Are Avocados Associated with Greater Risk or Reduced Risk of Cancer?

Avocado consumption can improve artery function, but what effect might guacamole have on cancer risk?

In my last video about avocados, The Effects of Avocados and Red Wine on Meal-Induced Inflammation, I described their anti-inflammatory effects and cholesterol- and triglyceride-lowering effects, but what about the Are Avocados Good for You? video I did years ago about the chromosome-damaging effects in a petri dish? That goes back to 1975, when a pesticide naturally produced by the avocado tree was discovered, thought to explain why lactating livestock suffer mammary gland damage after nibbling on the leaves. The toxin, named persin, was also found to be damaging to the heart, which is why you should never feed avocado to your pet birds.

But, if persin attacks mammary cells in animals, might it attack breast cancer cells in humans? As you can see at 0:52 in my video Are Avocados Healthy?, it did seem to have the same kind of cellular cytoskeleton-clumping effect in vitro that chemotherapy can have, demonstrating potent cell growth stopping and killing effects of the novel plant toxin among various lines of human breast cancer cells. So, researchers are thinking about how it might one day be used as chemo itself, but I’m thinking, Holy guacamole, Batman! Please tell me it doesn’t have toxic effects on normal cells, too.

We got an answer in 2010 with an evaluation of the genotoxicity—the toxicity to our chromosomes—of avocado extracts on human white blood cells in a petri dish. As you can see at 1:35 in my video, normally, less than 10 percent of our dividing cells have any chromosome abnormalities, but if you drip some avocado fruit extracts on them, up to half come out defective in some way. The researchers concluded that there’s something in avocado fruit that “can potentially induce significant genomic instability and some genetic damage in human lymphocytes in vitro,” that is, in white blood cells in a petri dish. If the same effect occurs in actual people, it could, for example, result in transforming cells into cancer. That is a big if, though. These were blood cells. You don’t inject guacamole into the vein. For something to get into our bloodstream, it first has to survive our stomach acid, get absorbed through our intestines, and then sneak past our liver’s detoxification enzymes. And indeed, persin may be affected, changed by acidic conditions. So, given all the differences between what happens in a petri dish and inside a person, it’s essential to carry out further studies “before making a final remark on the genotoxicity.” Sounds reasonable, but what do you do before these studies come out? I was concerned enough that I provisionally moved avocados from being a don’t-hold-back green-light food to a moderate-your-intake yellow-light food to err on the side of caution until we knew more.

Even if persin were utterly destroyed by stomach acid, what about oral cancer? As you can see at 3:01 in my video, avocado extracts at high enough concentrations can harm the growth of the kinds of cells that line our mouths. This was in a petri dish, though, where the avocado is coming in direct contact with the cells—but that’s also kind of what happens in your mouth when you eat it. However, it harms oral cancer cells even more. At 3:32 in my video, you can see a bunch of oral cancer cells. In the first image, the mitochondria, the power plants of the cells fueling cancer growth, are seen in red. In the second image, you can see they’ve been extinguished by the avocado extract—no more red-colored mitochondria. Since it does this more to cancerous cells than normal cells, the researchers conclude that avocados may end up preventing cancer.

What about the esophagus, which lies between the mouth and the stomach? Researchers similarly found that an avocado fruit extract appeared to inhibit cancer cell growth more than normal cell growth when it came to both colon cancer cells and esophageal cancer cells, as you can see at 3:53 in my video. But, rather than comparing the effects to normal colon and esophagus cells, they compared them to a type of blood cell, which, again, is of limited relevance in a petri dish study of something you eat.

