Wilhelmina Kalt and her team at the Agriculture and Agri-Food Canada research centre in Kentville, Nova Scotia, presented the results of two human trials in the paper. It was published in the American Chemical Society’s Journal of Agricultural and Food Chemistry.
One of the two trials monitored 72 subjects given two daily doses of blueberry, in the form of capsules (low dose) and juice (high dose), over a three-week period, with a three-week ‘washout’. In the second, 59 individuals were given a single daily high dose of blueberry over a 12-week period, and eight week ‘washout’. A placebo was used in both cases.
“In both [trials], neither dark adaptation nor night vision was improved by anthocyanin intake,” Kalt said in the paper. “However, in both trials anthocyanin consumption hastened the recovery of visual acuity after photobleaching.”
‘Photobleaching’ describes exposure of the retina to a very bright light.
According to Kalt, various pieces of in vitro, in vivo and clinical research carried out between the 1960s and 1980s, largely in Europe, appeared to suggest links between blueberries and vision improvements.
“Unfortunately, most of the early European clinical research that reported effects of bilberries on night vision did not employ a randomised, placebo-controlled study design,” said Kalt.
Four out of the five earlier trials which were placebo-controlled and randomised actually resulted in negative outcomes, she added.
In the new trials carried out by Kalt and her team, even the effect of recovery after ‘photobleaching’ was described as ‘moderate’.
“The magnitude of the effect would not likely translate into subjectively detectable improvement in everyday visual performance,” she concluded.
Regarding wider effects on dark adaptation, given the limited positive results from well-designed human studies in normal populations, the team considered that an epidemiological approach or interventional studies might be useful targeting specific pathologies.
The researchers highlighted one of the challenges with phytochemical trials more generally, in the fact that volunteers for nutritional studies tend to be nutrition conscious themselves, consuming a diet rich in fruit and vegetables. With blueberries or other produce, this means that effects which are not apparent with this well-nourished population may still be demonstrable in a less well-nourished population.
“Results of our study thus contribute to the evidence that nutritional studies related to vision should either target more vulnerable populations or utilise other investigational strategies,” the report found.
Source: Journal of Agricultural and Food Chemistry
“Blueberry Effects on Dark Vision and Recovery after Photobleaching: Placebo-Controlled Crossover Studies”
Authors: Wilhelmina Kalt, Jane E. McDonald, Sherry A.E. Fillmore, Francois Tremblay