By contrast, most other organs are well understood in their major and minor functions. The kidneys, for example, are a system governed by hydrostatic and osmotic pressures, that then communicate that information to the rest of the body via hormones. Likewise, the heart is a mechanical pump, and not much else.
The gut, however, is a complex interplay of bacteria, enzymes, neurotransmitters, foods, membranes and immune cells. While the cardiovascular system evolved in an effort to move fluid and nutrients around the body, the digestive system was an attempt to tame the wilds of the ocean so that we could harvest nutrients from it. As a result, the digestive system is more like a controlled ecosystem than a mechanical apparatus. After all, there are more bacterial cells in the gut than there are human cells in the rest of the body.
While researchers have a reasonable handle on certain parts of the digestive process, such as enzymatic digestion and absorption, the interplay between food, bacteria, and the immune system is still one of the biggest mysteries in modern science. Research into these relationships will help to answer at lot of questions, such as:
- What’s the benefit of probiotics? Is it just a question of outcompeting pathogenic bacteria? Why do some studies show benefit even from killed lactobacilli? Who should take what species of probiotics?
- How do beneficial bacteria interact with the immune system? How do they signal to the immune system? What does the immune system do upon receipt of these signals?
- How does bacterial digestion affect medications or herbs? Should some people take some over others?
- What’s the deal with food intolerances? Is it the food itself, or something else that people react to? Does it happen in the bloodstream or in the digestive tract? How can get decent lab tests for food intolerances?
- And finally, what the heck is IBS, really?
This is about as straightforward as trying to figure out how all of the birds, mammals, insects, bacteria, plants, fungi, viruses, weather patterns, pollution, human activity and magnetic forces in the entire Amazon basin affect one species of tree.
However, an article recently published in Nature has shed some light on one aspect of this question. It appears, based on a small study, that several distinct bacterial ecosystems exist in human digestive tracts. While this doesn’t yet offer explanations to many of the burning clinical questions we have, it represents a massive step forward in mapping the terrain of the digestive system. In the same way that discovering blood type in the early 1900s yielded vital clinical information that saved many lives, in addition to helping to push forward our understanding of the immune system, the discovery of enterotype is likely to yield clinical results and also help elucidate some of the subtleties of the immune system, and how it interacts with food and symbiotic bacteria.
Most surprising, I think, is that the bacterial populations were not organized around Lactobacillus, Bifidobacterium or other commonly-studied probiotics. Instead, they were organized around the species Bacteroides, Prevotella and Ruminococcus, names which to date have had little clinical significance. There is also some indication that these bacteria may, and I do mean may, direct the formation of an environment beneficial to certain bacterial strains. More information is likely to come out in the next few years as larger studies are performed – there is already an indication that enterotype 3, centered around Ruminococcus, may include multiple subtypes.