Gas

Every normal, healthy person has gas and gets rid of it from the body by either burping or passing it through the anus. This gas is mainly made up of: carbon dioxide, oxygen, nitrogen, hydrogen, and sometimes sulfur and methane.   

You can get gas from:
Swallowing air
- Every time you open your mouth to talk, eat, or drink, you are swallowing air. This air can exit your body via a burp or a belch. However, if you do not burp up or belch out the air you swallow, this air can travel down to your intestines and be released through the anus as gas.
Eating certain foods
- Normally, the food that you eat gets broken down in the small intestines. However, there are certain foods that do not get digested in the small intestine. As these undigested foods make their way down to the large intestine, a bit of these foods will get broken down by bacteria and gas is formed as a byproduct. This gas then gets released from your body by exiting the anus. The reason why gas may have an unpleasant odor is because the bacteria in your large intestines may also release sulfur during the breakdown of food and sulfur smells like rotten eggs.

Common foods that cause gas include: vegetables, legumes (beans, peas), sugars and sugar substitutes, whipped foods, carbonated drinks, fried and fatty foods, artificial sweeteners, milk and other dairy products.

Taking certain medicines
- Gas is a common side effect of certain medicines including Pysllium, lactobacillus, probiotics, narcotics, and penicillin.
- Having certain medical conditions. Gas is a common symptom of medical conditions including lactose intolerance, celiac disease (disease of the small intestines) and Irritable Bowel syndrome.