There is no hard scientific evidence that acid reflux causes constipation or diarrhea (or vice versa). However, many people experience these symptoms at the same time, so there is probably some sort of relationship.
Eg. this study has found that treatment of constipation has significantly improved acid reflux symptoms.
Low stomach acid and digestion
Stomach acid is an important part of the digestion process. Once food reaches the stomach, the stomach acid starts to break it down.
In 2-4 hours after eating stomach acids break down about 30% of the fat to fatty acids. Then food is pushed into the small intestine, the gallbladder releases bile, the pancreas releases its enzymes and the digestion process continues.
But how do the gallbladder and pancreas know when to kick in and release bile and enzymes? Turns out there is an enzyme – called cholecystokinin – that trigger this process:
Cholecystokinin is secreted by cells of the upper small intestine. Its secretion is stimulated by the introduction of hydrochloric acid, amino acids, or fatty acids into the stomach or duodenum. Cholecystokinin stimulates the gallbladder to contract and release stored bile into the intestine. It also stimulates the secretion of pancreatic juice and may induce satiety.(source)
Can you see the problem with low stomach acid levels?
Stomach acid is hydrochloric acid. Low stomach acid levels make digestion slower, less amino acids and fatty acids will be in the stomach.
So as food enters the small intestine, less cholecystokinin will be secreted, therefore less bile and pancreatic enzymes will be released. This makes the entire digestion process less effective.
What causes constipation?
Constipation is usually defined as having fewer than three bowel movements a week. The stool is hard and dry, and it is difficult or painful to pass.
There are a lot of potential causes of constipation, like stress, lack of physical exercise, not eating enough fiber, or not drinking enough, but the root of the problem is that the food moves too slowly through the digestive tract.
As we have seen above, low stomach acid levels might cause that.
Diarrhea – at the other end of the scale
If stomach acid levels are low, some bacteria might survive in the stomach and make it to the small intestine.
Bad bacteria might start to grow in the small intestine, causing all kinds of health issues, like stomach pain, nausea, bloating, etc.
This condition is called small intestinal bacterial overgrowth (SIBO).
Your body is trying to get rid of the bacteria like there is no tomorrow, and you have diarrhea.
Acid reflux and low stomach acid
While conventional medicine still says acid reflux is caused by too much acid in the stomach, more and more studies start to say, it might be quite possible that low levels of gastric acids can (also) trigger acid reflux and cause heartburn and other symptoms.
When digestion is slow, because of low stomach acid levels, food stays longer in the stomach, gas might be produced during digestion. This increases the pressure in the stomach and makes it easier for the acids to seep back into the food pipe.
Antacids, PPIs – let’s make things worse
More often than not, people with heartburn or other acid reflux symptoms are prescribed antacids or proton-pump inhibitors. Both of these make heartburn go away, so they must be great medications, right?
Well, not quite. The problem with these medications is that they only treat the symptom, not the underlying cause.
Heartburn, a burning sensation around the center of the chest, happens when stomach acid flows back (reflux) into the esophagus and irritates the lining of the food pipe.
These medications don’t prevent this acidic backflow, instead, they make the contents of the stomach less acidic by neutralizing stomach acids (antacids) or preventing stomach acid secretion (proton-pump inhibitors), so that esophagus irritation and chest pain goes away, problem solved.
However, as we have seen above, low stomach acid levels can cause multiple problems.
Nausea, vomiting, constipation, and diarrhea are just some of the common short-term side effects of antacids.
Acid reflux doesn’t seem to cause constipation or diarrhea.
However, low stomach acid levels – hypochlorhydria – might be a possible trigger for acid reflux, constipation, and diarrhea.
Acid reflux is often treated with medications that tend to cause either constipation or diarrhea as a side effect.
Dietary and lifestyle changes often help with acid reflux, you might want to try these first before taking medications. As an added benefit, you are less likely to have constipation or diarrhea if you are on a healthy diet.