Published online today in the British Ecological Society's Functional Ecology, the study claims to be the first to prove the connection between flavonoid consumption and stronger immune systems.
Flavonoids are a class of antioxidants found in plants that are especially concentrated in fruits and vegetables such as broccoli and other cruciferous vegetables, apples, beans and onions.
They have been shown to have various benefits including cancer reduction, heart health and free radical (oxidation) control, although their link with immunity is less established.
"In vitro studies have repeatedly shown that fruits containing flavonoids have a lot of health benefits.
But these studies don't really pin-point the effects to the flavonoids," explained the study's second author Dr Martin Schaefer.
"Our study is the first that really establishes the link between flavonoids and immunity because we didn't feed the entire fruit to the birds - we fed them an extract containing only flavonoids," he told NutraIngredients.com.
Although the findings are not transferable to humans, Dr Schaefer said they provide a "pretty good indication" that similar results could be expected in humans.
Healthy birds For the current study, researchers from the University of Freiburg and the Max Plank Institute for Ornithology in Germany offered a group of blackcaps a choice of two foods.
These were identical in all respects except for the amount of flavonoids they contained.
The birds were found to naturally select the food with added flavonoids, suggesting a "learned selection".
However, of particular interest to the functional food and dietary supplement industries are the researchers' second round of findings, which revealed that the flavonoids also impacted the birds' health.
Those birds that had consumed the flavonoid extracts were found to have a "significantly" higher immune response, measured by antibody production.
When injected with red blood cells from sheep, 54 percent of the supplemented birds had a positive immune response, compared to 30 percent of the non-supplemented birds.
The levels of flavonoids used were equivalent to the amounts that would have been obtained through eating 1-2 blackberries, bilberries or elderberries a day.
Because the birds weighed only 20g, this would roughly translate into several hundred berries a day for humans, said Dr Schaefer.
However, in extract form these levels would be easier to consume, he said.
Source: 'Fruit for health: The effect of flavonoids on humoral response and food selection in a frugivorous bird' , Carlo Catoni et al (2008).
Functional Ecology , doi: 10.1111/j.1365-2435.2008.01400.x