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:

Should You Get an Annual Physical Exam?

What are the risks and benefits of getting a comprehensive annual physical exam and routine blood testing?

The model of getting an annual physical exam dates back nearly a century in American medicine, but recently, many health authorities “have all agreed that routine annual checkups for healthy adults should be abandoned”—yet, the majority of the public still expects not only a comprehensive annual physical exam, but also extensive routine blood testing. “Given the gap between patients’ enthusiasm for and [the new] guidelines’ skepticism about annual head-to-toe examinations, what are physicians to do?” As I discuss in my video Is It Worth Getting an Annual Physical Exam?, “first, we must educate patients about preventive practices of proven and unproven benefit.” For example, the only routine blood test currently recommended by the USPSTF, the official preventive medicine guidelines setting body, is cholesterol.

The reason “why many physicians continue to perform annual examinations of patients’ hearts, lungs, abdomens and even reflexes and continue to order some of the tests that have been proven ineffectual or even harmful” is because, otherwise, the patient might leave unsatisfied with the visit. “Evidence suggests that the more thorough physicians are (that is, the more physical and laboratory examinations they perform), the better patients feel about their health and their physicians.” So, they are like “placebo clinical manoeuvers…But rather than performing unnecessary (and sometimes contraindicated) physical exams and laboratory tests during an annual visit, perhaps physicians should spend some of the time saved by telling their patients why they are not examining their abdomens, hearts and lungs”—that is, why are aren’t just going to go through the motions like some witch doctor. 

“Most important, we need to educate ourselves about the dangers of overdiagnosis….There will always remain a small possibility that our examination might detect some silent, potentially deadly cancer or aneurysm. Unfortunately for our patients, these serendipitous, life-saving events are much less common than the false-positive findings that lead to invasive and potentially life-threatening tests,” wrote a doctor from the Cleveland Clinic. He went on to share a story about his own father who went in for a checkup. Can’t hurt, right? His dad’s physician thought he felt what might have been an aortic aneurism, so he ordered an abdominal ultrasound. Can’t hurt, right? His aorta was fine, but something looked suspicious on his pancreas, so a CT scan was ordered. Well, that can hurt—it’s a lot of radiation—but thankfully, his pancreas looked fine. But…what’s that on his liver? It looked like cancer, which made a certain amount of sense since his dad had worked in the chemical industry. Realizing how ineffective the treatments were for liver cancer, he realized he was going to die.

His daughter was not ready to give up on him, though, and convinced him to see a specialist. Maybe if they cut it out, he could live a few more years. But first, they had to do a biopsy. The good news was he didn’t have cancer. The bad news, though, was that it was a benign mass of blood vessels, so when they stuck a needle in it to biopsy, he almost bled to death. He required ten units of blood—and ten units is about all you have! This resulted in pain, thus morphine, thus urinary retention, and thus a catheter, yet, thankfully, no infection. Just a bill for $50,000. 

The frustrating thing is that there wasn’t any malpractice or anything in the whole horrible sequence. Every step logically led to the next. “The only way to have prevented this [life-threatening] outcome would have been to dispense with the initial physical examination”—the “checkup” that couldn’t hurt, right?

“Why, then, do we continue to examine healthy patients? First of all, we get paid to do it.” His dad’s initial doctor only received a hundred bucks or so, but just think about all that “downstream revenue” for the hospital and all the specialists. Overdiagnosis is big business.

“Too many patients bear the costs and harms of unneeded tests and procedures,” but without annual check-ups, we doctors would miss out on all those opportunities for “open communication and interpersonal continuity…” Is that so? In that case, replied one physician, if you want communication, why not just take your patients out to lunch? 

“Of course, such lunches should fairly and ethically be preceded by an informed consent discussion that allows prospective diners to understand the risk that they will be infantilized, made dependent, and may well receive unnecessary and injurious diagnostic and therapeutic interventions as a consequence of that grilled cheese and soup”—particularly, I would add, if you’re feeding your patients grilled cheese, having already chalked up your first such “unnecessary and injurious” act!

So, if you don’t have any symptoms or issues, should you even have an annual check-up? That was the subject of my last video, Is It Worth Getting Annual Health Check-Ups?.

Should All Children Have Their Cholesterol Checked, too? Watch the video to find out!

Check out this video to Find Out If Your Doctor Takes Drug Company Money.

I sometimes stumble across these peripheral issues and fall down various rabbit holes. For example, I’ve got a whole series of videos on various diagnostic tests such as mammograms. I don’t want to get too far away from nutrition, but whenever I learn something new and interesting—particularly if there are conflicts of interest trying to muddy the waters—I feel compelled to try to share to help set the record straight.

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:

Should You Get an Annual Health Check-Up?

What are the risks and benefits of getting an annual check-up from your doctor?

