Smoothies are one of the easiest ways to pack more fruit into your day. Toss in a banana, add some berries, blend, and you have what looks like a perfectly healthy drink. But research from the University of California, Davis suggests that this popular combination may have an unexpected downside.

The issue is not that bananas are unhealthy. Instead, it comes down to how certain ingredients interact after they are blended together. In a study published in the Royal Society of Chemistry journal Food & Function , researchers found that fruits with high levels of an enzyme called polyphenol oxidase, or PPO, can sharply reduce the amount of flavanols your body absorbs from a smoothie.

Flavanols are natural plant compounds linked to heart and cognitive health.…