Propionate is a calcium salt that is naturally produced by bacteria. (In fact, propionate even lives in our gut and on our skin.) But it's often added to foods as a preservative to protect against mold and fungus. The FDA labels it as a "generally recognized as safe ingredient (GRAS)," and past studies have shown propionate has some positive effects—including a reduction in weight gain.
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But this most recent study raises concerns: Researchers fed both mice and people the ingredient, and noted that both experienced increased glucose production. Humans also experienced more insulin resistance. (Regular resistance to insulin contributes to type 2 diabetes and obesity.)
For the study, participants were either given a 500-calorie meal with 1 gram of propionate, which the study authors write is a standard dose for a "processed food-based meal," or a placebo each day. Researchers found that those participants who received the daily dose of propionate experienced both increased blood insulin levels and reduced insulin sensitivity.
"Propionate may activate a catecholamine-mediated increase in insulin counter-regulatory signals, leading to insulin resistance and hyperinsulinemia, which, over time, may promote adiposity and metabolic abnormalities," the researchers wrote. But they also added that more "evaluation of the metabolic consequences of propionate consumption is warranted."
So, what does this mean for the average consumer?
Propionate is in a lot of our foods: Many breads, butters, cheeses, and processed items have it. Even La Croix uses it. So while it's tough to avoid entirely, if you're concerned or would like to try to limit your intake, scan ingredients lists for propionate and its other common names, including propanoic acid, sodium propionate, calcium salt, and calcium propanoate.
And while 1 gram of propionate—the amount given to study participants each day—may be common in an average 500-calorie processed food-based meal, if you're eating a whole foods-focused diet, you're likely ingesting much less propionate daily than the participants.
In fact, the study's lead researcher Amir Tirosh, associate professor of medicine at Tel-Aviv University's Sackler Faculty of Medicine, told Medical News Daily that—because so much of the research was done on mice—it's too early to recommend that people avoid propionate.
"It will be premature to do so based on a single study," Tirosh says. "Therefore, we are not making such recommendations. Our research should serve as a proof-of-principle for the potential interference of propionate in normal metabolism, but most of the data was obtained in mice, and we need to be careful when translating these findings to humans."