Wednesday, September 5, 2007

Why do some foods give me gas?

-Anonymous, USA

I am sure many people will giggle at this question, but it is an excellent example of everyday science!

Gas is a natural by-product of consuming food and digesting it. The body relieves the build-up of gas pressure by belching (burping) or flatulence (farting). People are embarrassed to admit they burp or fart and some even claim that they do not pass gas. If this were true, they would inflate like a balloon and suffer terrible abdominal pains. Ouch!

Belching (burp!)

Belching is the release of air that gets trapped as we chew and swallow our food. If you drink carbonated beverages like Coke and 7-Up, you will burp the carbon dioxide gas that makes the drink fizzy. Some people can burp on demand simply by swallowing air.



Our anonymous friend has noticed that certain foods make them flatulent. Perhaps you may have noticed that beans, cabbage, high fiber breads and cereals give you gas. In general, this holds true for many people.

Let us take a look at how our bodies make gas.

The digestion process


Digestion begins once we put food in our mouth. The process of chewing physically mashes food and mixes it with enzymes (more on enzymes later) that fragment complex sugars. Once the food reaches the stomach, it is tossed around in very strong acid, breaking apart proteins, sugars, and fats into more manageable pieces.

Afterwards, the processed food is passed into the small intestine, where the nutrients and water are absorbed and the waste is passed on. The cells lining the intestines use enzymes to crack apart complex sugars so they can be absorbed easily.

Our bacterial friends

Inside our intestines live bacteria called Escherichia Coli, or E. coli for short. Our bacterial buddies assists in food absorption, waste processing and vitamin K production (a chemical that helps your blood to clot when you get cut). These bacteria are also responsible for making gas in your digestive tract, and…farts.

Here is a picture of E. Coli taken by an electron microscope.

Before you blame E. coli for all you flatulent woes, we need to understand why it makes gas.

Here is where things get interesting!


As I mentioned in the Antibiotics discussion, bacterial cells and our cells have similar machinery of life. Some of these machines are called enzymes.

Enzymes are proteins that perform the chemical reactions of life. There are many kinds of enzymes, each having a very specific function. Some enzymes only work with DNA, some with sugars, others only send signals.


Sugars are complex chemicals that are made by most living things, from bacteria to plants to humans!

Sugars are the universal source of chemical energy that makes life possible.

There are many different kinds of sugars with names like sucrose (table sugar), lactose (milk sugar), glucose, and fructose.

Here is a picture of table sugar from my kitchen.

Did you know that different sugars have different levels of sweetness?

It’s true!

A group of scientist figured out how to measure the relative sweetness of various sugars.

For example, they determine that fructose is about two times sweeter than sucrose (table sugar). This means that if you are making a sweet drink then you can use less fructose than sucrose to sweeten it. This is why most drink manufacturers use high fructose corn syrup instead of sucrose!

“Right-handed” sugars and “left-handed” sugars!

Many sugars can be “right-handed” or “left- handed.” This means they are mirror images of each other. An example of a mirror image is our hands.


Putting it all together!

Most sugars in nature are made in the “right-handed” version only. The enzymes in our intestines can only react with “right handed” sugars.

E. coli that live in our intestines can digest both right and left-handed sugars.

Foods like beans, cabbage and high grain breads and cereals contain “left-handed” sugars that only the bacteria can process.

When bacteria digest sugars, they make gases like carbon dioxide and methane. This is flatulence!

Now you know why certain foods cause gas!

Dr. Dave