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Thousands of years ago, some yeast floated down into our flour and drinks, and we were like, hmm; yummy, and we’ve been regularly exposed ever since. Not a problem for most people, but even non-disease-causing microbes could potentially trigger autoimmune diseases like Crohn’s disease in people who are susceptible, because their finely tuned immune balance is somehow off. Maybe that’s why bakers have the highest Crohn’s disease mortality, and, from a different data set, among the highest rates of Crohn’s disease. Perhaps a “hypersensitivity to…yeast” may be “play[ing] a role in Crohn’s disease.”

If you take people with Crohn’s, and remove from their diets the three foods to which they have the most antibodies to try to calm their disease, and then you add those foods back, you can “provoke the symptoms” back—restimulate the inflammation. So, anal fistula off those foods, nice and dry, but then starts oozing again once the foods are back. But then, the spigot’s turned back off once the foods are removed again—to which I say, “Why is that doctor not wearing gloves?”

Anyway, without a control group, you can’t exclude the possibility of a really powerful placebo effect. But there haven’t been any such randomized controlled trials—until now.

A brilliant design; they tested people with Crohn’s for antibodies to 16 different foods, and then randomized the people in two groups. Both groups were told to avoid four foods, but one group was told to avoid the four foods they most reacted to, and the other group told to avoid the foods they least reacted to. But, it was all done with “sealed envelope[s]”; so, no one knew who was in which group until the end. So, did it matter?

Yes, more than twice the probability of major clinical improvement in the group that was told to stay away from the things their blood reacted to. But, this wasn’t just yeast. In fact, it was the “exclusion of milk, pork, beef, and egg [that] was most strongly associated with [clinical] improvement”—leading the researchers to suggest that look, maybe instead of doing all the fancy blood tests, we should just tell our patients to cut out meat and eggs and see how they do, which would be consistent with both population studies that associate animal fat with an increased risk of inflammatory bowel diseases, as well as interventional studies, showing that a plant-based diet, in which you cut meat down to like one serving every two weeks, can drop relapse rates as low as an extraordinary 8% over two years.

Okay, but, what about the whole yeast question? Can’t you just stick some yeast up someone’s butt and see what happens? Why yes; yes, you can. They tested “rectal exposure to six” different foods in Crohn’s disease patients, including yeast. Kind of like a skin prick test, but instead of pricking the skin, they pricked the inside of people’s rectums with various foods. And, you can see in this example, the various prick sites—cabbage, milk, citrus, cereal, peanut, but whoa, yeast. Yeast gave the most significant reaction in Crohn’s patients.

So, it appears that baker’s yeast, which is the same yeast as brewer’s yeast and nutritional yeast, may indeed have disease-causing importance in Crohn’s disease. But the good news would then be hey, it “may [have] therapeutic relevance.” Maybe if you put Crohn’s patients on a yeast-free diet, they’d feel better. You don’t know until you put it to the test.

In fact, that’s exactly what the original study linking yeast and Crohn’s yeast suggested back in 1988. “A controlled trial of a yeast free diet for…Crohn’s disease [patients] may…be worthwhile.” So, why did it take years before such a study was done? Who’s going to fund it—Big Soda-Bread? Thankfully, there are charities, like the National Association for Colitis and Crohn’s disease, willing to put up the dough—the yeast-free dough, that is.

19 patients with Crohn’s disease ate their regular diet, and then switched to a yeast-exclusion diet. And, there was a significantly higher CDAI (Crohn’s disease activity index), assessing symptoms like abdominal pain and diarrhea during the period when they were eating yeast, compared to the yeast-free period. So, worse disease activity with yeast than without. Yeah, but what was this yeast-free diet? They didn’t just cut out bread and beer; they like switched people from dairy milk to soy milk, from white flour to whole wheat. Just cutting out milk can sometimes help with inflammatory bowel disease.

So, with so many dietary changes, how do we know what role the yeast played? This is how we know what role the yeast played. After placing them on the new yeast-free diet, they then challenged them with a placebo or a capsule of yeast. A tiny amount of yeast, like giving them a quarter-teaspoon of nutritional yeast a day, and that made them worse—suggesting “yeast may [indeed] be important in the [disease process] of Crohn’s disease.

Now, for the vast majority of people, yeast is not a problem, but “in susceptible individuals [it may trigger] an abnormal immune response” in the gut. But wait, I thought it was the paratuberculosis bug that was considered a trigger for Crohn’s disease. Well, hey, maybe infection with paraTB is what “induces [the] hypersensitivity response to dietary yeast.” Who knows? The bottom line is that people with Crohn’s disease should not go out of their way to add baker’s, brewer’s, or nutritional yeast to their diet.

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