By Amy Norton
WEDNESDAY, April 24, 2019 (HealthDay News) -- If you're watching your weight, you probably know to avoid sugary and fatty foods. But what about preservatives?
The additive, called propionate, is actually a naturally occurring fatty acid produced in the gut. When it's used as an additive in processed foods, it helps prevent mold.
But in the new study, researchers found that feeding mice low doses of propionate gradually caused weight gain and resistance to the hormone insulin -- which, in humans, is a precursor to type 2 diabetes.
And when the researchers gave healthy adults a single propionate dose, it spurred a release of blood sugar-raising hormones -- and a subsequent surge in insulin.
None of that proves propionate-containing foods raise the odds of weight gain and diabetes, said senior researcher Dr. Gokhan Hotamisligil, a professor at the Harvard School of Public Health.
"The point is not to say this additive is 'bad,'" he stressed.
Instead, Hotamisligil said, his team is interested in understanding the effects -- good or bad -- of the various "molecules" humans consume in their diets.
"There's a scarcity of scientific evidence on a lot of the things we put in our bodies through food," he said. "Propionate is just one example."
Still, Hotamisligil said, the findings do raise an important question: "Could long-time consumption of propionate in humans be a contributing factor to obesity and diabetes?"
When it comes to processed foods, the concern is usually directed toward ingredients like added sugar, sodium and trans fats. But there's also a host of additives that, according to the U.S. Food and Drug Administration, are "generally recognized as safe."
Dr. Emily Gallagher is an assistant professor of endocrinology at Mount Sinai Icahn School of Medicine in New York City.
She agreed it's important to dig into the potential metabolic effects of food additives.
"People may look at food labels and think they are making healthy choices," said Gallagher, who had no part in the study. "But without our knowledge, very small amounts of certain additives in food may be causing detrimental metabolic effects."
That said, it's too soon to point the finger at propionate, according to Gallagher.
She called these early findings "thought-provoking," but said longer-term studies are needed to better understand any health effects from the additive.
For the animal portion of the study, the researchers gave mice propionate in their water. The immediate effects included an increase in three hormones that spur the liver to produce glucose (sugar). Over time, chronic exposure to the additive caused the mice to gain weight and become resistant to the hormone insulin, which helps lower blood sugar levels.
The human portion of the study included 14 healthy people given a dose of either propionate or a placebo with a meal. Compared with the placebo meal, the additive caused the same hormonal response seen in mice, plus a surge in insulin in the blood.
Whether those effects over time could harm people's health is unknown.
Many factors, including overall diet and exercise, affect the risks of obesity and diabetes, Gallagher pointed out.
For now, she said, the findings support the general advice that we should be limiting processed foods, in favor of healthier, whole foods.
Hotamisligil agreed. "I'm not saying, if you don't eat propionate, you'll live forever," he said. "But these are the types of foods we should limit anyway."
The findings were published online April 24 in Science Translational Medicine.