What Meat and Eggs Do to Our Microbiome

As I’ve explored before, whether young or old, male or female, smoker or non-smoker, with high blood pressure or low blood pressure, high cholesterol or low, having high levels of a toxic compound called TMAO—trimethylamine oxide—in the bloodstream is associated with a significantly higher risk of having a heart attack, stroke, or dying over a three-year period. Where does TMAO come from? As I investigate in my video How to Develop a Healthy Gut Ecosystem, the choline in foods like eggs can be turned by gut bacteria into TMAO, which is then absorbed back into our system. And, the more eggs we eat, the higher the levels climb, as you can see at 0:41 in my video.

Given the similarity in structure between carnitine and choline, Cleveland Clinic researchers wondered if carnitine found in red meat, energy drinks, and supplements might also lead to TMAO production and put it to the test. As you can see at 1:00 in my video, if you feed omnivores, those who regularly eat meat, a steak, their TMAO levels shoot up. Those who eat strictly plant-based may start out with almost no TMAO in their system, presumably because they’re not eating any meat, eggs, or dairy. But, even if vegans eat a sirloin, still almost no TMAO is made. Why? Presumably, they don’t have steak-eating bacteria in their guts. Indeed, it was found that no TMAO is produced if you don’t have TMAO-producing bacteria in your gut. If you don’t regularly eat meat, then you’re not fostering the growth of the meat-eating microbes that produce TMAO.

This suggests that once we develop a plant-based gut ecosystem, our bacteria will not produce TMAO, even if we eat meat every once and awhile. However, we still don’t know how rapidly gut bacteria shift after a shift in our diet—but it does not appear to be all or nothing. If men eating the standard American diet are given two sausage, egg, and cheese biscuits before and after just five days of eating lots of similarly high-fat meals, their TMAO production boosts even higher, as you can see at 2:09 in my video. So, it’s not just whether we have the bad bugs or not. Apparently, we can breed more of them the more we feed them.

Meat-free diets, on the other hand, can also have been “demonstrated to have a profound influence on human metabolism.”Just by analyzing a urine sample, we can tell what kind of diet people eat, based on measurements like how low TMAO levels are in the urine of those eating egg-free vegetarian diets, as you can see at 2:26 in my video. At 2:43 in my video, you’ll see that we can even take the same people rotate them through three different diets, and determine who is on a high-meat diet, low-meat diet, or no-meat diet, based in part on the different compounds churned out by the different gut flora or different flora activity after just about two weeks on the different diets. It’s possible that some of the beneficial effects of whole plant foods may be mediated by the effects they have on our gut bacteria. At the same time, the standard American diet may increase the relative abundance of undesirables that produce toxic compounds including TMAO (as you can see at 3:07 in my video).

Strictly plant-based diets have gained acceptance as a dietary strategy for preventing and managing disease. Perhaps, in part, this is because of their rather unique gut flora, with less of the disease-causing bacteria and more of the protective species. So, all along, we thought the reason those eating plant-based had lower heart disease rates was because they were eating less saturated fat and cholesterol, but maybe their lower TMAO levels may also be contributing to their benefits, thanks to their reduced ingestion of carnitine and choline.

I talked about the egg industry response to the choline revelation in Egg Industry Response to Choline and TMAO. How has the carnitine supplement industry reacted? In response to the research implicating carnitine in TMAO production, the former vice president of AdvoCare—a multilevel marketing company that sells carnitine supplements like AdvoCare Slam while getting slammed with lawsuits finding them guilty of being “engaged in false, misleading or deceptive acts or practices” and forced to pay more than a million dollars—questioned whether there was a secret vegan conspiracy at the Cleveland Clinic. Restricting our intake of meat or carnitine supplements to prevent our gut bacteria from making TMAO, he argued, is like trying to prevent car accidents by restricting the sale of fuel.

Okay…but there are benefits to transportation. We’re talking about TMAO, which may be fueling our epidemic of heart disease, the number-one killer of men and women in this country. As far as I’m concerned, the more we can cut the fuel for that, the better.


