What is the mechanism of heartburn?

Question

My father recently forwarded me a factoid saying:

most of us believe that heartburn is caused by excess production of gastric acid. In fact, in most cases it is caused by improper functioning of the stomach muscles, which increases the pressure within the stomach. This is why antacids create a very short-lived relieve, while taking herbs which soothe the stomach muscles are a permanent solution to the problem.

To me, this sounds like bad advertising copy. But before I dismiss it completely, could they be partially or even completely right? What causes heartburn, is it acid overproduction, musculature problems, or something else?

Answer 1

WebMD has an interesting article on heartburn that states¹

With gravity's help, a muscular valve called the lower esophageal sphincter, or LES, keeps stomach acid in the stomach. The LES is located where the esophagus meets the stomach -- below the rib cage and slightly left of center. Normally it opens to allow food into the stomach or to permit belching, then closes again. But if the LES opens too often or does not close tight enough, stomach acid can reflux, or seep, into the esophagus and cause the burning sensation.

This would seem to be something of a union of both theories of heartburn. I was slightly doubtful - the article doesn't cite a particular source - but I was able to do a bit of digging on my own and found a few papers and other reliable sources that support this.

• Boeckxstaens (2005) states (abstract only)

  The two typical examples of dysfunction of the LOS are achalasia and gastro-oesophageal reflux disease (GORD). . . . GORD results from failure of the antireflux barrier, with increased exposure of the oesophagus to gastric acid. This leads to symptoms such as heartburn and regurgitation, and in more severe cases to oesophagitis, Barrett's oesophagus and even carcinoma.

• The Keck School of Medicine states

  The esophagus is a muscular tube that connects the pharynx in the throat to the stomach. There are two sphincters at each end, an upper esophageal sphincter and a lower esophageal sphincter. With normal swallowing there is an incredible coordination of neurologic and muscular events for the food or liquid to efficiently pass from the mouth and into the stomach. Any dysfunction in this process can manifest as difficulty swallowing solids and liquids, regurgitation of undigested food, chest pain, and even heartburn.

• Spechler et a. (1995) writes

  Heartburn, the main symptom of gastrooesophageal reflux disease (GORD), might be expected to occur infrequently in achalasia, a disorder characterised by a hypertensive lower oesophageal sphincter (LOS) that fails to relax.

It appears the dysfunction of the LES/LOS is considered to be a cause of heartburn, specifically causing GORD, which then leads to heartburn.

I'd like to emphasize that heartburn generally isn't a standalone illness, but a symptom of something else, just like a sore throat may be a symptom of, say, strep throat. This isn't always the case, but it generally is the case for instances of persistent heartburn.

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¹ The Mayo Clinic also has a rather accessible article for anyone who doesn't want to trudge through scientific papers.