A study I found to be pretty exciting looked at p-cresol, which is a “uremic toxin” and may also be toxic to the liver. “Found to be associated with autism,” it comes from eating high-protein diets, whereas if you eat a more plant-based diet, which is the only source of prebiotics like fiber and resistant starch, your levels go down. See, fermentation of carbohydrates in the colon, like fiber, is considered beneficial, whereas fermentation of protein, which is called putrefaction, is considered detrimental. So, if you switch people to a high-protein diet, within days, the excess protein putrefying in their gut leads to an increase in ammonia as well as p-cresol—in fact, a doubling of levels within a week. But, might phytonutrient-rich plant foods, like apples, cranberries, grapes, or avocados, protect the cells lining our colon “from the deleterious effects of p-cresol…in terms of cell viability, mitochondrial function, and epithelial integrity,” meaning protection against gut leakiness? At 5:12 in my video, I show the data on barrier function integrity. You can see that it is damaged by p-cresol, but rescued by all the cranberry, avocado, grape, and apple extracts. Mitochondrial function, however, was only improved by the cranberries and avocados, which also were the only ones that appeared to prevent the deleterious effect of p-cresol on colon cell viability. The bottom line, though, is that avocados appear to have beneficial effects on colon lining cells. Okay, but enough of these in vitro studies, already. Yes, an avocado extract can inhibit cancer cell growth in a petri dish, but unless you’re doing some unspeakable things to that avocado—like guacamole with benefits—there’s no way that avocado is going to come in direct contact with your prostate cells. So, what does this study mean?

This is why I was so excited to see the first study to actually look for a link between avocado consumption and prostate cancer. Actual human beings eating avocados! So, do avocado eaters have more cancer risk or less cancer risk? Men who ate the most avocado, more than about a third of an avocado a day, had reduced risk of prostate cancer—in fact, less than half the odds. So, with the data on improved artery function, lower cholesterol, and, if anything, an association with decreased cancer risk, I’d suggest moving avocados back up with the other green-light foods.


How Not to Die from Cancer is an overview on dietary approaches to cancer prevention and treatment, and I also have hundreds of videos on all of the common cancers.

What if you’ve already been diagnosed? Even if you already have prostate cancer, for example, it’s not too late to improve your diet. See: 

In health, 

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations:

Natural Dietary Treatments for Fibroids

The same diet that helps regulate hormones in women may also reduce exposure to endocrine-disrupting pollutants.

Fibroids are the most common benign tumors in women. They can grow to a foot in diameter and affect the majority of women before they hit menopause. Although fibroids tend to be asymptomatic, when symptoms do occur, they tend to manifest as heavy menstrual bleeding—so much so that women may get anemic and experience a lot of pain. So, what can women do? I discuss this in my video The Best Diet for Fibroids.

Up to half go into surgery and get their entire uterus removed. “Although hysterectomy is generally considered a safe operation, complications occur in a significant proportion of patients” and, obviously, you can’t have kids any more. The alternative is a variety of hormone-modulating drugs, which can shrink the fibroids and provide relief, but many of these drugs have significant side effects, like bone loss, so you really don’t want to be taking them for more than a few months. What’s the bottom line? “There is currently no evidence to support the routine use of medical treatment in women with uterine fibroids.” No wonder many women turn to “complementary and alternative treatments…including exercise, diet, herbs, and acupuncture.”

Women who exercise seven or more hours a week do seem to have lower risk of having fibroids than women who exercise less than around 20 minutes a day, but exercise has never been put to the test for treating fibroids. Likewise, to date, there isn’t a single randomized controlled trial of acupuncture for the treatment of fibroids to help guide us. 

In terms of herbs, there are two Asian herbal preparations that show promise—a five-herb combo called Guizhi Fuling and a Malaysian ten-herb formula that contains “secret ingredients” that must not be that secret since they’re just listed in the study, as you can see at 1:50 in my video—and they seemed to work as well as a leading drug. The problem is that traditional Asian herbal remedies may contain a few extra ingredients, like arsenic, mercury, and lead, which have been detected in most of the samples tested from Asian market and health food store shelves, and not just a little. Some, apparently, had really toxic amounts. So, these two Asian herbal preparations “may reduce fibroid size, but there is insufficient evidence to support the efficacy or safety of these treatments.” And, certainly, don’t try to apply caustic herbs internally, as this can lead to scarring, stenosis, and ulceration.

