With the dawn of the New Year, it is worth reflecting on the things that make us healthy and happy. Companionship, exercise, and good food are no doubt critical parts of a healthy lifestyle. We should be grateful also for the health benefits of dietary fiber, writes Carl Zimmer in the New York Times on January 1st. He summarizes two studies done by Frederik Bäckhed and Andrew Gewirtz that highlight the role of the gut microbiota in breaking down fiber, maintaining a healthy gut mucus barrier, and promoting a beneficial community of gut microbes. So the short answer for why fiber is good for us is…wait for it…the microbiome. The same answer applies to why low-fiber junk food diets are bad for us…yes…the microbiome. Why are some fats good for us and others bad for us?…you guessed it… the microbiome.
The studies highlighted in Zimmer’s article make clear that the gut microbiome is a key link in how the body processes nutrients and provides a mechanistic explanation of why fiber is good. What is left out is an evolutionary explanation for why the gut microbiota has these predictable effects on the body.
Helen Wasielewski, Athena Aktipis, and I tried to answer that question in a paper published in the NY Academy of Sciences, Conflict and cooperation between host and gut microbiota: Implications for nutrition and human health. We argued that empty calories tip the scale in favor of conflict between our bodies and resident gut microbes, whereas fiber-rich foods do the opposite.
Fiber rich foods permit resource sharing between host and microbiome that favors a mutualistic relationship. Fiber is fermented by microbes that produce short chain fatty acids as a fermentation product. Those short chain fatty acids themselves are taken up by gut epithelial cells as a source of energy
Whether diets promote or inhibit this mutually beneficial arrangement is vitally important. But junk food diets are bad for us only because of the absence of cooperation.
Conflict too is important, and conflicts between humans and gut microbes are built in to the relationship, in part because our genetic interests are not perfectly aligned. Junk food almost by definition is energy dense, yet low in compounds that inhibit microbe growth. Junk food is fuel that doesn’t come packaged with ingredients that kill off pathogens or pathobionts. Junk food, then is full of conflict.
On the other hand, many foods have ingredients that kill pathogens. Short chain fatty acids are an example. So are omega-3 fatty acids. So are plant polyphenols. Fiber too, inhbits pathogens indirectly because of SCFA and bacteriocin production by fiber fermenters. In other words, your gut microbiota can harm you or it can help you. Choosing the right diet is a microbe harm-reduction strategy.
The bottom line is: when we assess a food’s healthfulness we should consider how foods modify the conflict and antagonism inherent in our evolved relationship with our gut microbes. Certain foods are weapons in this fight. You can ask yourself, does this food that I am stuffing into my mouth fuel pathogens and promote their growth? Or does the food inhibit or kill pathogens? The answer will likely illuminate whether that food will lengthen or shorten your life!
Bon appetit! and Happy New Year to all!
Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine
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