Physical Activity, Cardiovascular Risk Factors, and Mortality Among Finnish Adults With Diabetes

Six independent cross-sectional population surveys were performed in the Kuopio and North Karelia provinces in eastern Finland in 1972, 1977, 1982, 1987, 1992, and 1997 (14). The survey was expanded to the Turku-Loimaa region in southwestern Finland in 1982, the Helsinki capital area in 1992, and the northern province of Oulu in 1997. In 1972 and 1977, a randomly selected sample (6.6%) of the population born between 1913 and 1947 was drawn. Since 1982, the sample was stratified by area, sex, and 10-year age-group according to the World Health Organization MONICA (Monitoring Trends and Determinants in Cardiovascular Disease) protocol (15). In the six surveys, the subjects included were 25–64 years of age, and in the 1997 survey, subjects between 65 and 74 years of age were also included. Subjects who participated in more than one survey were included only in the first survey cohort. The total sample size of the six surveys was 53,166. The participation rate varied by year from 74 to 88% (14). These surveys were conducted according to the ethical rules of the National Public Health Institute, and the investigations were carried out in accordance with the Declaration of Helsinki.

Patients who participated in any of the risk factor surveys and also reported having diabetes on the questionnaire at the baseline survey, who had had a hospital discharge diagnosis of diabetes between 1968 and 2000, or who were entitled for special reimbursement for antidiabetic drugs according to the National Social Insurance Institution’s Drug Register between 1964 and 2000 were considered as diabetic patients and were included in the present analysis. The National Hospital Discharge Register and Drug Register data were linked to the risk factor survey data with the unique identification numbers assigned to every resident of Finland. The Hospital Discharge Register has reported the codes for type 1 and type 2 diabetes separately since 1987. Antidiabetic drugs prescribed by a physician are free of charge in Finland and are subject to approval of the Social Insurance Institution based on a review of each case history. The physician confirms the diagnosis of diabetes applying the World Health Organization criteria: one or more classic symptoms plus fasting plasma glucose level ≥7.8 mmol/l (≥7.0 mmol/l from 1998) or result of oral glucose tolerance test ≥11.1 mmol/l; at least one increased plasma glucose concentration on a fasting plasma glucose level ≥7.8 mmol/l (≥7.0 mmol/l from 1998) or result of oral glucose tolerance test ≥11.1 mmol/l in the absence of symptoms; or treatment with a hypoglycemic drug (oral antidiabetic agents or insulin) (16,17). All approvals of patients receiving free-of-charge medication are entered into a register maintained by the Social Insurance Institution.

A total of 3,931 individuals were recognized as having diabetes. Of these, 106 subjects were excluded because they had type 1 diabetes, and 117 subjects were excluded because of incomplete data on physical activity, BMI, smoking, blood pressure, or serum cholesterol level. Therefore, 3,708 participants were included in the present study. To avoid the potential bias due to low physical activity associated with severe disease at baseline, subgroup analyses were performed after exclusion of 770 subjects who had been diagnosed with coronary heart disease (CHD), stroke, or heart failure before the baseline survey, who may have been physically inactive because of severe disease or disability at baseline, or who died during the first 2 years of follow-up.

