Lately, my household has begun to dip into preservation of vegetables (and fruit) via lactic acid fermentation. I recently obtained a copy of Fermented Vegetables by Kirsten and Christopher Shockey, and we’ve done a few trials with cabbage and onion (separately, though together might also be interesting!).

Onion

The book mentions that red onions are a good source of the flavonol quercetin, which can help to prevent or reduce cancer, and fermentation of red onions increases the amount of available quercetin. That’s fascinating! But: is it true?

I went hunting for research papers, and found plenty.

I skipped over the papers that showed that quercetin inhibits cancer cells and went for one that focused on whether a low dose of quercetin (read: amounts you might get from simply eating onions regularly) was effective. Effects of low dose quercetin: Cancer cell-specific inhibition of cell cycle progression. (Yes, there is at least some benefit from low doses of quercetin)

Next, the question of whether fermentation of onion increases quercetin. This study was done using yellow onion: Fermentation enhances the in vitro antioxidative effect of onion (Allium cepa) via an increase in quercetin content. (Survey says: yes)

So far, so good. But does red onion really have a higher quercetin content than its less-red siblings? Maybe? Sources of variability in the flavonoid content of foods

Another interesting paper I stumbled across discusses the clinical evaluation of whether just eating onions is a better way to deliver bioavailable quercetin vs taking an isolated supplement: Quercetin from Shallots (Allium cepa L. var.aggregatum) Is More Bioavailable Than Its Glucosides (spoiler: just eat the damn onions)

And finally, I was also curious to know if quercetin is useful in preventing and shortening COVID-19 viral infections. Here’s a fascinating discussion from 2022 about quercetin’s possible impact, not only as an anti-microbial and anti-viral compound, but also as an immuno-modulator and anti-inflammatory agent. Quercetin in the Prevention and Treatment of Coronavirus Infections: A Focus on SARS-CoV-2 (All of those? It appears so)