The following is a guest post from Emily Seitz who is doing a six-week internship with Melissa Gerharter.

Winter is almost here, which means cold and flu season is too. Since 70 percent of the body’s immune system lives in the digestive tract, maintaining a strong gastrointestinal system is essential for staying healthy.

There are billions of bacteria that naturally live in the intestines. Many of them are referred to as the “beneficial” or “friendly” bacteria because they defend against harmful bacteria that can attack the body.

Ideally, there should be more good bacteria than harmful bacteria in the intestinal tract. However, certain medical conditions, medications, stress or a poor diet can upset this balance by reducing the number of friendly bacteria. When this occurs, the digestive system can function irregularly, and the risk of catching a cold or the flu can increase.

One way to maintain a balanced digestive system is by eating a healthy diet rich in fruits, vegetables, whole grains and lean proteins. Particularly important are non-digestible carbohydrates, known as prebiotics. Prebiotics are used as an energy source and stimulate the growth of beneficial bacteria in the intestines. Prebiotics are found in foods such as chicory, barley, rye, onions and bananas.

Probiotics are also beneficial to digestive health. Probiotics are live microbial food supplements that have the potential to start colonies of beneficial bacteria in the digestive tract. Foods that contain probiotics are primarily dairy products that include yogurt with live cultures, kefir and cottage cheese. Non-dairy sources include soy sauce, miso and tempeh.

Eating a well-balanced diet that includes prebiotic and probiotic food sources is important for strengthening the immune system. However, don’t delay. The best time to start is now so that you can build up good bacteria and prepare your body for battle as the cold and flu season peaks this winter.

Emily Seitz is a senior dietetic student at Eastern Michigan University. She plans to graduate in April 2012. She is spending a six-week internship with Melissa Gerharter MS, RD, CSSD co-owner of Joust Strength and Fitness of Ann Arbor.