There is a rumor that eating bilberries improves eyesight or, at least, helps to maintain it.
Is it true?
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The potential positive effects of Bilberry (Vaccinium myrtillus or myrtille) as well as Blackcurrent (Ribes nigrum or cassis) on eye health and related neurological physiology is an area of active research.
The association with vision is ancient, but most articles just pay homage to Hildegarde of Bingen. I am yet to find an easy source for the few sentences in her medical and botanic works in Latin,Causae et Curae, where she wrote about this subject.
Modern studies focus on anthocyanosides in those fruits. Those compounds could have other interesting applications in neurology and several parts of metabolism.
Teams have begun to find positive correlations for preventive eye-related neurological use, but these studies are not conclusive for a human diet situation. At least one study has found that animals similar to humans in their digestive system store anthocyanins from alimentation in key organs of their bodies so that a key element of evidence exists between ingestion and use by the body.
Remember: many interesting natural substances are degraded by digestion and do not reach the blood in significant concentration level to do anything. That is one reason why special preparations (like pills, concentrations, compounds, etc.) of such substances are essential if you want to obtain medically-relevant effects.
This is a common trend in many studies of natural compounds: It is very difficult to go from something to eat or drink to something that would have a notable effect on your body, or from something that can work when delivered on site in vitro to something that can be taken in a self-administered manner. Many popular or newspaper reports forget to mention the concentration and equivalent quantity of products needed to reproduce the effect with conventional available products and so are misleading and very easy to contradict (apparently) by another study using other concentrations, other chemical forms, other means of applications.
This meta-study of research articles on bilberry related to night vision concludes that new studies should be conducted using the knowledge of all the shortcomings and discrepancies of the previous studies so that they can be compared. I hope this will happen.
The hypothesis that V. myrtillus anthocyanosides improves normal night vision is not supported by evidence from rigorous clinical studies. There is a complete absence of rigorous research into the effects of the extract on subjects suffering impaired night vision due to pathological eye conditions. Evidence from methodologically weaker trials and auxiliary evidence from animal studies, trials of synthetic anthocyanosides, and a recent randomized controlled trial of Ribes nigrum (black currant) anthocyanosides may warrant further trials of V. myrtillus anthocyanosides in subjects with impaired night vision.
So the provisional conclusion is that there are sufficient hints to explore the potential effects of these fruits on eye health but not enough evidence to conclude for saying anything else that would be useful for a layman or a physician. You cannot discard it as a myth.
I have not been able to found reports of dangerous effects of these fruits on health. As with many fruits, some substances of interest are degraded by heating, maturation or during storage. Eating them fresh and raw might increase the benefits but can be more difficult on the stomach (some varieties are quite acidic).
So, in the meantime those two fruits are pleasant to eat (beware of teeth staining and the difficulty of cleaning stained fabric), we have nice recipes for them so we do not have to wait scientific studies to eat them with moderation.