Anticancer Effects of Raw vs Cooked Garlic and Onion

Image Credit: Danielle Scott / Flickr

How can I preserve the anti-cancer effects of cooked garlic?

Awesome video, Dr. Greger! I am left with the question of the anticancer effects of raw vs. cooked garlic and onions. I know cooking reduces some of the nutrients but have raw and cooked alliums been tested in regard to the anti-cancer effects? In other words is it important to try to consume these raw notwithstanding their very strong flavors?

Michelle Rowe / Originally posted on #1 Anticancer Vegetable


The secret to maintaining the anti-cancer effects of garlic is to either eat it raw (think salsa, homemade dressings, pesto, etc) or crush the garlic first, wait 10 minutes, and then cook it.

You know those chemical flares? You bend them, two chemicals mix and a light-emitting reaction takes place? The same kind of thing happens in garlic. Floating around in the cytoplasm of garlic cells is a compound called alliin and packed away in tiny intracellular storage compartments (called vacuoles) is an enzyme called alliinase. When the garlic tissues are crushed, the two mix and alliinase turns alliin into allicin, the phytonutrient thought to be responsible for many of garlic’s health benefits. Cooking destroys the enzyme, though, so even if you crush your garlic, if it’s thrown immediately into the pan, little allicin may be produced.

Allicin is relatively heat stable, though, so if you chop your garlic and wait 10 minutes for the allicin to be formed, you can then cook it (the enzyme has already done its work) and presumably maintain many of the benefits.

A similar reaction happens in cruciferous vegetables like broccoli! Check out my videos The Best Detox and Sometimes the Enzyme Myth Is True.

Image credit: Danielle Scott / Flickr


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.

26 responses to “How can I preserve the anti-cancer effects of cooked garlic?

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  1. What do you think of this, is it true?

    Wow- very interesting….Everyone should read :)

    ONIONS! I had never heard this!!!

    In 1919 when the flu killed 40 million people there was this Doctor that visited the many farmers to see if he could help them combat the flu…
    Many of the farmers and their families had contracted it and many died.

    The doctor came upon this one farmer and to his surprise, everyone was very healthy. When the doctor asked what the farmer was doing that was different the wife replied that she had placed an unpeeled onion in a dish in the rooms of the home, (probably only two rooms back then). The doctor couldn’t believe it and asked if he could have one of the onions and place it under the microscope. She gave him one and when he did this, he did find the flu virus in the onion. It obviously absorbed the bacteria, therefore, keeping the family healthy.

    Now, I heard this story from my hairdresser. She said that several years ago, many of her employees were coming down with the flu, and so were many of her customers. The next year she placed several bowls with onions around in her shop. To her surprise, none of her staff got sick. It must work. Try it and see what happens. We did it last year and we never got the flu.

    Now there is a P. S. to this for I sent it to a friend in Oregon who regularly contributes material to me on health issues. She replied with this most interesting experience about onions:

    Thanks for the reminder. I don’t know about the farmer’s story…but, I do know that I contacted pneumonia, and, needless to say, I was very ill… I came across an article that said to cut both ends off an onion put it into an empty jar, and place the jar next to the sick patient at night. It said the onion would be black in the morning from the germs…sure enough it happened just like that…the onion was a mess and I began to feel better.

    Another thing I read in the article was that onions and garlic placed around the room saved many from the black plague years ago. They have powerful antibacterial, antiseptic properties.

    This is the other note. Lots of times when we have stomach problems we don’t know what to blame. Maybe it’s the onions that are to blame. Onions absorb bacteria is the reason they are so good at preventing us from getting colds and flu and is the very reason we shouldn’t eat an onion that has been sitting for a time after it has been cut open.


    I had the wonderful privilege of touring Mullins Food Products, Makers of mayonnaise. Questions about food poisoning came up, and I wanted to share what I learned from a chemist.

    Ed, who was our tour guide, is a food chemistry whiz. During the tour, someone asked if we really needed to worry about mayonnaise. People are always worried that mayonnaise will spoil. Ed’s answer will surprise you. Ed said that all commercially-made mayo is completely safe.

