Below is an excerpt from my colleague Chris Kresser, L.Ac.

What is the biggest factor that determines the health of the gut microbiome? I think it is the type of carbohydrate that you eat, and specifically, it’s the ratio of acellular to cellular carbohydrate. Now, that sounds very geeky, I know, but just give me a chance to explain it a little bit because it’s a very useful concept to understand, and I think it answers a lot of questions, and you’ll see what I mean as we go through this.

This concept of acellular versus cellular carbohydrate and the importance of it to the gut microbiome comes from one of my favorite research papers ever, which was written by Professor Ian Spreadbury, and he’s actually going to be a presenter at the Ancestral Health Symposium conference in New Zealand, the New Zealand version of that conference, which is happening in October of this year. And the paper is called “Comparison with ancestral diets suggests dense acellular carbohydrates promote an inflammatory microbiota, and may be the primary dietary cause of leptin resistance and obesity.” That’s a mouthful. The study will be in the show notes, so you can check it out. The full text is free, and you can read it if you’d like, but the basic idea is that all carbohydrates that were part of the ancestral diet, which would be tubers, fruits and vegetables, plant parts like stems and leaves, store their carbohydrates in fiber-walled, living cells, and those cells remain largely intact during the cooking process, and they also resist digestion or absorption in the small intestine, and therefore, the fiber remains intact all the way down to the colon, where it then becomes food for beneficial gut bacteria that are living in the large intestine. So those are the cellular carbohydrates, and they’re, like I said, found in all ancestral carbohydrate sources.

On the other hand, in the Western or industrialized diet, you have a lot of acellular carbohydrates. These are things like flour, sugar, and other processed starches that have no living cells. These industrial foods are much higher in carbohydrate density than anything the microbiota of our upper GI tract would have encountered during our long evolution. And these foods, because they have no living cells, they’re absorbed higher up in the GI tract, and they can stimulate the overgrowth of bacteria in the upper GI tract, AKA SIBO, or small intestinal bacterial overgrowth, and they preferentially will feed some species of bacteria over others, and that can in turn lead to an inflammatory gut microbiota.

Dr. Spreadbury makes the argument, which I agree with, that this difference in carbohydrate type could be the single most important difference between ancestral and industrialized diets. Now, notice I didn’t say carbohydrate quantity, the amount of total carbohydrate that you eat. I don’t think that’s the issue for most people. The issue is carbohydrate quality, and I know I’ve been beating this drum for a long time. I wrote about it in my book. Professor Spreadbury talks about it in the paper. He points to traditional cultures like the Kitavans and East African groups who eat from 35% to 70% of calories as carbohydrate, but all of these carbohydrates come from foods with living cells, like tubers or fruit or other plant parts, and the reason that industrialized diets are so harmful is not because of the overall amount of carbohydrates that they contain, as some people have argued, but because they don’t have those living cells, and as a result, they have a completely different impact on the gut microbiota than the ancestral carbohydrates, the real-food carbohydrates.

Steve Wright: Is it living, Chris? Or is it more of the complexity and the molecular structure of the differentiation? Because we can take those tubers and cook them. Now they’re not living anymore. But I was kind of under the understanding that you have a cell wall that contains all the parts of the cell and it’s a… complex isn’t necessary the perfect word here, but the actual molecule is still intact, whereas the processed foods, the molecule has been stripped down.

Chris Kresser: That’s right. Yeah, it’s been predigested, essentially, and broken down. And the cellular structure, like I said before, remains largely intact even during cooking, so the carbohydrates are locked into the fiber-walled cells, and that’s why in some cases the carbohydrate portion of that food is inaccessible, and that’s why it’s considered to be fiber. Fiber is not a nutrient technically for humans because we can’t absorb the carbohydrate that’s contained in those cells and make use of it ourselves, but those cells are instead nutrients for the bacteria that live in our large intestine because they survive in that form all the way down to the large intestine.

Another way to think about it is these real-food carbohydrates in these tubers and plant foods, especially the ones that are resistant to digestion, are not nutrients for us; they’re nutrients for our gut microbiota. On the other hand, the flour and sugar and processed starches, they are absorbed completely by us, and furthermore, they’re absorbed in a different part of the GI tract and they feed bacteria and can lead to an overgrowth of bacteria in that part of the digestive tract because they lack that cellular structure. And so they have a completely different impact on the gut microbiome. When you compare diets and you take one diet that has 25% or 30% of calories as carbohydrate in the form of these cellular carbohydrates, the tubers and the fruits and the vegetables, and then you have another diet that’s 30% of calories as carbohydrate in the form of flour and sugar, those are going to have a completely different impact on the body. And Spreadbury’s argument is the thing that’s mediating that difference in impact is the gut microbiome.