Occupational, commuting, and leisure-time physical activity was assessed using a self-administered questionnaire at baseline that was mailed to the participants in advance. A detailed description of the questions is presented elsewhere (13,18–23), and these questions were similar to those used and validated in the “Seven Countries Study” (24). The subjects reported their occupational physical activity according to the following categories: 1) light = physically very easy, sitting office work, e.g., secretarial work; 2) moderate = work including standing and walking, e.g., store assistant, light industrial work; 3) active = work including walking and lifting, or heavy manual labor, e.g., industrial work, farm work. The daily commuting return journey was categorized into three categories: 1) using motorized transportation, or no work (0 min of walking or cycling); 2) walking or bicycling 1–29 min; 3) walking or bicycling >30 min. Self-reported leisure-time physical activity was classified into three categories: 1) low = almost completely inactive, e.g., reading, watching television, or doing some minor physical activity but not of moderate or high level; 2) moderate = some physical activity >4 h per week, e.g., walking, cycling, light gardening, but excluding travel to work; 3) high = vigorous physical activity >3 h per week, e.g., running, jogging, skiing, swimming, and heavy gardening. Because our group has found that moderate and high occupational, commuting (≥1 min), or leisure-time physical activity independently and significantly reduces risk of total and CVD mortality among diabetic patients (13), physical activities were merged and regrouped into three categories: 1) low was defined as subjects who reported light levels of occupational, commuting (<1 min), and leisure-time physical activity; 2) moderate was defined as subjects who reported only one of the all three types of moderate to high physical activity; and 3) high was defined as subjects who reported two or three types of moderate to high physical activity.

Smoking, education, and medical history were also assessed using the self-administered questionnaire at baseline. Based on the responses, the participants were classified as never, former, and current smokers. Length of education was used as an indicator for socioeconomic status. At the study site, specially trained research nurses checked that the questionnaire was fully completed and measured blood pressure, height, and weight using a standardized protocol (15). Blood pressure was measured after 5 min of rest using a standard mercury manometer. Height was measured without shoes. Weight was measured with light clothing. BMI was calculated as weight in kilograms divided by the square of the height in meters. After measurement of blood pressure, a venous blood specimen was collected. Serum total cholesterol level was measured using the Lieberman-Burchard method in 1972 and 1977 and using an enzymatic method (CHOD-PAP; Boehringer Mannheim, Mannheim, Germany) since 1982. The enzymatic assay method gave 2.4% lower values than the Lieberman-Burchard method. The cholesterol values from 1972 and 1977 were corrected by this percentage. All samples were analyzed in the same laboratory at the National Public Health Institute.

The study cohorts were followed until the end of 2001 through computerized register linkage. Mortality data were obtained from Statistics Finland and were linked with the survey data using the personal identification number assigned for every resident in Finland. The International Classification of Diseases, 8th, 9th, and 10th revisions, were used for coding the causes of death. CVD was defined with codes 390–459 for the 8th or 9th revisions and I00–I99 for the 10th revision.

The statistical software package SPSS for Windows (version 11.5; SPSS, Chicago, IL) was used for statistical analysis. The Cox proportional hazards model was used to estimate single and joint effects of different levels of physical activity and other risk factors at baseline with the risk of mortality. The results were reported as hazard ratios (HRs) and 95% CIs for mortality. The subjects were classified according to smoking status (never, former, or current) and physical activity (low, moderate, or high) as well as according to the sex-specific tertiles of BMI, diastolic blood pressure, systolic blood pressure, and total cholesterol level. The prospective analyses were performed using dummy variables adjusted for the following covariates: age, sex, study year, education, BMI, systolic blood pressure, total cholesterol level, and smoking at baseline. Because the interactions between sex and each level of physical activity, BMI, blood pressure, cholesterol, and smoking on mortality were not statistically significant, men and women were combined in the analyses. The χ² log-likelihood ratio test was used to compare relative abilities of the different levels of physical activity on the risk of mortality.

During the average follow-up of 18.7 years, 1,423 deaths were recorded; of these, 906 were coded as CVD, 605 were coded as CHD, and 202 were coded as stroke. The general characteristics of the study population at baseline are presented in Table 1. In general, physically active diabetic patients were younger, had significantly lower BMI and systolic blood pressure, and were less often smokers compared with the inactive patients. Approximately 45% of the diabetic patients were obese (BMI ≥30 kg/m²), 78% had hypertension (diastolic blood pressure ≥90 mmHg and/or systolic blood pressure ≥140 mmHg), and 88% had hypercholesterolemia (cholesterol ≥5.0 mmol/l). The age-standardized total and CVD mortality rates showed a significant decrease from low to moderate physical activity and a small decrease from moderate to high physical activity.

