This Just In About Avocados

Health!  Wealth!  Happiness!  I’m Dr. Michael Greger and you’re listening to the Nutrition Facts podcast.  And while I can’t promise you all of those things, if you take a listen to the evidence-based nutrition found in this podcast, chances are you’ll learn something that you can use to make a positive change in your diet and in your health.  My job here is to bring you the information you need to make that reality possible.

On today’s show, we have late breaking news about…avocados!  Who would have thought avocados would make headlines? Well, today they do. When paired with greens, healthy sources of fat, such as from avocados, may maximize nutrient absorption, as many of the nutrients greens are famous for are fat-soluble, including beta-carotene, lutein, vitamin K, and zeaxanthin.  Try spelling that one in your next spelling bee! 

So, we had to ask, can guacamole lower your cholesterol as well as other whole-food sources of fat, such as nuts, or is it just avocado industry spin?

If you look at Avocado Board-sponsored reviews, they like to brag that “[a]vocados are the richest known fruit source of phytosterols,” which are cholesterol-lowering nutrients found in plant foods. The operative word, though, is fruit.

Yes, there are more phytosterols in avocados compared to other fruit, but the reason that’s such a misleading statement is that phytosterols are fat-soluble substances; most other fruits hardly have any fat in them at all. So, of course, avocados are going to come out on top, compared to other fruit. But, let’s compare phytosterol content of avocados to nuts and seeds. One avocado has about a hundred milligrams of phytosterols. But, on the same scale, sesame seeds and tahini have 400; pistachios, pumpkin seeds, and sunflower seeds have about 300; and almonds and almond butter, flax seeds, and Macadamia nuts have around 200. Even chocolate has about twice as many phytosterols as avocado.

Even though nuts and seeds have the highest levels overall, the studies that have been done on lowering cholesterol—lowering LDL (bad cholesterol) with phytosterols—have used supplements, starting at 600 mg up into the thousands. So, yeah, you can lower LDL cholesterol about 8% at up around 2,100 mg, but that would be 20 avocados a day. That would also be a lot of nuts. But, you can get an 8% drop in LDL just eating a palmful of nuts a day—a single ounce.

So, phytosterols are not the only components of nuts responsible for driving down cholesterol; there must be other components in nuts—like maybe the fiber, or other phytonutrients—that are contributing to the cholesterol-lowering effects. Hmm; I wonder if avocados have such components too. You don’t know, until you put it to the test.

There are studies dating back more than a half century that appear to show that if you add an avocado to people’s daily diets, their cholesterol drops, and then goes back up when you remove the avocados, then goes back down again. Pretty convincing data—until you see how the study was done. They didn’t just add an avocado, they swapped out animal fat. No wonder their cholesterol went down! And, that’s what nearly all avocado cholesterol studies are like.

Ten studies involving hundreds of people, and put them all together. And, it looks like adding avocados led to a significant drop in cholesterol and triglycerides—an average of about a 17-point drop in bad cholesterol. But, these were nearly all strictly substitution studies, where they removed saturated fat from people’s diets, and substituted in avocados. Well, of course, if you cut down on saturated animal fat, your cholesterol is going to drop.

You can tell this review was not funded by the avocado industry, because they point this out: “it is important to note that substituting avocados for saturated dietary fats as opposed to adding avocado to an already established baseline diet poses the greatest benefit.” Just adding avocado may confer no cholesterol benefits at all.

So, yeah, the avocado industry is right in saying that avocados are “a healthy substitute for butter/margarine, cheese, [and] cream cheese”—but that’s a pretty low bar.

In our next story we explore the effects of oatmeal, walnuts, extra virgin oil, and avocados on LDL cholesterol size.

When one sees headlines like, “Avocados Could Improve Your Cholesterol,” they’re largely talking about substitution experiments, where avocado is added to the diet by replacing animal fats. So, no wonder cholesterol goes down. So, for example, if you take people eating a standard North American diet, including animal fats—dairy and poultry are the two greatest contributors of cholesterol-raising saturated fat intake—just add avocado to their diet without doing anything else, and cholesterol does not go down. But, add avocado while reducing saturated fat intake, and cholesterol falls—but no more than just reducing saturated fat while adding nothing.

Okay, but what if you eat no meat at all, versus no meat with avocado added? They took people with sky-high cholesterol—up around 300—and switched them to a relatively low-fat vegetarian diet, with about 20% of calories from fat, versus a vegetarian diet with added avocado—bringing it up to more of a typical fat content: 30% of calories from fat. This group started out with LDLs through the roof, and while cutting out meat may have helped, cutting out meat and adding avocado seemed to help even more. And, it may help best with the worst type of LDL.

As I’ve touched on before, all LDL cholesterol is bad cholesterol, but large, fluffy LDL may only increase the odds of cardiac events—like heart attacks—31%, whereas small, dense LDL is even worse.

Feed people lots of oatmeal and oat bran, and not only does their LDL go down overall, but it specifically brings down the worst of the worst. Add walnuts to a low-fat diet, and not only does LDL go down, but the size distribution of the LDL shifts to a little more benign as well. And, if you put people on a plant-based diet with lots of fiber and nuts, you can get a massive 30% drop in LDL, comparable to a cholesterol-lowering statin drug. And, this includes the small, dense, most dangerous LDL. Note: this does not happen with extra-virgin olive oil. So, it’s not just a monounsaturated fat effect.

