Chapter 174
Food Additives to Avoid
The ultra-processed industrial foods that make up the majority of our diet contain too little fiber, but additives that have been shown to disrupt our gut flora. Even something as simple as salt can impair our microbiome. If you roughly double your sodium intake with an extra teaspoon a day, not only do blood pressure and the growth of pro-inflammatory cells that play a role in autoimmune diseases increase, but it rapidly reduces the beneficial Lactobacillus bacteria in the gut. In nine out of ten study participants who started out with Lactobacillus in their gut, these bacteria were completely wiped out by the extra salt within two weeks.
In see.nf/notsosweet you can learn more about the negative effects of sweeteners on the microbiome. The good news is that the original balance of gut bacteria can reestablish itself within a few weeks once you stop eating sweeteners. Harder is avoiding the consumption of emulsifiers, the most commonly used additives in foods. More details can be found in see.nf/emulsifiers, but the bottom line was that, of 20 commonly used emulsifiers, most have harmful effects, including carboxymethylcellulose and polysorbate 80. Two emulsifiers, by contrast, appear to be acceptable: soy lecithin as well as mono- and diglycerides.
Protein Putrefaction
Do you know the parody of the advertising slogan “Beef: It’s What’s for Dinner” — “Beef: It’s What’s Rotting in Your Colon”? I once saw that line on a T-shirt when I was out with a few friends, but then I was a total sourpuss—the pun is pure coincidence—and explained to everyone that meat is completely digested in the small intestine and never makes it all the way down into the colon. (It’s no fun hanging out with biology nerds.) But I was wrong! (About the meat in the colon, but not about the fact that I’m sometimes a science-obsessed killjoy.)
With the typical Western diet, an estimated up to 12 grams of protein escape digestion, and when they reach the colon, they can be converted into toxic substances like ammonia. This breakdown of undigested protein in the colon is called putrefaction, rotting. So a little meat can end up in the colon and rot there. The problem is that some of the byproducts of this rotting process can be toxic.
In see.nf/sulfide I explain that animal proteins generally contain more sulfur-containing amino acids like methionine, which can be converted into hydrogen sulfide in the human colon. This is one of the reasons why people who eat meat appear to have a higher risk of inflammatory bowel disease (see.nf/hsibd) and colon cancer (see.nf/hscancer). Sulfur-containing preservatives (sulfites and sulfur dioxide) in conventionally grown wine and dried fruit can also be a problem, but the sulfur-containing compounds in cruciferous vegetables are not.
Quiet, but deadly
There’s a reason hydrogen sulfide is referred to as the “stink of rotten eggs”: It’s blamed for the “foul-smelling intestinal gas” that can come with a low-carb diet. When fresh stool samples were compared, it turned out that one of the best predictors of whether stool stinks is whether the person eats meat. To reduce the stench, the Harvard Health Letter recommends eating less meat and eggs. When subjects are randomized to different amounts of meat, you find a clear correlation with sulfide concentration in stool. Compared with people eating a plant-based diet, habitual meat eaters were found to have up to 15 times higher sulfide concentration.
Reducing the TMAO Burden
Prebiotics like fiber and resistant starches can feed our probiotic beneficial bacteria such as Lactobacillus and bifidobacteria so that they produce beneficial postbiotics like butyrate and acetate. But with the wrong diet, we cultivate harmful bacteria that generate toxic postbiotics like TMAO.
TMAO is the abbreviation for trimethylamine N-oxide, evidence of interactions between the microbiome and disease. It was discovered when the blood of patients who had suffered a stroke or heart attack was compared with the blood of other people. (Learn the whole story in see.nf/tmaodiscovery.) Whether young or old, man or woman, smoker or nonsmoker, with high or low blood pressure, high or low cholesterol—if the TMAO level is high, you have a significantly higher heart attack or stroke risk and a greater likelihood of dying prematurely.
In mice, TMAO promotes atherosclerosis, because it promotes the accumulation of cholesterol and inflammatory cells in the artery walls. Two additional mechanisms for TMAO’s role in cardiovascular disease have been directly demonstrated in human intervention studies. One reason high TMAO levels increase the likelihood of stroke by 68 percent and quadruple the likelihood of dying from it is that it makes platelets stickier for clotting, meaning it does the opposite of aspirin. That leads to a prothrombotic (clot-promoting) state, while the reason TMAO impairs arterial function appears to be due to oxidative stress, since an intravenous vitamin C infusion can restore the TMAO-impaired function in middle-aged and older adults.
In the past, it was believed that the toxic effects of TMAO were limited to cardiovascular disease, but more recently it has also been associated with other diseases, from psoriatic arthritis to polycystic ovary syndrome (PCOS), including eight of our ten leading causes of death: cancer (ovarian cancer, colon cancer, and breast cancer), chronic obstructive pulmonary disease (COPD), dementia, diabetes, pneumonia, and kidney failure. See details in see.nf/tmaorisk. A systematic review with meta-analysis of 20 studies in which more than 30 000 people were followed for an average of five years found that higher TMAO levels are associated with nearly 50 percent higher risk of death from any cause.
Where does TMAO come from? Bad bacteria in the gut produce it when we eat a lot of choline, as concentrated, for example, in eggs and lecithin supplements, or carnitine, which is abundant in meat and some energy drinks. Within a few hours after eating eggs or meat, TMAO levels rise—unless you took antibiotics shortly beforehand and destroyed your gut flora. (It can take weeks for the harmful bacteria to recover from that.) Why not prevent the growth of the bad bacteria from the start by not feeding them in the first place, instead of swallowing medications? Researchers found that in vegans, virtually no TMAO was generated even after eating steak, presumably because a meatless diet hadn’t allowed steak-eating bacteria to accumulate.
Remarkably, people who eat a predominantly plant-based diet can even eat the equivalent of a 500-gram steak every day for two months, and only about half of them ramp up TMAO production; that shows how much their gut flora had changed. But it doesn’t have to mean: all or nothing. In see.nf/swap I show that TMAO levels can be lowered within a few weeks if, every day, you eat plant-based meat substitutes instead of two servings of regular meat.
Plant foods that are relatively rich in choline do not seem to create the same problem. Pistachios and Brussels sprouts, for example, can even lower TMAO levels. In my video see.nf/tmaoupdate I discuss the mixed effects of various plant foods as well as the pros and cons of carnitine dietary supplements in older age. In short, the best strategy for reducing the TMAO burden is probably to prevent the harmful bacteria from thriving in the first place. As the editorial in an endocrinology journal put it, TMAO should perhaps stand for “Time to Minimize intake of Animal prOducts” — “time to minimize the intake of animal products.”