A study published in the journal Nutrition Research found that blueberry powder improves immune and cardiovascular functioning. During the study, participants received a powder equivalent to 250 grams (about half a pound) of blueberries per day. After six weeks, researchers observed a 4% increase in the immune system’s natural killer (NK) cells. They also observed heart benefits, in particular a reduction in arterial stiffness. While other research has shown that blueberries have a wide range of positive health effects, such as reducing the risk of cognitive decline, this is the first study to directly examine blueberry consumption and NK cell count in humans.