Belly doing flip-flops again? For most people, abdominal upset is an occasional fact of life. “It affects everyone some of the time, and some people all of the time,” says Victor Sierpina, MD, professor of family and integrative medicine at the University of Texas Medical Branch, and author of The Healthy Gut Workbook (New Harbinger, 2010). In fact, one in three adults has a digestive disease, such as irritable bowel syndrome (IBS), gastroesophageal reflux disease (GERD), food intolerances, or leaky gut syndrome. The rest of us have periodic bouts of heartburn, diarrhea, constipation, gas, and bloating.

To understand why tummy troubles are so common, try to visualize this: The gastrointestinal tract—mouth, esophagus, stomach, small and large intestines—has an absorptive surface the size of a tennis court. “It’s one of the major routes where the outside world meets the inside of our body,” says Angelica S. Vrablic, PhD, manager of nutrition research for NBTY, a dietary supplement manufacturer. It’s exposed to toxins, allergens, viruses, and bacteria, as well as food. Good thing, then, that 70 percent of immune cells are in the GI tract, ready to defend against anything that comes in. The downside to this hypervigilance is that the digestive system’s reactions to invaders—real or perceived—often cause unpleasant symptoms.

Aging tends to aggravate the situation: Declining production of stomach acid—which helps activate enzymes—tends to mean weaker digestion. Ironically, you feel stomach acid more as you get older. “The valve between the stomach and esophagus gets weaker and floppy,” Vrablic says. This lets stomach acid splash up, leading to heartburn and reflux. Proton pump inhibitors—drugs like Prevacid and Prilosec—tend to worsen the cycle, and should be taken only as a last resort. Blocking stomach-acid production relieves irritation and may give the GI tract a chance to heal, but it also allows bacterial infections to thrive (which means food doesn’t get broken down as well) and may lead to more serious problems.