The age-, sex-, and study year–adjusted HRs associated with low, moderate, and high physical activity were 1.00, 0.59 (95% CI 0.52–0.68), and 0.49 (0.43–0.56) for total mortality and 1.00, 0.60 (0.51–0.71), and 0.49 (0.41–0.58) for CVD mortality, respectively (Table 2). The protective effect of physical activity did not appreciably change after further adjustment for education, BMI, systolic blood pressure, cholesterol, and smoking and after additional exclusion of individuals who had been diagnosed with CHD, stroke, or heart failure before the baseline study, who may have been physically inactive because of severe disease or disability at baseline, or who died during the first 2 years of follow-up. The combined measure of occupational, commuting, and leisure-time physical activity was a better predictor of mortality than any one of three types of physical activity alone (data not shown).

The highest tertiles of BMI, diastolic and systolic blood pressure, and current smoking were each significantly associated with an increased risk of total and CVD mortality, in comparison with the lowest tertiles of BMI, diastolic and systolic blood pressure, and never smoking (Table 2). The highest level of total cholesterol also increased CVD mortality. These associations remained significant also after adjustment for education, adjustment for other CVD risk factors, and additional exclusion of individuals who had been diagnosed with CHD, stroke, or heart failure before the baseline study, who may have been physically inactive because of severe disease or disability at baseline, or who died during the first 2 years of follow-up.

The joint associations of physical activity, BMI, diastolic and systolic blood pressure, cholesterol level, and smoking on the risk of mortality are shown in Table 3. The protective effect of physical activity on total and CVD mortality was fairly consistent at different levels of other risk factors. None of the interactions between physical activity and other risk factors on mortality were statistically significant. These associations did not appreciably change after additional exclusion of individuals previously diagnosed with CHD, stroke, or heart failure before the baseline study, who may have been physically inactive because of severe disease or disability at baseline, or who died during the first 2 years of follow-up (data not shown).

Our analysis addressed the joint association of physical activity and CVD risk factors for total and CVD mortality among patients with type 2 diabetes. Moderate or high levels of physical activity were associated with a significantly reduced risk of total and CVD mortality, independent of age, education, BMI, blood pressure, total cholesterol, and smoking. High levels of BMI, diastolic or systolic blood pressure, and current smoking were each significantly associated with an increased risk of total and CVD mortality, and high levels of cholesterol were also significantly associated with an increased risk of CVD mortality. The protective effect of physical activity was consistent in diabetic patients at any level of BMI, blood pressure, cholesterol, and smoking.

Consistent with our findings, several other studies have shown that hypertension, elevated total and LDL cholesterol levels, decreased HDL cholesterol level, and smoking were independent predictors of CVD mortality in diabetic patients (3–7). However, the association between obesity and CVD mortality among patients with diabetes is controversial. The results from the U.K. Prospective Diabetes Study indicated that high levels of BMI and waist-to-hip ratio were not major risk factors for coronary artery disease among patients with diabetes (4). The positive association of BMI and mortality among men with diabetes has been found in the Aerobics Center Longitudinal Study; however, this association became nonsignificant when fitness was included in the model (7). In the present study, high BMI at baseline was an independent predictor for both total and CVD mortality and was independent of physical activity and other risk factors. Obesity and central obesity have been found to be associated with insulin resistance and hyperinsulinemia (25).

