Of the 1,664 older adults included in this study, 347 (20.9%) had type 2 diabetes, 32 percent by history and 68 percent newly diagnosed by OGTT. The mean age was 73.7 years among those with diabetes and 69.1 years among those with normal glucose tolerance (p-value < 0.0001). As shown in , slightly more than half were women and 40 percent were non-walkers, although 63 percent of non-walkers indicated that they exercised > 3 times a week. Few participants were current smokers (13%) or reported three or more alcoholic drinks per day (11%). However, hypertension (71%) and a history of CHD (32%) were common. Mean fasting plasma glucose levels for adults with and without diabetes were 125.5 and 93.5 mg/dl, respectively (p-value < 0.0001), and mean post challenge levels were 237.5 and 105.2 mg/dl, respectively (p-value < 0.0001). also shows that adults with diabetes were significantly more likely than those with normal glucose tolerance to be male, older, former smokers, overweight or obese, hypertensive, consume ≥ three alcoholic drinks per day, have high triglycerides, low HDL cholesterol, and a history of coronary heart disease, and were less likely to report regular exercise. Walking was not significantly associated with diabetes status (p-value = 0.15).
Characteristics of Participants Stratified by Diabetes Status; Rancho Bernardo CA, 1984-87
compares characteristics after stratification by walking status. Walkers were significantly more likely than non-walkers to be male, report regular exercise, and have a history of CHD, but significantly less likely to be current smokers or have low HDL cholesterol. Although the mean age was similar by walking status (p-value > 0.10, means 69.8 for non-walkers, 70.7 for < 1 mile walkers, and 69.6 for ≥ 1 mile per day walkers), analyses with age as a categorical variable showed higher proportions of younger participants among non-walkers and higher proportions of older participants among walkers (p-value = 0.02). BMI, daily alcohol intake, hypertension, and triglycerides did not differ by walking status (p-value > 0.10).
Characteristics of Participants Stratified by Walking Status; Rancho Bernardo CA, 1984-87
During the 10-year follow-up period there were 152 CHD deaths, 143 other CVD deaths, and 269 deaths from other causes totaling 564 deaths from all-causes. The lowest age-adjusted all-cause death rate was among those with diabetes who walked a mile or more each day (rate = 26.5 per 100; 95% CI = 13.6, 39.4). When analyzed by cause of mortality the age-adjusted rate for other CVD mortality was also lowest among adults with diabetes who walked a mile or more each day (rate = 2.3 per 100; 95% CI = 0, 6.1). However, there was no difference in age-adjusted rates of CHD or other mortality by walking status (data not shown). Among adults with normal glucose tolerance there was little difference between age-adjusted all-cause and cause-specific mortality by walking status (data not shown). In comparison to other subgroup age-adjusted all cause mortality rates, those with the lowest mortality rates included never smokers (rate = 28.6), normal body weight as measured by BMI (rate = 32.0), those who drank on average 1-2 drinks per day (rate = 32.3), those who exercised 3 or more times per week (rate = 32.7), and those without hypertension (rate = 30.5) (data not shown).
There were no significant differences in distributions of covariates between those with and without complete data. Regression diagnostics for investigation of the pair-wise correlations and the variance inflation suggested no discernible multicollinearity among the variables in each of the models presented in .
Adjusted Hazard Ratios for Death from All Causes, Coronary Heart Disease, Non-Coronary Heart Disease Cardiovascular Disease, and Other Causes; Rancho Bernardo CA, 1984-1996
Covariate data were complete for 1,627 of the 1,664 participants (97.8%). Cox proportional hazard models adjusting for gender, age (continuous), smoking, BMI, average drinks per day, exercise, hypertension, triglycerides, high-density lipoprotein cholesterol, and history of coronary heart disease for all-cause and cause specific mortality are shown in . There was no evidence of a sex by walking/diabetes interaction (p-value = 0.77). After adjusting for other covariates, adults with diabetes who walked a mile or more each day were about half as likely to die in the next 10 years compared to adults with diabetes who did not walk this distance (HR = 0.54; 95% CI = 0.33, 0.88). Adults without diabetes who walked a mile or more each day were also at reduced risk for all-cause mortality, however this finding was not statistically significant (HR = 0.89; 95% CI = 0.67, 1.18) (). Other statistically significant independent predictors for all-cause mortality after adjustment for all other covariates included male sex (HR = 2.45; 95% CI = 2.03, 2.96), age in years (HR = 1.13; 95% CI = 1.11, 1.14), current smoking (HR = 2.02; 95% CI = 1.49, 2.73), and history of CHD (HR = 1.64; 95% CI = 1.38, 1.95); exercise ≥ 3 times per week was protective (HR = 0.69; 95% CI = 0.55, 0.88).
Only 32 percent of those with type 2 diabetes knew of their condition at the 1984-1987 clinic visit; the remainder were diagnosed by OGTT. The following reported results were consistent when stratified by known and unknown diabetes at baseline however, statistical significance was reduced due to smaller cell sizes. There was no statistically significant difference in reporting walking (p-value = 0.85) or exercising 3 or more times in the past week (p-value = 0.64) between those who did and did not know they had type 2 diabetes.
After adjusting for other covariates, walking at least one mile each day reduced the risk of other CVD death in those with diabetes (HR = 0.19; 95% CI = 0.04, 0.86) and those without (HR = 0.55; 95% CI = 0.32, 0.96) when compared to those who walked less (). In contrast, there were no statistically significant differences in risk of mortality attributed to CHD or other causes between walkers and non-walkers among adults with and without diabetes. Other risk factors for CHD, other CVD, and other causes of death included male sex, age, current smoking, and history of CHD. The only statistically significant protective factor for other CVD death was drinking 1-2 drinks per day, while the only protective factor for CHD death was exercising ≥ 3 times per week.
Plots of adjusted cumulative probability of death over the 10-year follow-up period are shown for adults with diabetes compared to adults without diabetes (). Almost 40 percent of those with diabetes died compared to 23 percent of those without diabetes.
Adjusted cumulative probability of all-cause mortality by diabetes status and all-cause mortality by walking and diabetes category during the period January 1, 1984, to December 31, 1996.
also shows the adjusted cumulative probability of death for adults for the six categories of walking and diabetes and appeared stable over the 10-year follow-up period. Those with and without diabetes who walked a mile or more each day had the lowest adjusted cumulative probability of death over the period. Proportional hazards assumption was confirmed by visual inspection of the curves as well as inclusion of a time interaction variable which was found to be insignificant (p-value=0.14).
Investigation of behavior change in the sub-sample of 731 participants who returned for a follow-up clinic visit in 1992-96 showed that 45 percent consistently reported being non-walkers, light walkers, and moderate walkers, and 71 percent consistently reported exercising three or more times per week. Of those whose walking habits changed, 30 percent increased to become light or moderate walkers from nonwalking, while 25 percent reduced their walking to become non- or light walkers from moderate walkers. Body mass index was stable (weighted kappa = 0.63).