Holidays are often a time of food indulgence, even for people who are generally careful about their diet the rest of the year. But there is more to eating on holidays than simply avoiding extra calories. What we eat affects the health of the entire body, including the immune system.

Because most of the body's immune system is concentrated in the gut and the trillions of healthy bacteria called the microbiome that live there - what we eat has a direct impact on how the immune system functions. "Many people don't think about the impact of their diet on immunity, but science says it's important," says Heather Mode, MD, MD, and founder of the Mode Center for Functional and Integrative Medicine in Norfolk, Virginia, and Philadelphia.

And that's especially important this time of year because the winter holidays are right around the time colds, flu and illnesses peak. While indulging in the occasional meal probably won't cause permanent damage, not being careful leading up to New Year's Day is definitely unhealthy and can make you more prone to illness. "It can really take just one month of being lax and paying attention to nutrition to have an impact on the microbiome," says Christine Kirkpatrick, a registered dietitian in the Cleveland Clinic's Department of Wellness and Preventive Medicine.