Do not exercise immediately after eating.

This is common advice, but it means life or death for a small group of people who suffer from food-dependent exercise-induced anaphylaxis.

The condition means that any form of exercise done after eating a food they are sensitive to will trigger off a potentially deadly reaction.

In Singapore, at least five people have been known to suffer from this rare condition, reported Shin Min Daily News. Of the five, three are male and the rest, female.

They were diagnosed between August 2006 and July 2008. During these two years, the five - aged between nine and 20 - had separately shown severe allergic reactions when they exercised after a meal and had to be admitted to a local hospital for emergency treatments.

Symptoms displayed included urticaria, or hives, hypotension and breathing difficulties. Two of the patients also reportedly lost consciousness.

The five had exercised 15 minutes to 21/2 hours after eating shellfish.

Exercise-induced anaphylaxis

Anaphylaxis is a very severe allergic reaction that affects the entire body.

This life-threatening condition may occur minutes to hours after exposure to an allergen.

The person may have skin rash or hives, a rapid, weak pulse, and nausea or vomiting.

MayoClinic.com reported that the massive amount of chemicals released by the immune system during anaphylaxis can cause one to go into shock; airways narrow and breathing becomes difficult, while blood pressure drops suddenly.

If left untreated, death may result.

Anaphylaxis can be triggered off by exercise. This condition, known as exercise-induced anaphylaxis (EIA) can cause the person to experience hives, itching, shortness of breath, and low blood pressure, usually five to 30 minutes after they begin exercising.

Jogging is the most common type of exercise that triggers off EIA, notes a medical doctor in About.com.

Some people experience EIA symptoms if they eat something they are sensitive to less then 24 hours before exercising. Called Food-dependent EIA, this condition occurs only if the person exercises after eating a food they are allergic to. The food or exercise alone does not produce symptoms, notes BBC.

Prompt treatment for anaphylaxis is critical as it can result in death. In order to prevent EIA symptoms, doctors advise people to avoid exercising alone and to only exercise on an empty stomach.

What the five patients ate
Patient #1: 19-year-old Chinese male
He had eaten some homemade prawn salad before setting off for a run. Halfway during his run, he was hit by a severe pain in his stomach. He also suffered from dizziness, diarrhoea and vomiting.

Patient #2: 18-year-old Chinese male
He ate food containing fish and prawns before going for a run. When he finished, his whole body broke out in hives.

Patient #3: 20-year-old Chinese female
She had eaten food cooked with prawns before going for a run. Halfway, her whole body started itching terribly and hives developed. She eventually lost consciousness.

Patient #4: 15-year-old Vietnamese male
He had eaten food containing shellfish in his school canteen before playing a basketball game. During the game, he broke out in hives and lost consciousness.

Patient #5: 9-year-old Chinese female
She had eaten seafood at home and went for a swim thereafter. After she completed her swim, her throat felt swollen and she experienced breathing difficulties. She also broke out in hives.