Now that your anti-cancer kitchen is brimming with small red and yellow onions, the obvious question is: Should you cook them or eat them raw?

The short answer: 

Cook onions lightly, then hedge your bets by eating a little raw onion in the same meal.   

The Sulfur Compounds

Like garlic and crucifers, onions contain enzymes that get activated when you cut the vegetables.  In a cascade of actions, those enzymes produce sulfur-based compounds that fight cancer and protect your DNA.  This chemical reaction is the plant’s protection from predators, explains Dr. Irwin Goldman, an onion expert and professor at the University of Wisconsin. When insects bite into the plant’s leaves, the enzymes get to work and generate pungent chemical warriors. 

But the enzymes are very sensitive to heat.

As you know from previous posts, you have to first cut alliums (the onion and garlic clan) to break their cell walls and thus release enzymes. Then you have to let the plants sit for a while so that the enzymes have time to produce sulfur compounds. 

How long do these plants have to sit? Goldman suggests leaving onions for a half hour; garlic needs about 10- 15 minutes, says another food scientist.  

Then, either eat the veggies raw—the best choice in the case of garlic-– or quickly cook onions.   Why cook them? 

The Phytonutrient Quercetin

While eating alliums raw may be the best way to optimize sulfur compounds,  onions have another trait going for them that its garlic cousin appears to lack.  Onions are rich in quercetin, a phytonutrient that acts as an antioxidant and anti-inflammatory,  inhibits harmful estrogens and helps keep cancer cells from using glucose and fatty acids as fuel.  Quercetin is also one of those important flavonols, which may act as proxidants in cancer cells to cause them to commit suicide.   

(Like many phytonutrients, quercetin concentrates in the plant’s outer layers.  That’s why you must peel onions ever so gently–see this short video.) 

Quercetin, it turns out, can benefit from some cooking, which releases the phyto from the plant’s matrix. Sauteeing onions , for example, increases the quercetin quotient.  Boiling, on the other hand,  decreases quercetin in the plant–but the good news is that it leeches into the cooking water, meaning you could just make a soup and drink the broth.   

How long should you cook onions?  Goldman says 4 to 5 minutes max, if you plan on eating the onions. If you’re making a soup, you can cook them much longer–just discard the plants.  

If you cook onions too long, his research has shown that they may cause your platelets, a type of blood cell, to clump together. (In the linked study, steaming for 10 minutes caused sticky platelets; he found the same result when broccoli was cooked for longer than 6 minutes.) That condition can, in turn, increase your risk of cardiovascular disease. And according to nutrition and cancer expert Dr. Jeanne Wallace, that condition can also promote metastasis by providing a hiding place for cancer cells.  

BOTTOM LINE: Putting this together with all the other anti-cancer cooking tips in this blog, what’s the bottom line?

Eat onions raw AND lightly cooked. Choose the small red and yellow onions grown in northern soils, peel them lightly, cut them small enough so that they cook quickly and let them sit.  Then cook them for 4- 5 minutes max, not on high heat.

For an extra bolus of sulfur, eat some raw garlic or onion in the same meal.  Why? Heat kills enzymes that produce healthy sulfurs. But in studies with cruciferous vegetables, enzymes in raw crucifers helped activate sulfurs in cooked crucifers consumed in the same meal. While comparable studies haven’t been done in onions, we’re practicing the precautionary principle and hedging bets.     

“But raw onions give me heartburn,” you might be crying. 

I hear you. In some people, they trigger the valve between the stomach and esophagus to open and close erratically. I’ve also heard the chlorophyll in green plants is a good antidote. Let our 3000plus readers know if that trick works for you.