In the famous PREDIMED study, those randomized to extra nuts got a significant drop in the smallest, densest LDL, but those randomized to the extra-virgin olive oil group did not. So, there appears to be some special components in nuts that lowers the worst of the worst. Do avocados offer similar benefits? We didn’t know…until, now.

“…[t]he first randomized controlled feeding trial” to look at avocados and LDL size; what they did was remove animal fat from people’s diet, and replaced it with either carbs, or avocado, or vegetable oils that had a similar fat profile to the avocado. So, the two latter diets were very similar diets, but one had the nutrients unique to the avocado, and the other didn’t. What happened?

Well, any time you drop saturated fat, you’re going to bring down LDL—whether you replace animal fat with plant fat (oil, in this case) or with carbs. But, what if you replace animal fat with the whole plant food avocado? An even better effect. And, to see why, they broke the LDL down into large versus small. They all brought the dangerous, large LDL down, but the avocado had the additional effect of also bringing down the super-dangerous small LDL. That’s where that extra drop came from.

So, it’s not just a matter of replacing animal fat with plant fat; there are additional benefits to the fiber and phytonutrients of whole plant foods, like avocados.

Oh, there’s something good in avocados? Well then, let’s just add avocado extracts to the meat! Incorporating avocado extracts into pork patties evidently reduces cholesterol oxidation products, “well documented” to be toxic, carcinogenic, and atherosclerotic—but less so, apparently, with some avocado mixed in.   

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

In my last video about avocados, I described the anti-inflammatory effects and cholesterol- and triglyceride-lowering effects. But what about that video I did years ago about the chromosome-damaging effects in a Petri dish? That all goes back to 1975, when a pesticide naturally produced by the avocado tree was discovered—thought to explain why lactating livestock suffered mammary gland damage nibbling on the leaves. The toxin was named persin, also found to be damaging to the heart, which is why you should never feed avocado to your pet birds.

But hey, if it attacks mammary cells in animals, might it attack breast cancer cells in humans? It did seem to have the same kind of cell 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, they’re thinking about how it might one day be used as chemo itself. But, here I am thinking, “Holy guacamole, Batman; please tell me it doesn’t have toxic effects on normal cells, too.”

In 2010, we got an answer: an “Evaluation of [the] Genotoxicity,” the toxicity to our chromosomes, of avocado extracts on human white blood cells in a Petri dish. Normally, less than 10% of our dividing cells have any chromosome abnormalities, but drip some avocado fruit extracts on them, and up to half come out defective in some way. They conclude that there’s something in avocado fruit that “can potentially induce significant genomic instability and some genetic damage” in human 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’s a big if, though.

These were blood cells. You don’t inject guac into the vein. For anything 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. And, so given all the differences between what happens in a Petri dish and a person, “it is essential to carry out [further] studies…before making a final remark” about its toxicity. Okay, but what do you do before these studies come out? I was concerned enough, I provisionally moved it from a stuff-your-face green-light food, to a moderate-your-intake yellow-light food, until we knew more, to err on the side of caution.

Even if persin was utterly destroyed by stomach acid, what about oral cancer?  At high-enough concentrations, avocado extracts can harm the growth of the kinds of cells that line our mouths. Yeah, but this is in a Petri dish, where the avocado is coming in direct contact with the cells. But that’s kinda what happens in your mouth when you eat it. But, it harms oral cancer cells even more. Here’s a bunch of oral cancer cells. Those red dots are the mitochondria—the power plants of the cells fueling cancer growth, extinguished by the avocado extract. But, since it does this more to cancerous than normal cells, they end up concluding avocados may end up preventing cancer.

What about the esophagus, which lies between the mouth and the stomach? They similarly found that an “avocado fruit extract [appeared to inhibit] cancer cell growth” more than normal cell growth, when it came to colon cancer cells, or esophageal cancer cells. But, rather than comparing the effects to normal colon and esophagus cells, they compared to a type of blood cell, which, again, is limited relevance in a Petri dish study of something you eat.

This study was pretty exciting, though. It looked at p-cresol, which is a “uremic toxin,” may also be toxic to the liver, has been found associated with autism, and it comes from eating high-protein diets, whereas if you eat a more plant-based diet, 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, you switch people to a high-protein diet, and 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 like protection against gut leakiness?

Here’s that data on barrier function integrity, damaged by p-cresol, but rescued by all the cranberry, avocado, grape, and apple extracts, though mitochondrial function was only improved by the cranberries and avocados, and they were also the only ones that appeared to prevent the “deleterious effect of p-cresol” on colon cell viability. But, bottom line, avocados appear to have beneficial effects on colon-lining cells; so, that’s a good sign.

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 the avocado is going to come in direct contact with your prostate cells. So, what does this study mean?

That’s why I was so excited to see the first to actually look for a link between “avocado consumption [actual human beings eating avocados] and prostate cancer.” So, do avocado eaters have more cancer risk, or less cancer risk? We’ll find out…right now!

Men who ate the most avocado, more than about a third of an avocado a day, “reduced [their] 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 it back into the green zone.

To see any graphs, charts, graphics, images or studies mentioned here, please go to the Nutrition Facts podcast landing page.  There you’ll find all the detailed information you need – plus links to all the sources we cite for each of these topics.

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Thanks for listening to Nutrition Facts. I’m Dr. Michael Greger.

This is just an approximation of the audio content, contributed by Allyson Burnett.