I am trying to find the association of BMI on the age at onset of hypertension in diabetes vs non-diabetes patients. Or in other words, does diabetes (if the patient is already diabetic before the hypertension diagnosis) modify the association of BMI and age at onset of hypertension.
To find this, I have tried a multiple linear regression model with onset age of hypertension as dependent variable and BMI as independent variables adjusted for other covariates.
I am getting a slope of -0.33 * BMI for non-diabetic hypertensives and -0.65 * BMI for diabetic hypertensives. i.e. slope is almost two fold for the diabetic hypertensives compared to non-diabetics.
So I was concluding that the average rate of change in onset age of hypertension with respect to increase in each unit of BMI is more in diabetic patients than in non-diabetic patients.
Now, counter intuitive to this result the mean onset age of diabetic hypertensive patients (55 years) is higher than non-diabetic hypertensives (46 years).
Shouldn't the diabetic hypertensive patients have lower mean onset age than the non-diabetic hypertensives since the slope is steeper for diabetic hypertensives?
Can anyone tell me if there is any flaw in this result and conclusion.
Here is the plot