Should You Be Concerned about Bovine Leukemia Virus in Milk?

Decades ago, concern was raised that the milk of dairy cows frequently contains a leukemia-causing virus—more specifically, bovine leukemia virus (BLV), the leading cancer killer among dairy cattle. Most U.S. dairy herds are infected with the cancer virus. “Thus the question of whether dairy cows naturally infected with BLV release infectious virus into milk is an important public health consideration” and the subject of my video Is Bovine Leukemia Virus in Milk Infectious?.

Researchers at the University of Pennsylvania decided to put it to the test. And indeed, infectious virus was demonstrated in the milk of 17 of the 24 cows tested, indicating that “humans are often orally exposed to BLV.” Just because we’re exposed to it doesn’t mean it’s causing human disease, though. How do we know BLV can even infect human cells? We didn’t until 1976 when it was discovered that BLV can indeed infect human, chimpanzee, and rhesus monkey cells. Nevertheless, that still doesn’t mean BLV necessarily causes cancer in other species.

Researchers can’t lock human infants in a cage and feed them infected milk, but they can cage infant chimpanzees. Chimps Bois and Roger were fed infected milk, developed leukemia, and died. Until then, we didn’t even know chimps could get leukemia. The fact that BLV-infected milk appeared to transmit or induce leukemia in our closest living relatives certainly did raise the stakes, but human beings are not chimpanzees. Yes, our DNA may be 98 percent identical, but we may share 60 percent of our DNA with a banana. We need human studies.

We can’t do interventional trials in this case, thanks to those pesky Nuremberg principles, but what about observational studies? Do cattle farmers have higher rates of cancer? Apparently so. This finding led some to suggest that “milk- and egg-borne viruses may be highly important in the pathogenesis [or development] of human leukemia and lymphoma,” but farmers may be exposed to all sorts of potential carcinogens, such as pesticides. Large animal veterinarians may also have more leukemia and lymphoma, but some are also “particularly lax in the use of X-ray protective equipment,” so it didn’t necessarily have anything to do with viruses.

We needed so-called serology studies, testing people’s blood for antibodies against the virus, which would prove human exposure, and we got them. Ten different studies looked for BLV antibodies in cancer patients and non-cancer patients, creamery employees versus office employees, veterinarians, unpasteurized milk drinkers, and more. “Not one of these studies found a single individual with antibodies to BLV…” As a result, in 1981, the case was closed: “Therefore, there is strong serological evidence to indicate that BLV is not transmissible to man.” However, the strength of the evidence is only as strong as the strength of the test. Chimpanzees Bois and Roger didn’t develop detectable antibodies either, and they died from BLV.

The tests available a handful of decades ago were not really sensitive. “Clearly, the question of whether BLV poses a public health hazard deserves thorough investigation” using highly sensitive molecular probes. It would take a few decades for us to get such an examination, and I discuss those landmark findings in my videos The Role of Bovine Leukemia in Breast Cancer and Industry Response to Bovine Leukemia Virus in Breast Cancer.


Thankfully, feline leukemia virus does not appear to be transmissible. See Pets and Human Lymphoma.

In health,
Michael Greger, M.D.

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

The Healthiest Way to Eat Paleo

There have been about a half dozen studies published on Paleo-type diets, starting around 20 years ago. For example, in what sounds like a reality TV show: ten diabetic Australian Aborigines were dropped off in a remote location to fend for themselves, hunting and gathering foods like figs and crocodiles.

In Modern Meat Not Ahead of the Game, my video on wild game, I showed that kangaroo meat causes a significantly smaller spike of inflammation compared to retail meat like beef. Of course, ideally we’d eat anti-inflammatory foods, but wild game is so low in fat that you can design a game-based diet with under 7 percent of calories from fat. Skinless chicken breast, in comparison, has 14 times more fat than kangaroo meat. So you can eat curried kangaroo with your cantaloupe (as they did in the study) and drop your cholesterol almost as much as eating vegetarian.

So, how did the “contestants” do? Well, nearly anything would have been preferable to the diet they were eating before, which was centered on refined carbs, soda, beer, milk, and cheap fatty meat. They did pretty well, though, showing a significantly better blood sugar response—but it was due to a ton of weight loss because they were starving. Evidently, they couldn’t catch enough kangaroos, so even if they had been running around the desert for seven weeks on 1,200 daily calories of their original junky diet, they may have done just as well. We’ll never know, though, because there was no control group.

Some of the other Paleo studies have the same problem: They’re small and short with no control groups, yet still report favorable results. The findings of one such study are no surprise, given that subjects cut their saturated fat intake in half, presumably because they cut out so much cheese, sausage, or ice cream. In another study, nine people went Paleo for ten days. They halved their saturated fat and salt intake, and, as one might expect, their cholesterol and blood pressure dropped.

The longest Paleo study had been only 3 months in duration, until a 15-month study was conducted—but it was done on pigs. The pigs did better because they gained less weight on the Paleo diet. Why? Because they fed the Paleo group 20 percent fewer calories. The improvement in insulin sensitivity in pigs was not reproduced in a study on people, however. Although, there were some benefits like improved glucose tolerance, thanks to these dietary changes: The Paleo group ate less dairy, cereals, oil, and margarine, and ate more fruits and nuts, with no significant change in meat consumption.