Well, what about diet? In one of the largest studies of diet and fibroids, fibroid tumors were “associated with beef and ham consumption, whereas high intake of green vegetables seems to have a protective effect.” The researchers figured that the “association between levels of estrogen, diet, and breast and endometrial [uterine lining] cancers also may help us understand” why. Indeed, “[f]or breast and endometrial cancers, a direct association with the frequency of consumption of meat and ham was observed…whereas protection was conferred by high intake of vegetables and fruits.” Thus, there may be these shared risk factors between estrogen-responsive malignant tumors, like breast cancer, and estrogen-responsive benign tumors, like fibroids.

We know the presence of fibroids seems to correlate with an increase in the amount of estrogens flowing through your body, for example, and that women eating vegetarian diets have significantly lower levels of excess estrogen. Researchers are using this knowledge to try to explain why there are lower rates of endometrial cancer—that is, lining-of-the-uterus cancer—and possibly breast cancer among vegetarian women, but it could also help explain the fibroid findings. “The incidence of breast cancer among vegetarian American women (Seventh Day Adventists) is 60 to 80 per cent of the incidence among American women in general, and the incidence among women in Africa and Asia is even lower.” Why might vegetarian women have lower estrogen levels? A famous study in the New England Journal of Medicine concluded that it was their “increased fecal output, which leads to increased fecal excretion of estrogen,” resulting in lower blood levels. Double the fecal output, in fact, as you can see at 4:07 in my video.

And, you can put it to the test. Maybe the same reason African-American women have more fibroids is the same reason they have worse breast cancer survival: too much estrogen in their bloodstream due to a less than optimal diet. So, researchers designed a study to see what would happen if they were switched to a more plant-based, higher fiber diet. Compared with the Caucasian women, the African-American women started out with much higher estrogen levels, again helping to explain their increased mortality from breast cancer. But, after they were put on a healthier diet, all of their levels came down, “suggest[ing] that a substantial reduction in breast cancer risk can be achieved” by adopting a diet centered around more whole plant foods. The same also appears to be true for fibroids, especially eating lots of cruciferous vegetables—broccoli, cabbage, and Chinese cabbage—as well as tomatoes and apples.

Women who underwent premature puberty, starting their periods before age 11, may also be at increased risk of fibroids later in life, and we know that higher childhood red meat intake is associated with earlier age of starting one’s period, though total protein and animal protein in general may contribute. For example, girls who eat meat tend to start their periods about six months earlier than vegetarian girls. Those who eat meat analogues like veggie burgers and veggie dogs start their periods nine months later on average, and a similar puberty normalizing influence was found with consumption of whole plants foods, such as beans. 

It could also be the endocrine-disrupting pollutants that build up the food chain. Researchers took samples of internal abdominal fat from women and found there appeared to be a correlation between the presence of fibroids with the levels of a number of PCBs in their fat. So, does that mean fish-eaters have higher risk of fibroids? Researchers did find a small increase in risk associated with the intake of long-chain omega-3 fats, mostly from “dark-meat fish consumption,” by which they meant fish like sardines and salmon. This could be because of “the endocrine-disrupting chemicals commonly shown in fish,” or it could just be a statistical fluke. It would be consistent with the increased risk seen among “sport-fish consumers.” 

Recognizing that diet and endocrine-disrupting persistent organic pollutants have been associated with a variety of gynecologic conditions, including fibroids, researchers looked at consumers of fish fished out of the Great Lakes and found a 20 percent increased risk for every ten years they had been eating the fish. In the most comprehensive study to date, researchers compared pollutant levels in fat samples from women with fibroids to fat liposuctioned out of women without fibroids. They didn’t just find higher levels of PCBs in fibroid sufferers, but also long-banned pesticides, like DDT and hexachlorocyclohexane, PAHs, which are polycyclic aromatic hydrocarbons formed when coal is burned, tobacco is smoked, and meat is grilled, as well as heavy metals, arsenic, cadmium, lead, and mercury. These levels correlated not only to fibroids, but also to seafood consumption or excess body fat. So, the researchers determined that “shedding excess weight and limiting seafood consumption would confer a protective effect” on fibroid tumor development by minimizing exposure to environmental pollutants as much as possible.

