I've read online that we shouldn't lie down after eating because it'll contribute to acid reflux. However, I personally never feel the presence of acid reflux if I lie down after eating. What are the other negative effects? Are there any serious long-term potential side effects aside from some possible heartburn (which I personally never experience)?
Advice against lying down after a meal is usually given to those who already have acid reflux (WebMD) and not to everyone.
Reflux comes from excessive relaxation of the lower esophageal sphincter (LES) or diaphragmatic sphincter (the later occurs in hiatal hernia), which results in a free flow of acid into the esophagus when you lie down. If your LES functions properly, you will not likely have acid reflux if you lie down.