A follow-up study also failed to find improved glucose tolerance in the Paleo group over the control group, but did show other risk factor benefits. And no wonder! Any diet cutting out dairy, doughnuts, oil, sugar, candy, soda, beer, and salt is likely to make people healthier and feel better. In my video Paleo Diet Studies Show Benefits, you can see a day’s worth of food on the Standard American Diet, filled with pizza, soda, burgers, processed foods, and sweets, versus a Paleo diet, which, surprisingly, has lots of foods that actually grew out of the ground.

But the Paleo diet also prohibits beans. Should we really be telling people to stop eating beans? Well, it seems hardly anyone eats them anyway. Only about 1 in 200 middle-aged American women get enough, with more than 96 percent of Americans not even reaching the minimum recommended amount. So telling people to stop isn’t going to change their diet very much. I’m all for condemning the Standard American Diet’s refined carbs, “nonhuman mammalian milk”, and junk foods, but proscribing legumes is a mistake. As I’ve noted before, beans, split peas, chickpeas, and lentils may be the most important dietary predictor of survival. Beans and whole grains are the dietary cornerstones of the longest living populations on Earth. Plant-based diets in general and legumes in particular are a common thread among longevity blue zones around the world.

The bottom line may be that reaching for a serving of kangaroo may be better than a cheese danish, “but foraging for…[an] apple might prove to be the most therapeutic of all.”


I’ve reported previously on Paleo’s disappointing results in Paleo Diets May Negate Benefits of Exercise.

The underlying philosophy behind “caveman” diets may be flawed in the first place. See:

So, What’s the Natural Human Diet? Watch the video!

The wild game video I mentioned is Modern Meat Not Ahead of the Game. Kangaroo is kind of the Australian version of venison. Note that it also matters how the animals are killed. See Filled Full of Lead and Lead Contamination in Fish and Game.

And, for more on the musical fruit, 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, year-in-review presentations:

Beer Phytoestrogens

Why do alcoholic men develop so-called man boobs and other feminine traits? We know estrogens produce feminization, and our liver clears estrogens from the body. As such, the original theory was that alcohol-induced liver damage led to the retention of excess estrogens. The problem was that when researchers measured estrogen levels, they weren’t elevated. What’s more, even those with cirrhosis of the liver appeared to clear estrogens from the body normally, and men’s testicles started shrinking even before serious liver disease developed.

So, alternative explanations were considered. If it’s not due to estrogens produced endogenously, meaning within the body, maybe alcoholics are being exposed to “exogenous estrogenic substances from dietary sources”—perhaps from phytoestrogens in the plants that alcoholic beverages are made from. The discovery that plants could contain hormonal compounds was made back in 1951 by two Australian chemists charged with finding out the cause of an “epidemic of infertility in sheep that was ravaging their nation’s wool industry.” It took them ten years, but they finally figured out the cause: a compound called genistein, present in a type of clover, and the same phytoestrogen found in soybeans.

You can read about the dreaded clover disease on scare-mongering websites, but you’ll note they never talk about the difference in dose. To get as much as the sheep were getting from clover, you’d have to drink more than 1,000 cartons of soymilk a day or eat more than 8,000 soy burgers or about 800 pounds of tofu a day.

This is not to say you can’t overdo it. There are two case reports in the medical literature that describe feminizing effects associated with eating as few as 14 to 20 servings of soy foods a day. But at reasonable doses, or even considerably higher than the one or two servings a day Asian men eat, soy phytoestrogens do not exert feminizing effects on men.

So, back in 1951, we realized plant compounds could be estrogenic. Two German researchers realized that perhaps that’s why women who handle hops start menstruating, and, indeed, they found estrogenic activity in hops, which is the bittering agent used to make beer. They found trace amounts of the soy phytoestrogens, but in such tiny quantities that beer would not be expected to have an estrogenic effect. In 1999, however, a potent phytoestrogen called 8-prenylnaringenin was discovered in hops, which I discuss in my video The Most Potent Phytoestrogen Is in Beer. In fact, it’s the most potent phytoestrogen found to date, fifty times more potent than the genistein in soy, “provid[ing] an obvious explanation for the menstrual disturbances in female hop workers in the past.” Today, we have machines to pick our hops, so our only exposure is likely via beer consumption, but the levels in beer were found to be so low that they shouldn’t cause any concern.

Then in 2001, a study on a hops-containing “dietary supplement for breast enhancement” raised the concern that another phytoestrogen in hops called isoxanthohumol might be biotransformed by our liver into the more potent 8-PN, which would greatly augment the estrogenic effect of hops. This study was conducted on mice, though. Thankfully, a study using human estrogen receptors found no such liver transformation, so all seemed fine…until 2005. “[T] he liver is not the only transformation site inside the human body.” The human colon contains trillions of microorganisms with enormous metabolic potential. It’s like a whole separate organ within our body, with a hundred livers’ worth of metabolizing power. So, let’s effectively mix some beer with some poop and see what happens.

Indeed, up to a 90 percent conversion was achieved. Up to then, “the concentration of 8-PN in beer was considered too low to affect human health. However, these results show that the activity of the intestinal microbial community could more than 10-fold increase the exposure concentration.” This can explain why you can detect 8-PN in the urine of beer-drinkers for days: Their gut bacteria keep churning it out. Obviously, the amount of straight 8-PN in beer is not the only source of estrogen effects given this conversion. So, a decade ago, the question remained: Might drinking too much beer cause estrogenic effects and feminize men? See my video What Are the Effects of the Hops Phytoestrogen in Beer? for the update.


Other videos on phytoestrogen include:

What about GMO soy? See GMO Soy and Breast Cancer.

For menstrual health videos, 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, year-in-review presentations: