a useful means for obese NIDDM patients not only to reduce body weight but also to improve insulin sensitivity. Diabetes Care 1995; 18: 775-84 Correspondence and offprints: Dr Andrea Kriska, Graduate School of Public Health, Department of Epidemiology, University of Pittsburgh, Pittsburgh, PA 15261, USA.