Proanthocyanidins are a class of polyphenolic compounds found in a variety of plants, most notably in grape seeds, pine bark (as in Pycnogenol®), and certain berries. Scientific interest in proanthocyanidins has focused on their antioxidant properties and their effects on vascular health, including the capillaries. Several studies have shown that proanthocyanidins can help to strengthen capillary walls, reduce capillary permeability, and improve microcirculation. These effects are believed to result from their ability to protect collagen and elastin in blood vessel walls from free radical damage, as well as to inhibit enzymes that degrade these structural proteins.

Clinical trials—primarily with grape seed extract and Pycnogenol®—have demonstrated benefits in conditions involving capillary fragility, such as chronic venous insufficiency, edema, and retinopathy. For example, a number of randomized controlled trials have reported reduced swelling and improved capillary resistance in subjects with venous insufficiency who were given proanthocyanidin-rich extracts. However, while these findings are promising, the quality and size of many studies are modest, and more large, high-quality trials are needed to confirm efficacy and fully elucidate mechanisms.

In summary, there is moderate scientific evidence supporting the use of proanthocyanidins for capillary health, particularly in improving capillary strength and reducing permeability, but the evidence is not yet definitive.