Okay, so a plant-based diet may be best, but is there a plant in particular that has been shown to be particularly powerful?

Plant-based compounds with disease-preventive properties, dietary phytochemicals are found in whole grains, fruits, vegetables, beans, split peas, chickpeas, and lentils, herbs, spices, nuts, and certain beverages. As I discuss in my video The Best Food for Fibroids, we know they can help regulate the initiation, promotion, and spread of cancerous tumors, so what about benign tumors like fibroids? Most anti-cancer drugs on the market now were originally derived from plants or plant products, so why not try to use plants to target the inflammation or blood supply of fibroids? Might fibroids be a consequence of chronic inflammation within the body? We know that women with fibroids are more likely to eat more beef and ham, and fewer fruits and green vegetables, but whole plant foods don’t just have anti-inflammatory effects but antioxidant effects as well. “If the generation of free radicals exceeds the protective effects of antioxidants, oxidative damage will occur,” which has been implicated in a variety of disease states, including gynecological conditions such as fibroids. 

If you collect fresh fibroids, as well as normal uterine tissue from hysterectomy surgeries, the fibroid cells have significantly fewer antioxidant enzymes, as you can see at 1:20 in my video, so might antioxidant-rich foods help? Well, if you drip some strawberries onto cells in a petri dish, you can apparently kill of some fibroid tumor cells, while leaving normal uterus cells alone. But, what good does that do us? That’s only relevant if we can show those strawberry compounds get absorbed through our gut and achieve high enough concentrations in uterine tissue. The same with curcumin, the component of the spice turmeric. One of its so-called “miraculous” properties is suppressing the growth of uterine fibroid cells, but, again, that was just in vitro. Yes, an inhibitory effect was found and at concentrations that don’t compromise the growth of normal, regular uterine tissue, but my patients are people, not petri dishes. 

It’s pretty neat to find out what happens to human fibroid cells as you drip higher and higher concentrations of green tea compounds on them in a test tube, as you can see for yourself at 2:19 in my video, but I care less about what happens in vitro or in mice, whether or not they have any clothes on—one study looked at “a nude mice model”—but there were no randomized, controlled clinical studies until 2013. 

Subjects were randomized to green tea extract or placebo for four months. In the placebo group, fibroid volume increased by 24 percent. That’s what fibroids do; they continue to grow. However, those randomized to the green tea group showed a reduction in total fibroid volume—and not just by a little. There was a dramatic decrease, shrinking by almost a third, which is a highly significant difference, as you can see at 3:02 in my video. Okay, but did the women feel any better? Yes, they experienced a dramatic decrease in symptom severity, as well. Month after month, nothing much happened in the placebo group, but those taking the pills that looked the same but happened to contain green tea compounds had consistent improvement and felt lessening symptoms, each month better than the last, as well as an improved health-related quality of life, month after month, that was significantly better than control. What’s more, their blood counts got better too. With all that continued excess blood loss every month, the blood levels kept decreasing in the placebo group, but they reversed in the green tea group. So, anemia also significantly improved, because average blood flow significantly diminished. And, all this—the fibroid shrinkage, less pain, better periods—was achieved with “no adverse effects.” 

So, not only were the results comparable to those for the drugs that are commonly used—again, without the side effects—but the results were also comparable to uterine artery embolization, where they try to cut the blood supply to the fibroid, which is great—unless they accidentally cut the blood supply to the rest of the uterus and cause uterine necrosis, one of many reported major complications. Others include death, not only of the fibroid, but also of the patient, along with other potential complications that may arise from accidentally clogging off non-target arteries. In my book, a side-effect-free solution as good as a more invasive procedure is potentially better than. The researchers conclude that green tea compounds show “promise as a safe and effective therapeutic agent for women with symptomatic UFs [uterine fibroids]. Such a simple, inexpensive, and orally administered therapy can improve women’s health globally.” 

