Cheese and bread

Recent studies have shown that a common preservative in cheese and bread can negatively affect your metabolism.

 In recent years, scientists have begun to warn cautiously about the subtle harm that certain food additives can cause people. A new study suggests that one such supplement may be the dangerous food propionate. Experiments in mice and humans have found that taking propionate can have negative effects on metabolism, including increased insulin resistance. Propionate, or propionic acid, is ubiquitous in our world. It is naturally produced by many bacteria, including those that live in our intestines and skin. It is added as a preservative to animal feed and human foods such as cheese, baked goods and artificial flavorings. But while propionate is one of many additives on the Food and Drug Administration's list of foods deemed safe to eat (GRAS), some studies have shown that propionate is not completely harmless to the body.

For example, in animals, it is thought that propionate can raise blood sugar levels through the liver, which converts glucose from non-carbohydrate fuel sources. This ability is even used by ruminant species such as cows to regulate healthy blood sugar levels. In humans, the relationship with propionate is more mysterious. Some studies have shown that propionate and other short-chain fatty acids, which are naturally produced by gut bacteria, can actually help us maintain a healthy metabolism, suppress appetite and reduce the risk of obesity. But other studies have shown that people who have more propionate in their system are more likely to be obese. To better understand the short-term effects of propionate consumed from food, the authors of this current study, published in Science Translational Medicine, conducted feeding experiments with both mice and humans. The human portion of the study was a randomized, placebo-controlled, double-blind study with 14 healthy volunteers. "We verified that in mice it leads to an increase in blood glucose levels. But the most interesting thing we determined in these experiments is that a single dose of propionate can increase levels of hormones in the body that are designed to stimulate glucose production from the liver," writes study author Gekhan S. Hotamyshlygil, director of the Sabri Ulker Nutrition, Genetics and Metabolic Research Center at Harvard Chan University, School of Public Health.

"There are times when it's necessary, like when you're starving or have dangerously low blood sugar. But here, propionate almost fools the body into thinking it needs to produce glucose when it doesn't."And in humans, these higher hormone levels also seem to cause insulin resistance, which means that their bodies don't also respond to the insulin signal of lower blood sugar levels. Over time, chronic insulin resistance is known to contribute to the development of type 2 diabetes and other metabolic disorders such as obesity. In experiments on mice, mice given low doses of propionate for long periods of time also gradually gained more weight than mice that did not consume propionate. But Hotamyshlygil and his team don't say their findings should immediately cause anyone to avoid cheese and bread.

"We don't want to make loud statements and recommendations right now," he said. "This is a proof of principle study, just illustrating that we can actually identify these molecules and study their biology, which will then stimulate further work." At this point, more research is needed to figure out exactly how propionate can cause these metabolic changes, Hotamyshlygil added. In humans, for example, they found evidence that the effect of propionate is through the brain and nervous system through adrenaline production, rather than through direct effects on the digestive system or liver. This research should include human studies with a large number of volunteers from different laboratories.