The researchers found that a blueberry-supplemented diet did not improve sight in the dark, but they did help participants recover normal vision after exposure to a bright light.

"The enhancement, however, was small and not likely noticeable to most healthy people," said Wilhelmina Kalt from Canadian federal agriculture department, Agriculture and Agri-Food Canada.

But the researchers added that anthocyanins, which are pigment molecules in blueberries and other plants, might improve visual health among people who have existing eye disorders, though this remains to be demonstrated with well-designed studies.

Health food advocates believe that blueberries for not only promote heart health, better memory and digestion, but also improve night vision.

Wilhelmina Kalt and colleagues noted that studies published decades ago provided the first hints that blueberries might improve people's night vision.

Later lab experiments appeared to shore up these early findings. For example, anthocyanins, which are pigment molecules in blueberries and other plants, encourage the regeneration of key molecules in the eye involved in perceiving light.

But reviews of the earlier clinical research that tested the effect of blueberries on night vision in human participants revealed that the studies were poorly controlled. The study appeared in the Journal of Agricultural and Food Chemistry.

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