You probably associate getting enough fiber—the non-digestible “roughage” we get from plant foods—with good elimination. You may also know that fiber helps maintain a healthy weight by filling you up so you eat fewer calories, or even that fiber helps to maintain good cholesterol levels. But how can eating fiber strengthen your immunity?

The answer is that fiber nourishes the trillions of beneficial bacteria (probiotics) that live in our intestines, where they are a crucial part of our immune systems. Anywhere from 60-80% of immune system activity takes place in the intestines, where probiotics attack pathogens with powerful antimicrobial substances. These substances are often as powerful as the strongest antibiotic medications, but they don’t kill off good gut bacteria as is done by antibiotic drugs.

According to Rachel Begun, MS, RDN, quoted in a recent issue of Environmental Nutrition, various aspects of modern life have altered gut microflora, damaging immunity and leading to significant increases in immune system disorders like allergies and autoimmune illnesses. “Fortifying the gut microflora with probiotics,” she says, “should be one of your top health priorities, as this promotes a stronger immune system.”

Although Ms. Begun doesn’t specify which aspects of modern life have led to widespread gut immunity damage, antibiotic drugs bear most of the blame. Contributing to the damage are some other drugs, stress and poor diet. But even one course of antibiotics can do permanent damage to your microbial population, and most of us are in this boat.

What to do? A recent article in the Journal of the Academy of Nutrition and Dietetics updating Registered Dieticians on the subject recommended a high-fiber, plant-based diet as the best way to positively impact gut microflora. Studies have shown fiber-rich plants produce greater numbers and more diversity of microorganisms in the gut.

What we’ve found works best for most people is a combination of probiotics (from fermented foods and/or probiotic supplements) and getting about 35-40 grams of fiber per day (more if you’re diabetic or trying to lose weight).