The berries do not lower blood sugar, but their constituents help improve the strength and integrity of blood vessels and reduce damage to these vessels associated with diabetes and other diseases, such as atherosclerosis (calcium and fat deposits in arteries). Bilberry extracts also may reduce the tingling sensations in the extremities associated with diabetes. Several studies have shown that bilberry extracts stimulate blood vessels to release a substance that helps dilate (expand) veins and arteries. Bilberries help keep platelets from clumping together, which, in turn, thins the blood, prevents clotting, and improves circulation. Bilberry preparations may be of benefit in certain eye conditions, including diabetic retinopathy, cataracts, night blindness, and degeneration of the macula, the spot in the back of the eye that enables sharp focusing.