Physicians and patients have come to expect the annual check-up as a routine part of care. “However, considerable research has not demonstrated a substantial benefit,” so a “revolt is brewing against the tradition of periodic” check-ups. “Even the Society for General Internal Medicine advised primary care physicians to avoid ‘routine general health checks for asymptomatic adults.’”

As I discuss in my video Is It Worth Getting Annual Health Check-Ups?, routine check-ups do seem to make sense. But, historically, medical practice has included all sorts of interventions that seemed to make sense, such as hormone replacement therapy for menopause—that is, until it was put to the test and found to increase risks of breast cancer, blood clots, heart disease, and stroke. “History repeatedly shows that good intentions and ‘common sense’ kill in the name of prevention (for example, prone sleeping recommendation for infants).” Indeed, doctors killed babies by making the so-called common sense recommendation that infants sleep on their tummies, whereas we now know “Face Up to Wake Up.” “We should always demand evidence rather than succumb to delusion.”

“We check our cars regularly, so why shouldn’t we also check our bodies…?” Well, unlike cars, our bodies have self-healing properties. To see if the benefits outweigh the harms, researchers decided to put it to the test.

“What are the benefits and harms of general health checks for adult populations?” The bottom line is that check-ups were “not associated with lower rates of all-cause mortality, mortality from cardiovascular disease, or mortality from cancer,” meaning they weren’t associated with living longer or a lower risk of dying from heart disease, stroke, or cancer. So, general check-ups may not reduce disease rates or death rates, but they do increase the number of new diagnoses. And, the “[h]armful effects of some tests and subsequent treatment could have balanced out possible beneficial effects of others.”

Possible harms from check-ups include “overdiagnosis, overtreatment, distress or injury from invasive follow-up tests, distress due to false positive test results, false reassurance due to false negative test results, possible continuation of adverse health behaviours due to negative test results, adverse psychosocial effects due to labelling, and difficulties with getting insurance” (now that you have a pre-existing condition), not to mention all of the associated costs. 

Take diabetes, for example. Wouldn’t it be great if we detected cases of diabetes earlier? Perhaps not, if you were one of the people given Avandia, the number one diabetes drug that was then pulled off the market because instead of helping people, it appeared to be killing them. Adverse drug events are now one of our leading causes of death. When it comes to lifestyle diseases like type 2 diabetes, maybe we should focus instead on creating healthier food environments. This is what one of my favorite organizations, Balanced, does to help prevent the diabetes epidemic in the first place.

How many times have you tried to inform someone about healthy eating and evidence-based nutrition, only to have them say, “No, I don’t have to worry. My doctor reassured me I’m fine. I just had a check-up, and everything’s normal.” As if having a normal cholesterol is okay in a society where it’s normal to drop dead of a heart attack, the number one killer of men and women. It would be one thing if you went to see a lifestyle medicine doctor who spent the check-up giving you the tools to prevent 80 percent of chronic disease, but given the way medicine is currently practiced, it’s no wonder why the history of routine check-ups “has been one of glorious failure, but generations of well meaning clinicians and public health physicians struggle to allow themselves to believe it.” But, “policy should be based on evidence…” 

Poor diet may be “on par with tobacco smoking as the most common actual causes of death,” yet the medical profession is inadequately trained in nutrition. Worse, nutrition education in medical school appears to be declining. If you can believe it, there is actually a “shrinking of formalized nutrition education” among health professionals, so the advice you get during your annual check-up may just be from the last tabloid your doctor skimmed while in the supermarket check-out line.

“And screening appointments should not be regarded as a form of ‘health education,’” read one medical journal editorial. “People who are obese know very well that they are, and if we have no means of helping them…then we should shut up.” Well, if you really have nothing to say that will help them, maybe you should shut up, especially those doctors who say they “have no idea what constitutes a ‘healthy’ diet”—although we do know that veggies and nuts are a good start.

Won’t a check-up allow your physician to do a comprehensive physical exam and routine blood testing? I discuss that, as well as the pros and cons, in my vide Is it Worth Getting an Annual Physical Exam?.

Did I say lifestyle medicine? Yes! Learn more about this exciting growing field in Lifestyle Medicine: Treating the Causes of Disease and Convincing Doctors to Embrace Lifestyle Medicine. Make sure your doctor is a member of the American College of Lifestyle Medicine (and even better certified by the American Board of Lifestyle Medicine).

Still don’t understand how there can be risks? See Why Prevention Is Worth a Ton of Cure. Unfortunately, physicians and patients alike wildly overestimate the benefits of pills and procedures. See, for example, The Actual Benefit of Diet vs. Drugs.

The fact is Physicians May Be Missing Their Most Important Tool.


And what about mammograms? See my video series:

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: