A new study of mice suggests bacteria may help to cause anxiety and depression in overweight people.

The research raises questions about whether changing organisms living inside our bodies or changes in diet could help treat these conditions.

A 2017 study found that one-third of the world's population is overweight or considered obese.

When people become obese, this changes the processes by which the body uses food to make energy, build tissue and remove waste material. Left untreated, these changes can lead to diabetes.

Diabetes is a serious condition. It affects how your body uses blood sugar. Obese people also have higher rates of anxiety and depression than other individuals.

Ronald Kahn studies diabetes at Harvard Medical School in Massachusetts. He says researchers have wondered if people suffer from depression because they are obese or because of their metabolism.

"And we asked the question, 'Maybe the metabolic link is at least partly fueled by the microbiome,'" Kahn said.

The microbiome is the community of bacteria, fungi and other microorganisms living in your stomach. It changes with your diet. And Kahn says those changes may affect both body and mind.

To test the theory, he and his research team fed mice a high-fat diet and studied their behavior as the animals became obese.

In one test, they put mice in a box divided into a darkened area on one side and a light area on the other side. They found that the more nervous, anxious mice liked to spend time on the dark side.

"And a mouse on a high fat diet will spend more time in the dark than a mouse on a normal... diet will. So, they have more sign of anxiety," Khan said.

The anxiety went away when the mice were given antibiotic drugs.

The researchers said this suggests that the bacteria were helping make the mice nervous.

The researchers then wanted to see if they could produce the same effect by giving the same bacteria to animals raised in a bacteria-free environment, with no microbes of their own.

They found that it did. Microbes from obese mice made the bacteria-free mice anxious. Microbes from obese mice given antibiotics did not.

"It was actually quite a surprise," Khan said. "Even though we had seen some effects on metabolism in the rest of the body, I was very surprised how dramatic and how clear the effects were also on the brain and on behavior."

A report on the study appears in the journal Molecular Psychiatry.

Kahn noted, however, that the new study does not mean antibiotics are the cure for depression. The drug kills both good and bad microbes. Also, drug misuse is making these powerful medicines less effective.

Another important point, he said, is that what happens in mice does not necessarily happen in humans, or it may only happen for some people.

Gregory Simon is a mental health specialist at the Kaiser Permanente Washington Heath Research Institute. He said that it is also important to remember there is much more going on with people than just their microbes.

"The difficulty is, both of these things, depression and obesity, are complicated things that have multiple, multiple factors influencing them," Simon said.

He says genetics, environment, social influences and our microbes all help shape people and their behavior.

I'm Phil Dierking.

Steve Baragona wrote this story for VOANews Phil Dierking adapted their story for Learning English. George Grow was the editor.