The goal of this randomized, double-blind, placebo-controlled study is to assess whether the food supplement Endocalyx lowers blood pressure in patients with treatment resistant hypertension.

During this intervention study, subjects will receive either a placebo or Endocalyx for 12 weeks. The main objective of this study is to assess whether the food supplement Endocalyx lowers blood pressure in patients with treatment resistant hypertension. Secondary objectives are: * Change from baseline in office blood pressure and 24-hour blood pressure profiles in subjects with treatment resistant hypertension. * To assess whether sodium intake, sex or kidney function modulates the effect of Endocalyx on blood pressure. * Change from baseline in total vessel density, perfused vessel density, proportion of perfused vessels and microvascular health score * To assess the effect of Endocalyx on total peripheral resistance. * The effect of Endocalyx on quality of life. * To estimate the potential impact of Endocalyx on long-term cardiovascular protection and health care costs.