Normally when you swallow, your lower esophageal sphincter — a circular band of muscle around the bottom part of your esophagus — relaxes to allow food and liquid to flow down into your stomach. Then it closes again.
However, if the lower esophageal sphincter relaxes abnormally or weakens, stomach acid can flow back up into your esophagus, causing heartburn. The acid backup is worse when you're bent over or lying down.
Risk factors
Certain foods and drinks can trigger heartburn in some people, including:
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Alcohol
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Black pepper
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Chocolate
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Coffee
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Fatty food
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Fried food
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Ketchup
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Mustard
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Orange juice
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Peppermint
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Soft drinks
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Tomato sauce
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Vinegar
Complications
Heartburn that occurs frequently and interferes with your routine is considered gastro-oesophageal reflux disease (GORD). GORD treatment may require prescription medications and, occasionally, surgery or other procedures. GORD can also cause serious complications.
See your doctor if your heartburn becomes more frequent or no longer responds to over-the-counter medications. Your doctor can determine if your heartburn is likely to be a symptom of GORD.
References:
http://www.emedicinehealth.com/heartburn/article_em.htm
http://www.medicinenet.com/heartburn_reflux/article.htm
https://en.wikipedia.org/wiki/Heartburn
http://www.health.com/health/gallery/0,,20527745,00.html