For more background on TMAO, see Carnitine, Choline, Cancer, and Cholesterol: The TMAO Connection, then find out How to Reduce Your TMAO Levels.

Our gut flora are what we eat. Check out:

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:

Balancing the Risks and Benefits of Vitamin C Supplements

Mainstream medicine has long had a healthy skepticism of dietary supplements, extending to the present day with commentaries like “Enough is enough.” In an essay entitled “Battling quackery,” however, published in the Archives of Internal Medicine, it’s argued that we may have gone too far in our supplement bashing, as evidenced by our “uncritical acceptance” of supposed toxicities; the surprisingly “angry, scornful tone” found in medical texts using words like “careless,” “useless,” “indefensible,” “wasteful,” and “insidious”; and ignoring evidence of possible benefit.

“To illustrate the uncritical acceptance of bad news” about supplements, the authors discussed the “well-known” concept that high-dose vitamin C can cause kidney stones, as I highlight in my video Do Vitamin C Supplements Prevent Colds but Cause Kidney Stones? Just because something is well-known in medicine, however, doesn’t mean it’s necessarily true. In fact, the authors couldn’t find a single, reported case.

We’ve known that vitamin C is turned into oxalates in the body, and, if the level of oxalates in the urine gets too high, stones can form, but, even at 4,000 mg of vitamin C a day, which is like a couple gallons’ worth of orange juice, urinary oxalates may not get very high, as you can see at 1:10 in my video. Of course, there may be the rare individuals who have an increased capacity for this conversion into oxalates, so a theoretical risk of kidney stones with high-dose vitamin C supplements was raised in a letter printed in a medical journal back in 1973.

When the theoretical risk was discussed in the medical literature, however, the researchers made it sound as if it were an established phenomenon: “Excessive intake of vitamin C may also be associated with the formation of oxalate stones.” Sounds less like a theoretical risk and more like an established phenomenon, right? That statement had seven citations supposedly suggesting an association between excessive vitamin-C intake and the formation of oxalate kidney stones. Let’s look at the cited sources, which you can see from 1:47 in my video. One reference is the letter about the theoretical risk, which is legitimate, but another listed citation, titled “Jaundice following the administration of niacin,” has nothing to do with either vitamin C or kidney stones. What’s more, the other five citations are just references to books. That may be acceptable if the books cited primary research themselves, but, instead, there was a kind of circular logic, where the books just cite other books citing that theoretical risk letter again. So, while it looks as if there’s a lot of evidence, they’re all just expressing this opinion with no new data.

By that time, there actually were studies that followed populations of people taking vitamin C supplements and found no increased kidney stone risk among men, then later, the same was shown in women. So, you can understand the frustration of the authors of “Battling quackery” commentary that vitamin-C supplements appeared to be unfairly villainized.

The irony is that we now know that vitamin-C supplements do indeed appear to increase kidney stone risk. The same population of men referenced above was followed further out, and men taking vitamin-C supplements did in fact end up with higher risk. This has since been confirmed in a second study, though also of men. We don’t yet know if women are similarly at risk, though there has now also been a case reported of a child running into problems.

What does doubling of risk mean exactly in this context? Those taking a thousand milligrams or so of vitamin C a day may have a 1-in-300 chance of getting a kidney stone every year, instead of a 1-in-600 chance. One in 300 “is not an insignificant risk,” as kidney stones can be really painful, so researchers concluded that since there are no benefits and some risk, it’s better to stay away.

But there are benefits. Taking vitamin C just when you get a cold doesn’t seem to help, and although regular supplement users don’t seem to get fewer colds, when they do get sick, they don’t get as sick and get better about 10 percent faster. And, those under extreme physical stress may cut their cold risk in half. So, it’s really up to each individual to balance the potential common cold benefit with the potential kidney stone risk.


What about intravenous vitamin C? I’ve got a whole video series on that, including:

If you’re not taking vitamin C supplements for pharmacological effects and just want to know how many vitamin C-rich fruits and vegetables to eat every day, check out my video What Is the Optimal Vitamin C Intake?.

