Among 161 subjects who completed the baseline survey and were given an accelerometer to wear for three weekdays and one weekend, three subjects did not return the accelerometer to the study coordinator, and three subjects did not have any recorded data in the returned accelerometers. Among 155 subjects with recorded accelerometer data, 24 subjects had less than two days of valid accelerometer wear (i.e., ≥10 h wear time for a given day). This resulted in 131 subjects who had two or more days of valid accelerometer wear for the final analysis. The subjects with and without two valid days of accelerometer wear were comparable in age, gender, race/ethnicity (non-Hispanic white, non-Hispanic black, Hispanic), body mass index, smoking status (non-smokers, former smokers, current smokers), alcohol drinking (nondrinkers, monthly or less, ≥2–4 times/month) and dietary folate intake (<median vs. ≥median).
The mean minutes per day of moderate physical activity in the 131 subjects with two or more days of valid accelerometer wear were 17.5, whereas the mean minutes per day for vigorous physical activity was 0.6. Thus, the combined physical activity, i.e., moderate plus vigorous, was 18.1 min per day. Because the level of vigorous physical activity was low, the level of combined physical activity assessed in this population represented primarily the level of moderate physical activity. Women had significantly lower levels of physical activity than men (11.6 vs. 29.6 min/day for combined physical activity, p < 0.0001). The levels of physical activity were also different among different racial/ethnic groups. Among non-Hispanic whites, the mean min/day for combined physical activity were 28.3, which was significantly higher than that in non-Hispanic blacks and Hispanics (15.3 and 15.7, respectively) (p = 0.01).
presents the mean minutes per day for moderate, vigorous and combined physical activity by different population characteristics among men and women separately. Among women, the level of combined physical activity declined significantly with age and body mass index (p = 0.04 and 0.004, respectively). Non-Hispanic whites had a higher level of physical activity than non-Hispanic blacks (21.0 vs. 11.0 min/day, p = 0.09) and Hispanics (21.0 vs. 9.5 min/day, p = 0.04). Among men, the level of physical activity also declined with age. Non-Hispanic whites and Hispanics had similar level of physical activity (33.6 and 31.1 min/day, respectively), both higher than that in non-Hispanic blacks (23.2 min/day). Overweight (BMI: 25–30 kg/m2) men appeared to have the highest level of physical activity (32.3 min/day) whereas normal weight (BMI <25 min/day) men had the lowest level of physical activity (19.0 min/day). However, none of the differences in men reached statistical significance. In both men and women, the level of physical activity did not differ significantly by education, alcohol drinking, cigarette smoking and dietary folate intake.
Duration of moderate and vigorous activity (minutes/day) by characteristics of the study population in the North Texas Healthy Heart study, 2006–2008
The median level of global genomic DNA methylation as measured by LINE-1 was 73.9%. When study participants were categorized into low and high levels of LINE-1 methylation according to the median, levels of combined physical activity were slightly higher among subjects with high levels of LINE-1 methylation (≥median) as compared to those with low levels of LINE-1 methylation (<median) (19.7 vs. 16.5) but the differences were not statistically significant (p = 0.39). In both males and females, levels of physical activity were not significantly different according to levels of LINE-1 methylation ().
We then analyzed the association between physical activity and global genomic DNA methylation first in a linear regression model where levels of physical activity were categorized into seven groups with a 5-min/day interval (0–5, 6–10, 11–15, 16–20, 21–25, 26–30, >30 min/day). Compared to those with daily physical activity ≤5 min/day (i.e., the reference group), individuals who had physical activity 26–30 min/day had a significantly higher level of global leukocyte methylation (β = 2.54, 95% CI: 0.67, 4.42) (). Because the regression coefficient was close to 0 for individuals with physical activity 6–10 min/day we combined this group with the reference group. We also combined the 16–20 min/day group with the 21–25 min/day group due to the small sample size in each group for further analysis (N = 7 and 8 in each group). presents the findings from linear regression models assessing the association between physical activity and global genomic DNA methylation. Compared to those with physical activity ≤10 min/day, individuals who had physical activity 26–30 min/day had a significantly higher level of global genomic DNA methylation in leukocytes (β = 2.52, 95% CI: 0.70, 4.35).
Figure 1 Association between physical activity and global genomic DNA methylation (LINE-1) in the North Texas Healthy Heart Study 2006−2008. Regression coefficients (βs) and 95% confidence intervals (CIs) were generated from a simple linear regression (more ...)
Differences in global genomic DNA methylation (%) according to duration of physical activity in the North Texas Healthy Heart study, 2006–2008
To control for potential confounding effect of age, gender, race/ethnicity, social economic status and other lifestyle factors, we analyzed the association between physical activity and global genomic methylation in multivariate linear regression models (). Multivariate linear regression model 1 was adjusted for age (continuous), gender and race/ethnicity (non-Hispanic whites, non-Hispanic blacks and Hispanics). Multivariate linear regression model 2 was additionally adjusted for central obesity (yes/no), the ratio of fat mass to fat-free mass, the ratio of subcutaneous fat to visceral fat, cigarette smoking (nonsmokers, former and current smokers), alcohol drinking (never, monthly or less and ≥2–4 times/month), dietary folate equivalent (quartiles) and education (high school or less and some college/college graduate). In both multivariate models, the association between physical activity and global methylation were attenuated and became statistically insignificant.
Because gender and race/ethnicity was each significantly associated with DNA methylation in our study population, we further stratified the association by these factors to explore potential effect modification. Limited by a small sample size, we analyzed physical activity as a dichotomous variable in interaction models using 25 min/day as the cut-point. The associations between physical activity and global methylation appeared to be similar between men and women (among men, β = 0.14, 95% CI: −1.97, 2.26; among women, β = 0.52, 95% CI: −1.30, 2.34) (p for interaction = 0.86). Among non-Hispanics, levels of physical activity were positively associated with global methylation and the association was borderline significant (β = 1.50, 95% CI: −0.08, 3.08) in the fully adjusted model; there was no apparent association between physical activity and global methylation among Hispanics (β = −0.79, 95% CI: −4.68, 3.12) (p for interaction = 0.29) ().