Exercising shortly after eating certain foods can cause a severe allergic reaction for some people.
For a small group of people in Singapore, exercise is not something their doctor would recommend lightly.
One example is 15-year-old Dan (not his real name), who was rushed to hospital two years ago with hives, facial swelling and tightness in the throat. He was also wheezing.
He had never had an allergy to prawns, which was what he had eaten at dinner.
Nor had he ever reacted badly to playing basketball, which was what he had done about half an hour after eating the prawns.
Dr Elizabeth Tham, an associate consultant at the division of paediatric allergy, immunology and rheumatology at National University Hospital, realised that it was a combination of the two that had brought on the attack.