For the first just model SBP as a function of the raw BMI values (possible with a transform to make SBP nonlinear in BMI (SBP doesn't increase infinitely with BMI).

For "want to show yearly increasing SBP&BMI..."? Can you reword this, I'm not quite sure I understand what you mean.

This is my result.

In this table I didn't perform piecewise regression.

Instead of pieciwise regression, I performed 40 (8*5) simple linear regression (SBP on BMI) for all combination of year * BMIgroup.

I thought direct comparison of these coefficient is not statistical , because each group is different and independent each other.

Nevertheless, the result shows that the larger the BMI category, the larger the coefficient, and as the year passes, the coefficient increases.

So to prove these two things, I considered piecewise regression.