Eating blueberries improves vascular function. One cup per day is effective, but half a cup does not not produce the same benefit. Adults with metabolic syndrome that ate one cup rather than only a half-cup or matched placebo improved flow-mediated dilation (FMD) by about 1.5 percentage points in a six-month randomized trial. FMD measures how effectively an artery widens when blood flow increases, essentially a "stress test" for the endothelium. It's relevant because studies have shown it is a strong predictor of future cardiovascular disease events. The one-cup dose is consistent with other studies showing cardiovascular, metabolic, and brain-health benefits of blueberries.

Bananas contain unusually high levels of polyphenol oxidase, or PPO, an enzyme responsible for browning that also degrades polyphenols. That's why, on the occasion I happen to make a smoothie, I usually use a lower-PPO ingredient such as avocado instead. A high-PPO banana smoothie reduced peak blood levels of epicatechin-derived metabolites by 84% in a small, crossover experiment. A low-PPO mixed-berry smoothie preserved exposure. This was primarily a cocoa-epicatechin experiment rather than a direct test of blueberry anthocyanins or health outcomes. But PPO can oxidize epicatechin and blueberry chlorogenic acid, and the resulting compounds indirectly degrade anthocyanins. Bananas are healthy. But I think leaving them out of a food combination where they can interact too directly with polyphenols is a sensible optimization.