In this 2013 study, participants given 0.45 g of a standardized bilberry extract showed lower post-prandial glucose. Note that the beneficial effects were slightly delayed, which researchers thought might be due to the fact that the extract was ingested via capsule.
As always, questions remain concerning purity and dosage of products available for sale. This press release describes a case where a widely sold commercial bilberry extract was not what it claimed to be. Caveat emptor.
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