Is there anything we can put into our mouth that really might help prevent colds? These videos will point you in the right direction:

And, if you’re interested in learning about the most important steps you can take to prevent and treat kidney stones, look no further than my videos How to Prevent Kidney Stones with Diet and How to Treat Kidney Stones with Diet.

What about high-oxalate vegetables such as rhubarb, spinach, beet greens and swiss chard? I’d encourage a moderation in intake. If you’re going to take my advice to ideally eat cups of dark green leafy vegetables a day I’d recommend sticking with other greens such as kale or collards.

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:

The Foods to Avoid to Lower Stroke Risk

“Stroke remains one of the most devastating of all neurological diseases,” killing about 5 million people a year worldwide, and is “the leading cause of permanent disability in the USA.” But the good news is that about 80 percent of stroke risk may be due to basic lifestyle factors: primarily, improving our diet, stopping smoking, and getting regular exercise.

The best way to stop smoking, evidently, is to have a heart attack. Certainly, once dead, you can’t smoke. Of those who survive a heart attack, strong, repeated advice from their doctor may persuade up to two-thirds to quit and never smoke again in any form as long as they live. “Yes, quitting smoking is very difficult. It doesn’t matter; it has to be done. If you were walking along the lakeshore and one of your grandchildren is drowning, it doesn’t take will power to go into the lake; it just has to be done.” It’s like a healthy diet: Some things just have to be done. Getting up at night to feed a baby can be difficult, too, but it’s not a matter of having willpower—some things in life just have to be done. After all, what we regularly eat every day is indeed a matter of life and death.

For stroke prevention, that means eating a more plant-based diet, like a traditional Mediterranean diet centered around whole grains, fruits, vegetables, lentils, beans, and nuts, as I discuss in my video Best Foods to Reduce Stroke Risk. A vegetarian or vegan diet may also work, but it must be accompanied by a regular, reliable source of vitamin B12, meaning B12-fortified foods or supplements. “Unfortunately, recommending taking B12 supplements may meet opposition among vegetarians because misconceptions regarding this nutrient are prevalent. Many individuals still hold on to the old myth that deficiency of this vitamin is rare and occurs only in a small proportion of vegans…Future studies with vegetarians should focus on identifying ways of convincing vegetarians to routinely take vitamin B12 supplements in order to prevent a deficiency.” The research is clear on that.

What is it about plant-based diets that make them beneficial for stroke prevention? In my video How to Prevent a Stroke, I talked about the role of fiber, which potentially leads to about a 1 percent drop in risk for every 1 gram of fiber ingested per day. Or, even better: A 12 percent drop in risk is associated with every extra 10 grams of fiber a day. In fact, fiber from whole grains is associated with a lower chance of dying not only from heart attack and stroke, but also cancer, diabetes, and respiratory diseases, as well as a lower risk of dying from infections or other causes––in other words, a lower risk of dying prematurely from all causes combined. Why? Perhaps because of the anti-inflammatory effects of fiber, which could explain how it could help across the board. Or, it could be that eating fiber means eating fewer pro-inflammatory foods. Those who eat more whole plant foods, which are where fiber is found, may be eating less processed and animal foods. In fact, the study immediately preceding the meta-analysis of fiber was a meta-analysis on meat, which looked at red meat and processed meat, and found about a 10 percent increased risk for stroke associated with each three and a half ounce daily portion, which is about the size of a deck of playing cards, or about 10 percent increased risk for every “half-deck” of processed meat.

Perhaps this occurs because of the heme iron—the blood and muscle iron—in meat, or because of “its pro-oxidative properties.” (No association was found between stroke and non-heme iron, which is the type of iron that predominates in plants.) Or, perhaps it’s because of some of the toxic pollutants like PCBs that can build up in animal fats. We’ve known, for example, that living next to a toxic waste dump might increase stroke risk, but only recently have we realized that dietary exposure even at so-called safe levels might increase stroke risk—and increase it by as much as eight or nine times for those with the highest levels of these pollutants in their bloodstream.


For more on how to reduce stroke risk with diet, see:

What does vitamin B12 have to do with stroke? Watch my video Vitamin B12 Necessary for Arterial Health to find out.

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: