While previous research supported blueberries' potential brain benefits, a 2015 double-blind study, funded in part by the US Highbush Blueberry Council, found that blueberries might also have heart benefits. Published in the journal Nutrients, the study divided 44 adults with metabolic syndrome into two groups: one group received a smoothie with 22.5 grams of freeze-dried blueberry powder twice daily for six weeks; the other group received a placebo twice daily for six weeks. The placebo consisted of a smoothie nearly identical to the blueberry one but minus the blueberry powder and plus 14 grams of added sugar. The blueberry powder smoothie contained no added sugar.
In total, individuals in the blueberry group consumed 45 grams of blueberry powder daily, which is equivalent to about two cups of fresh blueberries. Here's what the researchers discovered after comparing the participants’ metabolic markers at the beginning and end of the six-week study:
Participants in the blueberry group had significant improvements in their resting endothelial function compared to those in the placebo group. The endothelium is a layer of cells that line blood vessels and other areas of the body; dysfunction of the endothelium is associated with cardiovascular events such as heart attacks.
No differences in blood pressure or other metabolic markers, such as body composition, fasting glucose, and insulin sensitivity, were observed between the blueberry and placebo groups.
While the results (specifically regarding endothelial function) are consistent with other research showing that blueberries may have a positive effect on cardiovascular function, they're inconsistent with at least one other study which found that blueberries may help lower blood pressure. One possible explanation for this discrepancy in the blood pressure findings is that most of the participants in the 2015 study were on blood pressure-lowering medications, which could have skewed the results. In addition, it's important to note that part of the difference in endothelial function between the two groups in the 2015 study resulted from a worsening of endothelial function in the placebo group. It's unclear whether the added sugar in the placebo smoothie contributed to this, thereby exaggerating the benefits from the blueberry powder.