Mixed

Mendelian randomization using semiparametric methods (fractional polynomial or piecewise linear) reveals that the causal relationship between BMI and systolic blood pressure is nonlinear, showing a steep increase up to a BMI of ~32 kg/m2 and a flatter or slightly negative slope thereafter, while the relationship with diastolic blood pressure increases almost linearly up to ~40 kg/m2.

For individuals with a BMI between 25 and 32, blood pressure tends to rise sharply with each unit of BMI. However, once BMI exceeds 32, the rate of blood pressure increase may slow down or even slightly decrease, suggesting a complex biological interaction rather than a simple linear progression. For Diastolic Blood Pressure, the increase remains relatively linear up to a BMI of 40. This implies that weight management strategies might have different impacts on blood pressure depending on the starting BMI and the type of blood pressure being monitored.