(CBSNews) - The food you eat and the medicines you take can alter your gut bacteria in ways that either help or harm your health, two new studies suggest.

Foods like fruits, vegetables, coffee, tea, wine, yogurt and buttermilk can increase the diversity of bacteria in a person's intestines. And that diversity can help ward off illness, said Dr. Jingyuan Fu, senior author of one of the studies.

"It is believed that higher diversity and richness [in gut bacteria] is beneficial," explained Fu. She is an associate professor of genetics at the University of Groningen in the Netherlands.

On the other hand, foods containing loads of simple carbohydrates appear to reduce bacterial diversity in the gut, Fu and colleagues found. These include high-fat whole milk and sugar-sweetened soda.

In addition, medications can also play a part in the makeup of your gut bacteria. Antibiotics, the diabetes drug metformin and antacids can cut down on gut bacterial diversity, the researchers found. Smoking and heart attacks also can have a negative effect, the team said.

Each person's intestines contain trillions of microorganisms, which doctors refer to as the "gut microbiome," said Dr. David Johnson. He is chief of gastroenterology at Eastern Virginia Medical School in Norfolk, Va., and a past president of the American College of Gastroenterology.

The gut microbiome plays an essential but little-understood role in human health, said Johnson, who was not involved with the new studies.

"It's the largest immune system in the body," Johnson explained. "These bacteria have a very dramatic and prominent role in determining health and disease."