Table 4. Associations between body mass index/fat mass index and activity levels as tested both by conventional epidemiological approaches and through the application of instrumental variable analysis using a 32-SNP weighted allelic score as an instrument.
| Adiposity | Activity | N | Linear Regression | Instrumental Variable Regression (Weighted Allelic Score with 32 SNPs) | |||||||
| Z-Score Value Coefficient (95% CI)a | Difference in Activity, Raw Units (95% CI)a | p-Value | F-Statistic | Partial R 2 | Z-Score Value Coefficient (95% CI)a | Difference in Activity, Raw Units (95% CI)a | p-Value | p-Value (DWH)b | |||
| BMI (kg/m2) | Total physical activity (counts/min) | 4,296 | −0.12 (−0.15, −0.10) | −22.3 (−27.6, −17.0) | 1.6×10−16 | 124.85 | 0.03 | −0.18 (−0.36, 0.00) | −32.4 (−63.9, −0.87) | 0.04 | 0.52 |
| Moderate-to-vigorous activity (min/d)c | −0.17 (−0.20, −0.14) | −2.6 (−3.1, −2.1) | 3.7×10−29 | −0.18 (−0.36, −0.01) | −2.8 (−5.5, −0.08) | 0.04 | 0.89 | ||||
| Sedentary time (min/d) | 0.05 (0.02, 0.08) | 3.5 (1.5, 5.5) | 5.0×10−4 | 0.20 (0.02, 0.38) | 13.2 (1.3, 25.2) | 0.03 | 0.10 | ||||
| FMI (kg/m2) | Total physical activity (counts/min) | 4,244 | −0.18 (−0.21, −0.15) | −32.3 (−37.6, −27,1) | 1.5×10−33 | 108.34 | 0.03 | −0.20 (−0.39, −0.02) | −36.1 (−69.3, −2.8) | 0.03 | 0.82 |
| Moderate-to-vigorous activity (min/d)c | −0.22 (−0.25, −0.19) | −3.4 (−3.8, −2.9) | 2.4×10−48 | −0.20 (−0.39, −0.02) | 3.1 (−5.9, −0.24) | 0.03 | 0.86 | ||||
| Sedentary time (min/d) | 0.09 (0.06, 0.12) | 5.8 (3.8, 7.7) | 1.4×10−8 | 0.22 (0.03, 0.41) | 14.6 (1.9, 27.3) | 0.02 | 0.16 | ||||
Regression results were adjusted for age.
a
Coefficients are displayed as sex-specific z-scores for both measures of adiposity and activity levels and have also been rescaled to give more meaningful outcomes relating to the raw units of these variables. The raw-unit difference was computed by multiplying the z-score value by the SD of the variable, taken from Table 1.
b
p(DWH) is the p-value of the Durbin form of the DWH test, which examines the difference between the estimates from linear regression and instrumental variable analysis.
c
Moderate-to-vigorous activity was log transformed for analysis.