Relatively safe doesn’t mean risk-free, however. Although there were no liver function abnormalities detected, this was a small study. If you give green tea extract pills to a thousand women for a year, like they did in the Minnesota Green Tea Trial for breast cancer, the livers in about 1 in 17 women started to get inflamed and a few became serious. Now, the dose they used in this study was twice that of the fibroid study and it’s not completely clear if the pills were the only cause, but, in general, we should try to avoid extracts and instead get nutrition from foods as grown—or at least from foods as grown that are then dunked in hot water, like green tea. 

The researchers had to use pills in this study, because they wanted it to be a double-blind study and it’s hard to create a placebo tea that looks, smells, and tastes like the real thing. I don’t think we should take green tea extract pills, though. We should drink green tea. The problem is that the dose the researchers used was about 11 cups a day, which would be a lot of caffeine. You could choose decaf, though, and it’s not outside the realm of possibility to drink a couple quarts of tea a day, especially if doing so may shrink your fibroids so much you can keep your uterus. But, for all we know, five cups of tea a day would work or maybe even three cups or one cup. No other dose has been tested, so we just don’t know. But, you can test it in your own life. If you have fibroids, it couldn’t hurt to add a few cups of green tea to your daily diet and see if you start feeling better.

And, for even more on fibroids, see Should Women with Fibroids Avoid Soy? and Talcum Powder and Fibroids.

For more on contaminated herbal products, see Get the Lead Out and Some Ayurvedic Medicine Worse Than Lead Paint Exposure.


I’ve got dozens and dozens of videos on the effects of diet on estrogens, such as:

In health, 

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations:

Are Lectins in Food Good or Bad for You?

Might lectins help explain why those who eat more beans and whole grains have less cancer?

Lectins are to blame for the great “white kidney bean incident” of 2006 in Japan. One Saturday evening, a TV program introduced a new method to lose weight. The method was simple: toast some dry, raw, white kidney beans in a frying pan for three minutes, grind the beans into a powder, and then dust it onto rice. Within days, a thousand people fell ill, some with such severe diarrhea and vomiting they ended up in the hospital. Why? Lectin poisoning. Three minutes of dry heat is not enough to destroy the toxic lectins in kidney beans. If you don’t presoak them, you need to boil large kidney beans for a full hour to completely destroy all the lectins, though if you first soak them overnight 98 percent of the lectins are gone after boiling for just 15 minutes and all are gone by half an hour, as you can see at 0:44 in my video Are Lectins in Food Good or Bad for You?. And, indeed, when researchers tested the white beans, they found that toasting them for three minutes didn’t do a thing. It’s no wonder people got sick. But, 95 percent of the lectins were inactivated after boiling them for three minutes and completely inactivated after ten minutes of boiling. Evidently, “‘Do not eat raw beans’ is a traditional admonition in Japan to prevent intestinal problems”—and now we know why.

While canning may completely eliminate lectins from most canned beans, some residual lectin activity may remain in canned kidney beans, though apparently not enough to result in toxicity. And, ironically, “How doses of lectins may be beneficial by stimulating gut function, limiting tumor growth, and ameliorating obesity.” What? I thought lectins were toxic.

For as long as people have speculated dietary lectins are harmful, others have conjectured that they may be protective. “If this theory is correct, appropriate lectins by mouth should be of use in the prophylaxis [prevention] (and possibly treatment) of colon cancer.” Or, of course, we could just eat our beans.

Interest in the purported antitumor effect of plant lectins started with the discovery in 1963 that lectins could distinguish between cancer cells and normal cells. Researchers at Massachusetts General Hospital found a substance in wheat germ—the lectin in whole wheat—that appeared to be “tumor cell specific,” clumping together the tumor cells, while the normal cells were left almost completely alone. In fact, it is so specific that you can take a stool sample from someone and, based on lectin binding to the colon lining cells that get sloughed off into the feces, effectively predict the presence of polyps and cancers.