    “It doesn’t even have to be refrigerated. No harm in refrigerating it, but it’s not really necessary.” He explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment. He then talked about the summer picnic, with the bowl of potato salad sitting on the table, and how everyone blames the mayonnaise when someone gets sick.

    Ed says that, when food poisoning is reported, the first thing the officials look for is when the ‘victim’ last ate ONIONS and where those onions came from (in the potato salad?). Ed says it’s not the mayonnaise (as long as it’s not homemade mayo) that spoils in the outdoors. It’s probably the ONIONS, and if not the onions, it’s the POTATOES.

    He explained onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion.. He says it’s not even safe if you put it in a zip-lock bag and put it in your refrigerator.

    It’s already contaminated enough just by being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!). Ed says if you take the leftover onion and cook it like crazy you’ll probably be okay, but if you slice that leftover onion and put on your sandwich, you’re asking for trouble. Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down.

    Also, dogs should never eat onions. Their stomachs cannot metabolize onions.

    Please remember it is dangerous to cut an onion and try to use it to cook the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning.

    1. Much of this is not based in fact. Food scientists don’t give factory tours, your tour guide was probably a guy making minimum wage with a high school education. The pH of mayo is *not* so low that it can’t grow bacteria, and pretty much anything wet will grow bacteria when moved from cold to hot, because bacteria grows under warm conditions. The onion stuff is also wholly untrue. Onions *do not* attract bacteria from the surrounding environment, they don’t become toxic after they’re cut, and they actually contain some compounds that are inhibit bacterial growth. Furthermore, a dog’s stomach can certainly digest onions, but you should never feed them onions because they contain a chemical that is toxic to dogs.

      Please consider fact-checking before repeating information that comes from a random source (i.e., tour guide, blog or small outlet news story lacking citations, etc). Health misinformation can stop with you.

      1. If this was true about onions becoming toxic, at 71 and a lifelong leftover onion eater, I’d be gone and buried by now.

  2. I’d love to hear what Dr Greger thinks about Alisa Wiegel’s post too! Is any of that true. I’ve always been a big fan of raw onions and eat them daily so I store a large red onion sliced in half and then cut from it for daily salads and sandwiches. Is there harm in this approach?

    1. Snopes? The founder embezzled company funds using them to cheat on his wife with a prostitute. Not where I would go for for advice on anything other than infidelity

  3. What about those bottles at most grocery stores of garlic already chopped & marinating in it’s own juices; that should be full of allicin, correct?

  4. Seeing the virus in the onion with a 1919 microscope is a big, big hoax! Just read more about viruses, how big they are, and how are they isolated and tested. Flu virus could not be “seen” in 1919. Flu virus could be tested by inoculating fertile chicken eggs.

  5. “Since cooking can inactivate alliinase, some scientists recommend letting garlic stand for 10 minutes after chopping or crushing before cooking it.”
    “The formation of thiosulfinates (alliicin) is very rapid and has been found to be complete within 10-60 seconds of crushing garlic.”
    Source: Oregon State University.
    Is this conflicting?

    1. Hi Jesse,

      I am a volunteer for Dr. Greger. Thank you so much for your question.

      Putting garlic in a food processor will not, to the best of my knowledge, destroy any phytochemicals or associated benefits. In fact, putting garlic in a food processor may actually improve the health effects of garlic. Putting it in a food processor would be especially important if you plan on cooking the garlic afterward. This is effective for onion, shallots, garlic, and others in that same plant family, as there are enzymes within these foods that convert phytochemicals to their active form. When the food is cooked, the enzymes are deactivated and cannot work. This is much the same way that broccoli works!

      I hope this helps!

  6. Julie,

    The answer is past is best: Processing into paste was more favorable to the preservation of allicin than the other processes. The amount of allicin lost during the process to obtain paste for the different varieties was less than 9.5%, and it reached a maximum loss of 22% for the commercial varieties during storage (180 days).

    And part of the rest of the story, what if any liquid is present along with the product, with oil being the worst for preservation of allicin:

    And now to the cut version… will find that the chemical conversion to the active allicin degrades quickly and is not the best approach.

    Dr. Alan Kadish moderator for Dr. Greger

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