Several previous studies, including the Aerobics Center Longitudinal Study (8), the Nurses’ Health Study (9), the Whitehall Study (10), the National Health Interview Survey (11), and the Health Professionals’ Follow-Up Study (12) have indicated that regular leisure-time physical activity is associated with reduced CVD and total mortality among patients with diabetes or impaired glucose tolerance. Four of these studies also indicated that walking had a similar inverse association with the risk of CVD and total mortality as vigorous leisure-time physical activity (9–12). Our previous study reported that moderate and high occupational and leisure-time physical activity as well as daily walking or cycling to and from work independently and significantly reduced risk of total and CVD mortality among patients with diabetes (13). In the Aerobics Center Longitudinal Study, a steep inverse relation between fitness and mortality was found in men with diabetes, and this association was seen in all BMI and body fatness groups (7). Regular physical activity may reduce CVD and total mortality among patients with diabetes by several mechanisms. In the general population, regular physical activity can improve insulin sensitivity, decrease blood pressure, increase HDL cholesterol levels, decrease triglyceride levels, induce weight loss and improve weight maintenance (26,27), and also reduce the risk of metabolic syndrome (28) and type 2 diabetes (18,20,29,30). Among diabetic patients, regular physical activity is also associated with improved insulin sensitivity, glycemic control, and improved levels of CVD risk factors (31,32). Nevertheless, the present study confirms previous findings that physical inactivity is an independent risk factor for CVD and total mortality in patients with diabetes. The American Diabetes Association, the National Cholesterol Education Program Export Panel, and the International Diabetes Federation (European Region) recommend physical activity for the primary and secondary prevention of CVD complications among patients with diabetes (32–34).

The present study indicated that the protective effect of physical activity on mortality appeared at different levels of weight, blood pressure, and serum total cholesterol and among smoking and nonsmoking individuals. Regular physical activity may be more important in mortality risk than many of the traditional CVD risk factors. Even though the results of an observational cohort study alone cannot prove a causal relationship, there is no doubt that physical activity should be considered an integral part of treatment among patients with type 2 diabetes.

There are several strengths and limitations in our study. First, we had a homogenous study population, and the number of participants was large. Second, the mean follow-up of 18.7 years was sufficiently long and the CVD events were ascertained without losses of follow-up. Third, in addition to leisure-time physical activity, occupational and commuting physical activities were included in the analysis. Fourth, we excluded patients with type 1 diabetes from the analysis. Finally, we performed additional analyses excluding subjects who were diagnosed with CHD, stroke, and heart failure before the baseline study, subjects who may have been physically inactive because of severe disease or disability at baseline, or subjects who had died during the first 2 years of follow-up and thus avoided the potential bias from a relative excess in early mortality and severe disease at baseline that might have resulted in low physical activity in such patients with diabetes.

The major limitation of our study is that we did not have data on the severity of diabetes, glucose control, and type of drugs used for the treatment for diabetes. Another limitation was the self-report of physical activity. Use of a questionnaire to assess habitual physical activity is always a crude and imprecise method. We had no data on possible changes in physical activity during follow-up. Misclassification, particularly overreporting of the amount of physical activity at baseline and changes in the activity during follow-up, most probably leads to an underestimation of the association between physical activity and the outcome. The results from the Aerobics Center Longitudinal Study have shown that the relative risk of total mortality for low fitness was substantially greater than that for physical inactivity (8), suggesting that self-reports probably underestimated the true association between sedentary lifestyle and mortality.

In conclusion, our study confirmed that a moderate or high level of physical activity was associated with a reduced risk of total and CVD mortality among patients with type 2 diabetes. Obesity, high blood pressure, and current smoking were independently associated with an increased risk of total and CVD mortality in patients with diabetes, and a high serum total cholesterol level was also associated with an increased risk of CVD mortality. The favorable association of physical activity with longevity was observed regardless of the levels of BMI, blood pressure, cholesterol, and smoking. Regular physical activity can be recommended to patients with diabetes whether or not they have other known CVD risk factors. However, because the results of an observational cohort study cannot be interpreted as a causal relationship, a randomized clinical trial is still warranted to test the effects of physical activity on long-term health outcomes among patients with type 2 diabetes. Such a trial is not easy to conduct.

This study was supported by grants 46558, 53585, 204274, and 205657 from the Finnish Academy.