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Organic onions richer in flavonoids

A six-year comparison of organic versus conventional onions shows higher levels of phytochemicals thought to promote health

by Alla Katsnelson
June 28, 2017

Credit: Shutterstock
Organically grown Red Baron onions (shown) have higher levels of flavonoids, on average, than conventionally grown ones.

Many grocery store shoppers believe that organic vegetables are healthier than their conventional counterparts, but studies comparing nutritional content have produced mixed results. Now, researchers have completed the first-ever multiyear field test of a crop grown with organic methods versus conventional ones. They report that organic onions contain a significantly higher amount of two types of flavonoids—compounds that some studies have shown may protect against chronic health conditions like cardiovascular disease, diabetes, and cancer (J. Ag. Food Chem. 2017, DOI: 10.1021/acs.jafc.7b01352).

Previous studies of differences between conventional and organic produce have relied on vegetables bought off the shelf or have encompassed just a couple of years of side-by-side growth, making it difficult to rule out the effects of annual differences in weather or other factors. In this study, researchers from Teagasc Food Research Centre wanted to do a longer, better-controlled study by growing the same vegetables in the same location over six seasons using conventional versus organic methods. Between 2009 and 2014, the researchers grew red and yellow onions—Red Baron and Hyskin varieties—using both organic farming practices as defined by the European Union and conventional methods, which involved the use of mineral fertilizers and synthetic pesticides. Then they analyzed the compounds in freeze-dried extracts from the onions using high-performance liquid chromatography and other techniques.

Not surprisingly, the red onions had higher levels of the red-pigmented flavonoid anthocyanin, while the yellow onions had higher levels of quercetin, which imparts a yellowish pigment. Annual weather factors like rainfall and temperature also contributed to variability in phytochemical content.

Overall, however, across the six years of the study, the total flavonoid content was 10-50% higher by dry weight in the organic onions. Consistently higher levels of individual flavonoids were also present in the organic onions, and the extracts had higher antioxidant activity in two assays that measure radical scavenging activity.

The team’s analysis attributed the flavonoid content increase in the organic onions primarily to the differences in soil management and pest control practices. Under organically farmed conditions, the plant experiences more stress as it grows, explains Dilip K. Rai, the study’s senior author. Without synthetic pesticides, for example, microbial attacks increase. Plants generate secondary metabolites such as anthocyanin and quercetin as a defensive mechanism, he says.

Rai points out that the study’s findings align with a major 2012 meta-analysis that found no nutritional differences between organic and conventional produce but noted that flavonoids and other polyphenol compounds increase in organic farming. “We have to make a distinction between nutritional compounds” — like protein and minerals that we need for growth — “and health-promoting ones,” he says.

Pinning down whether the increase translates to real health benefits is difficult, however. “That’s the million-dollar question,” says Rai. Thunder Jalili, a nutritional scientist at the University of Utah who studies the physiological effects of phytochemicals, notes that a handful of studies have shown that taking quercetin reduces blood pressure. The doses given are generally much higher than could be consumed from onions, however.



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Keith (June 28, 2017 11:14 PM)
My own experience with growing onions is you don't need pesticides to grow them. The bugs don't eat them. You may need a little herbicide to keep down the weed competition. If there is a little science here they would tell us exactly what pesticides were used. Pesticide X may in fact suppress the biochemical pathway to synthesize flavonoids whereas pesticide y or z has no impact on that pathway. If this were a science magazine it would provide details. Pesticides (and Phamaceuticals) are highly engineered molecules that are designed to disrupt biochemical pathways. Switching herbicides could end the problem in which case organic is identical to conventional produce which is the conclusion that every other study has shown. Also I would be interested in the effect on yield. I wonder if the conventional onions are bigger so the concentration of flavonoids in smaller organic onions are higher but the total amount of flavonoids produced are the same.
Donald E. Clark PhD (June 29, 2017 12:18 PM)
J. Agricultural and Food Chemistry is a peer reviewed scientific journal of the American Chemical Society.
Corinna Wu (June 29, 2017 2:03 PM)
There is a link to the original research article in the first paragraph. You can find more details about the study there.
Steve Stumbras (September 18, 2017 4:44 PM)
Keith, I think you are spot on, but it seems like you then went on to contradict the wisdom you shared. If a mineral and a microbe synthesize to create a phytonutrient and soils with balanced minerals and benificial biology are more suitable for the production of these metabolites; doesn’t the use of “cides” repress the production of these health producing metabolites?
Melody Russo (June 29, 2017 7:56 AM)
This is a very timely article for me as I'm studying with Institute of Integrative Nutrition this year and just heard from presenters about the organic vs. conventional nutritional content study referenced herein. This paper emphasizes the value of other healthful properties, the soil and the organic method.

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