Fructose, Viruses, and the Microbiota

Among the bacteria that live in our digestive tracts are ones termed “lysogenic,” which means that they actually contain dormant virus DNA.  When exposed to certain metabolic conditions, this DNA is activated and produced viruses:  “Approximately half of intestinal viruses are derived from lysogens, suggesting that these bacteria encounter triggers that promote phage production.”[i]  (Remember: bacteriophages (or phages, for short) are bacteria killing viruses.  You can read more about them here, for example.) It turns out that the dietary sugar, fructose, is one of the triggers and researchers at the University of Wisconsin-Madison have also discovered the mechanism behind this effect.

Our old friend, L.reuteri, is a lysogenic bacterium. When exposed to a fructose-enriched diet, it produces the short-chain fatty acid (SCFA), acetic acid.  This, in turn, triggers the production of viruses. In fact, this production of phages does indeed reduce survival of L.reuteri but on the other hand, it also may benefit it by killing other, competing bacteria.

Interestingly, by the way, the other SCFAs our gut bacteria produce (acetic, propionic and butyric being the main ones) have the same effect in terms of stimulating the production of phages.

The exact purpose of this metabolic phenomenon is as yet unknown.  (I wonder if it’s not a checks-and-balances kind of thing:  a way to keep the body from developing an overgrowth of bacteria? That is, the more bacteria present, the higher the levels of SCFA’s produced, which in turn, reduces the number of bacteria present?  We know that SCFAs are critically important to the health of the microbiome and the host, after all. Time will tell, I guess.)

Fructose is, of course, found naturally in fruit and (unless you have a rare condition wherein you are unable to metabolize it), is perfectly harmless.  However, it’s interesting to consider that since the 1970s, and the ever-increasing inclusion of high-fructose corn syrup (HFCS) into our diets, our consumption of this sugar has increased fourfold.[ii]   Does this have any bearing on the microbiota alterations associated with so many diseases whose incidence continue to rise exponentially?

I took a quick look around and not surprisingly, found a big mixture of information on HFCS.  On the one hand, the FDA states, “We are not aware of any evidence, including the studies mentioned above, that there is a difference in safety between foods containing HFCS 42 [42% fructose syrup] or HFCS 55 [55% fructose syrup] and foods containing similar amounts of other nutritive sweeteners with approximately equal glucose and fructose content, such as sucrose, honey, or other traditional sweeteners. The 2010 Dietary Guidelines for Americans recommend that everyone limit consumption of all added sugars, including HFCS and sucrose. FDA participated in the development of the Dietary Guidelines and fully supports this recommendation.”[iii]  On the other hand, it’s also easy to find articles in the medical literature that state things like, “Scientific evidence has identified that the excessive consumption of products made from high-fructose corn syrup is a trigger for obesity, whose prevalence increased in recent years. Due to the metabolic characteristics of fructose, a rapid gastric emptying is produced, altering signals of hunger-satiety and decreasing the appetite.”[iv]

Whether or not HCFS is “unhealthy” in and of itself…it does not appear so.  But the AMOUNT consumed is a huge issue.  100 grams of an apple has 5.9 grams of fructose, according to the US Department of Agriculture.  A 14 ounce bottle of Coke (414 ml) contains about 29 grams of fructose.[v]

I find this subject particularly interesting so will most certainly keep an eye out.  We know that diet is one of the main factors in the health (or lack thereof) of the biome.  I’ll keep you updated.

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[i] Oh, JH, et. al. Dietary fructose and microbiota-derived  short-chain fatty acids promote bacteriophage production in the gut symbiont Lactobacillus reuteri.  Cell Host & Microbe. 2019;25(2):273-284. doi.org/10.1016/j.chom.2018.11.016

[ii] https://scienceblog.com/505882/fructose-can-trigger-viruses-in-the-guts-microbiome/

[iii] https://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm324856.htm

[iv] Loza-Medran, SS, Baiza-Gutman, LA, Ibanez-Hernandez, MA, Cruz-Lopez, M, Diaz-Flores, M. Molecular alterations induced by fructose and its impact on metabolic diseases.  Revista Medica del Instituto Mexicano del Seguro Social. 2019;56(5):491-504.

[v] https://www.goranlab.com/pdf/Ventura%20Obesity%202010-sugary%20beverages.pdf