SIBO (Small Intestine Bacterial Overgrowth) is when good bacteria are in the wrong place: specifically, bacteria that should be in the large intestine are instead in the small intestine, or when there are more bacteria in the small intestine than there should be.

GERD, heartburn, or reflux involves hydrochloric acid in the esophagus, rather than trapped inside the stomach, where it’s supposed to be. This happens because the lower esophageal sphincter (LES) requires a low pH to trigger its closure. If the pH is too high (meaning acid levels are too low), the sphincter will not close effectively. Any amount of acid in the esophagus will trigger the symptoms of GERD—so while it feels like there’s too much acid, the exact opposite is actually the case.

Considering these two conditions involve different parts of the digestive system, you might not think they’re necessarily related—but they are, in two key ways.

Low Stomach Acid —> GERD

As mentioned above, contrary to conventional wisdom, GERD is caused by low, not high, stomach acid. This makes sense when you consider that while stomach acid production declines with age, incidence of GERD simultaneously rises. It makes even more sense when you consider that H Pylori, the ubiquitous organism most often associated with GERD, actually decreases stomach acid secretion.

Just as bile helps to keep bacteria in check in the small intestine, one of the primary jobs of HCl is to prevent bacterial overgrowth in the stomach, as well as in the upper part of the small intestine too. As you might expect, low stomach acid therefore encourages bacterial overgrowth.

Also, digestive juices tend to trigger one another like a chain reaction, beginning with salivation. In the same way, HCl triggers the pancreas to release enzymes that help to break down macronutrients, including carbohydrates. Lower HCl leads to lower pancreatic secretion, which leads to poorer digestion of carbohydrates. When we don’t break down our carbs well enough to absorb the nutrients, the bacteria in our intestines do so for us — and what you feed, grows. This is another way that low HCl can lead to SIBO.

SIBO —> Increased LES Pressure

Once you have bacterial overgrowth in the small intestines, they ferment your carbs and produce hydrogen and/or methane gas as part of the process.

Gas in the small intestine doesn’t escape as flatulence—it’s closer to the other end, where it can escape as belching. Belching and eating are the only two times when the lower esophageal sphincter should open—but in GERD, it opens at other times as well, leading to a backsplash of HCl into the esophagus. It’s well accepted that GERD occurs due to increased LES pressure, secondary to increased intra-abdominal pressure (IAP). Generally accepted causes of IAP include lying down, obesity, or overeating—but if there’s a lot of gas in the small intestine, that will certainly do it too.

The Upshot

GERD can cause SIBO. SIBO can also at least exacerbate, if not cause existing GERD.

It’s all interrelated!