Drinking our morning coffee is one of life's little pleasures. Unfortunately, coffee is notorious for inciting acid reflux, GERD, and worsening peptic ulcers. This is especially the case with more heavily roasted coffees and instant coffee.
What is it about coffee that makes it so problematic? The underlying reasons for the irritability of coffee are a little mysterious. People often cite the acidity of coffee, and it does seem that coffee that tastes less acidic is less harsh on the stomach. However, considering the stomach has a pH that makes coffee appear comparatively mild, there is likely more to this than meets the eye. Furthermore, there are many foods that taste acidic that don't elicit the same response. Malic acid in green apples is very tart and lemons are barely edible for the amount of citric acid they contain, yet eating these fruits doesn't typically cause acid reflux (they may make ulcers sting, but that's another story).
The classic stimulant caffeine almost certainly contributes to acid reflux to some extent. Caffeine definitely relaxes the esophageal sphincter, which is a muscle whose function is to separate stomach acid from the esophagus. The stomach is full of concentrated HCl, which would damage the lining of the stomach except for the presence of a thick alkaline mucus bilayer maintained by prostaglandins and certain nutrients such as Vitamin U. However, decaffeinated coffee can still cause acid reflux so there's more to it than just caffeine.
Stomach acid has several functions, from inhibiting the growth of bacteria to unraveling dietary protein and providing the right pH for the proteolytic actions of pepsin. What isn't widely known is that the stomach isn't full of acid at all times. In fact, eating stimulates the production of the hormone gastrin, which via a chain of events results in the secretion of HCl into the stomach. The key dietary component that stimulates gastrin release is protein. Scientists were curious to understand what is it about protein that triggers this response. Protein consist of 20 types of amino acids that have different properties. The aromatic amino acids phenylalanine and tryptophan were by far the most stimulatory (the other aromatic amino acid tyrosine was not tested due to solubility issues).
The fact that aromatic amino acids were most stimulatory may be quite revealing. Other amino acids all have an acidic group (in fact, some even have two), so it's not acid per se that is the issue. It would seem that the aromatic side chain is the effector (chemically, aromatic simply means that it has a benzene ring). This is where coffee comes in. Coffee has over 2000 compounds, some of which have benzene rings just like the aromatic amino acids. These are collectively referred to as cinnamic acids, and are present in many vegetables, fruits and other plant-based products. Examples include caffeic acid, p-coumaric acid, ferulic acid, and esters thereof. Though it hasn't been demonstrated conclusively, one may wonder whether some of the adverse gastronomical effects of coffee may in part be due to the fact that coffee is an extract containing a wide array of compounds that bear an uncanny resemblance to known acid producers. As extracts, cinnamic acids in coffee are easily accessible. Furthermore, it has been estimated that coffee is the richest sources of cinnamic acids in the Western diet at up to 1 g per day.
Can Vitamin U help with acid reflux? As the mode of protective action afforded by Vitamin U is via the stimulation of mucus secretion in the stomach, Vitamin U should protect the lining of the stomach to some extent. There is evidence that Vitamin U can help maintain mucosal integrity in other parts of the alimentary canal such as the esophagus. However, the mucus lining the esophagus is thin and not built to withstand concentrated hydrochloric acid. The protective effects conferred by mucoprotectants such as Vitamin U are most effective when used in conjunction with dietary modification that avoids the worse offenders like coffee. Drinking lightly roasted low-acid coffees still have plenty of caffeine and are less likely to cause problems.