Experimental design

The study was conducted using a single-blind, controlled, randomized, cross-over study design. Data collectors, outcome adjudicators, and data analysts were blinded while participants could not be blinded due to the nature of the test products. Volunteers reported trice to the test facility after an overnight fast. After 20-min rest in the supine position, brachial and femoral artery blood flow (echo-Doppler), forearm and leg resistance artery blood flow (venous occlusion plethysmography) and forearm microvascular perfusion (near infra-red spectroscopy) were regularly assessed. After baseline measurements, volunteers randomly received either a single dose of 100 ml tea, beetroot juice or control (hot water), all containing 75 g glucose. Based on the cross-over design of our study with three different interventions, there were six different possibilities regarding the order of interventions. Volunteers were randomly allocated by an independent statistician into six treatment allocations (Williams design, balanced for first order carryover effects). Subsequently, all measurements were repeated (every 30-min) across a three hour period. Blood was taken every 30 min for post-hoc analysis of glucose and insulin. Between the subsequent testing days, a wash-out period of two to seven (on average four) days was provided.