Subsequently, it was discovered that lectins couldn’t only distinguish between the two types of cells, but also extinguish the cancer cells, while largely leaving the normal cells alone. For example, that same white kidney bean lectin, as you can see at 2:53 in my video, was found to almost completely suppress the growth of human head and neck cancer cells, liver cancer cells, breast cancer cells, and cervical cancer cells (at least most of the way), within about three days—but that was in a petri dish. Those petri dish studies are largely the basis of the evidence for the antitumor activity of plant lectins. How do we even know dietary lectins are absorbed into our body?

Colorectal cancer is one thing. The fact that lectins can kill off colon cancer cells in a petri dish may be applicable, since lectins we eat may come in direct contact with cancerous or precancerous cells in our colon, “providing a mechanism” by which bean consumption may help in “the prevention and treatment of colorectal cancer.” Even more exciting is the potential for effectively rehabilitating cancer cells. The “loss of differentiation and invasion are the histological hallmarks of malignant cells,” meaning that when a normal cell transforms into a cancer cell, it tends to lose its specialized function. Breast cancer cells become less breast-like, and colon cancer cells become less colon-like. What these researchers showed—for the first time—is that the lectin in fava beans could take colon cancer cells and turn them back into looking more like normal cells. As you can see at 4:13 in my video, before exposure to the fava bean lectins, the cancer cells were growing in amorphous clumps. But, after exposure to the fava bean lectins for two weeks, those same cancer cells started to go back to growing glandular structures like normal colon issue. Therefore, dietary lectins or putting them in a pill “may slow the progression of colon cancer,” potentially helping to explain why dietary consumption of beans, split peas, chickpeas, and lentils appears to reduce the risk of colorectal cancer based on 14 studies involving nearly two million participants. Okay, but what about cancers outside of the digestive tract?

“Although lectin containing foods,” like beans and whole grains, “are frequently consumed cooked or otherwise processed, these treatments may not always inactivate the lectins…For example, lectins have been detected in roasted peanuts….” Peanuts are legumes, and we don’t tend to eat them boiled but just roasted or even raw. Are we able to absorb the lectins into our system? Yes. As you can see at 5:12 in my video, within an hour of consumption of raw or roasted peanuts, you can detect the peanut lectin in the bloodstream of most people. Same with tomatoes. Some of the non-toxic lectin in tomatoes also makes it down into our gut and into our blood. Wheat germ agglutinin, the wheat lectin known as WGA, doesn’t seem to make it into our bloodstream, though, even after apparently eating the equivalent amount of wheat germ in more than 80 slices of bread. And, if you ate something like pasta, the boiling in the cooking process might wipe out the lectin in the first place anyway.

In terms of phytochemicals in the fight against cancer, lectins are able to “resist digestion resulting in high bioavailability,” potentially allowing “the cellular mechanisms of the host to utilize the full potential of the…dramatic anti-cancer benefits” lectins have to offer. But, these dramatic benefits have yet to be demonstrated in people. We do know, however, that population studies show “that the consumption of a plant-based diet is strongly associated with a reduced risk of developing certain types of cancer.” People eating a plant-based diet could just be eating fewer carcinogens, but plants do have all those active components that do seem to protect against the “initiation, promotion, or progression” of cancer. So, maybe lectins are one of those protective compounds. We know people who eat more beans and whole grains tend to get less cancer overall, but we’re just not sure exactly why. Now, you could say, “Who cares why?” Well, Big Pharma cares. You can’t make as much money on healthy foods as you can on “lectin based drugs.”

Interested in learning more about lectins? Check out my videos Dr. Gundry’s The Plant Paradox Is Wrong and How to Avoid Lectin Poisoning.


Lectins remind me of the story about phytates. Other components of beans and whole grains, phytates were thought at first to be harmful, but, more recently, evidence is coming to light that suggests the opposite may be true. Check out Phytates for Rehabilitating Cancer Cells and Phytates for the Prevention of Osteoporosis.

What else may explain the protective effect of beans? See, for example, Gut Dysbiosis: Starving Our Microbial Self. Soybeans may be particularly protective against certain cancers, as you can see in BRCA Breast Cancer Genes and Soy.

In health,

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations: