Ask the Doctors

Dear Doctor: My cousin says she heard that eating sugar pretty much wipes out the beneficial bacteria in our gut. Please tell me that's not true. I'm an avid baker and love sweets and don't think that I can give them up.

Dear Reader: We can reassure you that the report your cousin is referring to doesn't claim that sugar out-and-out destroys the gut microbiome. But don't celebrate with a home-baked brownie just yet. We're afraid that the new research does contain some bad news for people who have a sweet tooth. According to the study, which was published in the Proceedings of the National Academy of Sciences at the start of this year, high levels of fructose and glucose in the diet wreak havoc on a certain protein that is necessary for beneficial bacteria to colonize the gut.

Why does that matter? The latest research continues to make clear that good health hinges on each of us maintaining a robust and diverse gut microbiome. Made up of bacteria, fungi, viruses and other microorganisms that number in the trillions, the gut microbiome plays a decisive role in digestion, the absorption of vitamins and other nutrients, and the optimal function of the immune system. Whether directly or indirectly, the thousands of species of microbes we host in our bodies affect most of our physiologic functions. At the same time, we have a direct effect on these populations, including through what we eat.

In addition to the word "probiotics," which refers to beneficial gut bacteria, the term "prebiotics" has entered common usage. This refers to the portion of the diet that contains nutrients that are available to those trillions of gut microbes. Dietary fiber, which is made up of long chains of simple sugars bonded together to make a large and complex molecule known as a polysaccharide, sails through the small intestine largely undigested. That fiber reaches the part of the colon known as the distal gut, which houses the lion's share of the gut microbiome. Not only does that dietary fiber provide nutrients to the gut microbiome, but it affects the growth and colonization of the microbial communities.

When it comes to monosaccharides, or simple sugars, like fructose and glucose, which are routinely added to a wide range of prepared and processed foods, it was believed that they were absorbed in the small intestine and never made it to the distal gut. However, it is now known that both fructose and sucrose do reach the distal gut. When they do, they have a negative impact on good bacteria like Bacteroides thetaiotaomicron (B. theta for short), which are associated with a lean and healthy body. Instead of providing food, simple sugars in the distal gut stop the production of a key protein that B. theta needs in order to maintain and expand its presence. Without that protein, B. theta populations become significantly diminished.

It's important to note that this research was done on mice. How or even whether it translates to the human microbiome is not yet known. But considering the many health problems clearly linked to added sugar, including diabetes, obesity and cardiovascular disease, cutting out simple sugars makes sense.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

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