Researchers at the University of British Columbia have shown that our gut bacteria can feed on these large molecules — something thought to not be possible — thanks to enzymes that normally help us break down dietary fiber.

“Researchers assumed that these thickening agents, which are artificial derivatives of natural cellulose, just pass right through the digestive system unaltered,” says Dr. Deepesh Panwar, a postdoctoral fellow at the Michael Smith Laboratories and lead author of the study published in the Journal of Bacteriology. “But our study provides a first glimpse at how these food additives are actually digested by our gut bacteria thanks to natural polysaccharides in our diets.”

The complex structure of these cellulose derivatives is what makes them valuable as thickening agents in popular products like ketchup, salad dressing and even toothpaste. This structure is also why gut bacteria have a harder time breaking them down — and why in higher concentrations, they’re even used as laxatives.

This new in vitro study, however, shows that if our gut bacteria are ‘primed’ with natural polysaccharides — long chains of sugars found in fruits, vegetables and cereals — the cellulose derivatives can be digested. This is because the natural polysaccharides activate enzymes that are produced on bacteria cell surfaces that can also break down artificial cellulose molecules.

The findings don’t challenge the fact that these compounds are safe to consume, proven by years of testing and history of use. However, the new research suggests that more work should be done to explore the physical, chemical and biological effects of the digestion of cellulose derivatives by gut bacteria.

One reason researchers may not have seen this before is because bacteria in the lab are often exposed to polysaccharides, including cellulose derivatives, in isolation, instead of in combinations that mimic our diet. On their own, these cellulose derivatives can’t activate the same enzymes, preventing their digestion.

“It was really unexpected for us to see that these cellulose derivatives are in fact used as a source of sugar for bacterial growth,” says Dr. Harry Brumer, a professor in the Michael Smith Laboratories and Department of Chemistry. “It is always a surprise when a new finding goes against the conventional wisdom, in this case showing that these common additives are not just inactive thickeners.”

Dr. Brumer also notes that the next steps in this research will be to look for this ability in a wider range of human gut bacteria, and eventually explore potential effects on nutrition in people.

So, next time you pair a green salad with a sweet dressing, know that your gut bacteria are hard at work helping to break down all parts of your meal.