If solving a mystery is like peeling an onion, then the opposite is also true, as the true nutrition benefits of different allium varieties are not revealed at first glance. When evaluated narrowly in terms of conventional nutrients, onions may seem like nutrition zeroes — but it’s their lesser-known phytochemicals that make them nutrition heroes. We’ve rounded up highlights from current research, below:

Red Onions: Among allium vegetables, red onions have relatively high levels of the same anthocyanins that are sometimes credited for the HDL “good” cholesterol benefits of red wine. Red onions’ have twice the antioxidant (ORAC) score of their white and yellow counterparts; representing the content of compounds called polyphenols, ORAC is a laboratory index of the potential for a food to furnish antioxidant value once consumed, a concept still under scrutiny for its potential use on food labels. While polyphenol compounds are often concentrated in fruit and veggie peels, removing the inedible outer layers of the red onion only reduced the ORAC score by 20%. Toss red onions on the grill and in salads.

Yellow and White Onions: Provide for our diets a significant source of vitamin C and a polyphenol compound called quercetin, found in basic research to have potential anti-cancer effects. This may help explain why Cornell researchers showed in a lab study that extracts from several common onion varieties significantly inhibited human colorectal and liver cancer cell growth. 

Shallots: A study in the Journal of Agriculture and Food Chemistry demonstrated that among ten other onion varieties, shallots had the highest ORAC score. In preliminary research, Taiwanese scientists found that shallot compounds could have anti-carcinogenic effects. Shallots are perfect for salad dressings and also sauces — as in Spaghetti Squash with Marinara Sauce.

Scallions: Alternatively known as green or spring onions, they’re actually the fresh shoots of the white onion harvested early. Scallions are tops in terms of vitamin K: just a half cup provides 210% of your daily value of this nutrient needed for strong bones. Promising research shows vitamin K could also help inhibit cancer cells, which may explain why a recent National Cancer Institute study found that men who ate the most scallions had a 70% reduced risk of prostate cancer. Try our Unstuffed Cabbage an unusual way to enjoy scallions.

Leeks: Best known for their bone-building, bacteria-busting prebiotic fiber, which helps guard against foodborne infection and increase the absorption of important minerals. As a good or better source of seven essential vitamins (Vitamins A, C, K, B6 and folate) and minerals (iron and manganese) leeks are definitely the most all-around nutrient dense of all onion varieties. 

Published February 1, 2010