Blueberries Improve Endothelial Function, but Not Blood Pressure, in Adults with Metabolic Syndrome: A Randomized, Double-Blind, Placebo-Controlled Clinical Trial

Randomized, double-blind, placebo-controlled trial in adults with metabolic syndrome, prehypertension, and prediabetes. Participants were middle-aged and obese, with mean age 55 ± 2 years, mean BMI 35.2 ± 0.8 kg/m2, and 91% using antihypertensive medication.

Adults in the active arm consumed 22.5 g of freeze-dried blueberry powder in a smoothie twice daily for 6 weeks, for a total of 45 g/day by oral ingestion. The smoothies also contained milk and yogurt; the comparator was a matched placebo smoothie.

Daily blueberry intake for 6 weeks did not improve blood pressure or insulin sensitivity, but it did improve endothelial function versus placebo. Reactive hyperemia index increased from 1.94 ± 0.11 to 2.17 ± 0.14 in the blueberry arm, while the placebo arm showed a decline; the unadjusted between-group change in RHI was 0.23 ± 0.14 versus -0.23 ± 0.13 (p = 0.024), and the adjusted change was 0.32 ± 0.13 versus -0.33 ± 0.14 (p = 0.0023). Twenty-four-hour systolic and diastolic blood pressure changes were small and similar between groups, and no adverse events were reported.

The intervention was short at 6 weeks and the population was relatively specific: obese adults with metabolic syndrome who were largely taking antihypertensive medication. The trial was not powered here with an active-arm sample size reported in the source packet, and no physical activity data were available. Dietary restrictions and counseling to reduce caloric intake may also have influenced cardiometabolic outcomes.