What makes breath smell?

Breath only smells when certain aromatic chemicals dissolve within it. Unfortunately many of these chemicals are regularly produced when proteins from food are broken down by bacteria in the mouth. Examples are:

* methyl mercaptan (colourless gas found in foods like nuts and cheese, smells like rotten cabbage)

* putrescine (gas produced when the body breaks down amino acids in food, smells of decaying meat)

* hydrogen sulphide (gas produced by bacteria in the colon and when the body breaks down amino acids in food, smells like rotten eggs).

These chemicals can be absorbed into your bloodstream from the bowel and then circulated around the body until they are excreted via the lungs in breath.

In this way, garlic rubbed into the soles of your feet can later be detected in trace amounts in your breath.

Morning breath

Practically everyone has a degree of halitosis first thing in the morning.

When we sleep, saliva flow drastically reduces while your tongue and cheeks move very little. This allows:

* food residues to stagnate in the mouth

* dead cells to accumulate that would normally be shed from the surface of your tongue, gums and the inside of the cheeks.

As bacteria in your mouth start to work on and breakdown these residues, they generate an unpleasant smell.

Although normal, anyone with nasal congestion who mouth-breathes at night is more likely to be affected.

Morning breath usually disappears after breakfast and daily brushing, because saliva starts to flow again and any leftover residues are washed away and swallowed.

Occasional bad breath

The most common causes of occasional halitosis include smoking cigarettes or cigars, drinking alcohol or eating certain foods.

Culprits that are often to blame include onions, garlic, curries, cured foods like salami and cooked foods like kippers.

Smoking also reduces the flow of saliva in its own right and so worsens bad breath.

Crash dieting or fasting can also lead to halitosis. When the body no longer has a supply of carbohydrates, it first breaks down glucose stored in the muscles and liver.

After a few hours, the body begins to break down its fat stores. The waste products of their metabolism, known as ketones, give breath a distinctive sweet and sickly smell.

You can smell this on the breath of anyone who has vigorously exercised but who hasn't eaten enough carbohydrates before or after their workout.

People on strict high-protein diets experience the same effect for similar reasons.