Any thoughts on if drinking coffee (espresso) on an empty stomach is a bad idea? I usually reach for 1-2 shots in the morning on an empty stomach and my first meal wouldn't be until at least a few hours after waking.
I haven't noticed any ill effects but I'm wondering if this is a bit of an acid load on my stomach/digestive system.. has only recently struck me as an issue as scheduling has pushed by my first meal of the day mon-fri.
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The pH of espresso is similar to the pH of regular coffee, which is 4.9 to 5.2 ( http://www.answerbag.com/q_view/1985048 ). The pH of the human stomach is 2 to 3 ( http://www.suite101.com/content/the-human-stomach-a52808 ). Since pH is a log scale, the stomach is normally 100 to 1000 times as acidic as a cup of espresso. So the acid is probably not a problem.
If you're prone to gastro-esophageal reflux (GERD), the one thing I would be cautious about is the caffeine. Caffeine can aggravate GERD ( http://digestive.niddk.nih.gov/ddiseases/pubs/gerd/ ), and a serving of espresso typically contains 58 to 185mg of caffeine ( http://www.energyfiend.com/caffeine-content/espresso ). A shot (1 to 2oz.) of espresso contains a similar amount of caffeine as a 7oz. serving of regular coffee ( http://coffeefaq.com/site/node/18 ).
Since espresso is similar to regular coffee in pH and caffeine content, it's probably no worse for you in the morning (or any other time of day) than regular coffee.
There may be some compounds in some types of coffee that could increase gastric acid secretion but it's not clear yet. Veronica Somoza and Thomas Hofmann are the big names who are currently doing research on this topic. See http://pubs.acs.org/doi/abs/10.1021/jf903614d, for example.