YOU CAN spot the truly seasoned backpacker: he collects stories about his bodily functions. He can — and will — tell you how many times he vomited on a night bus in Laos, or about the occasion when he didn’t know whether to squat or kneel after an attack of Delhi belly, or the best type of food to mix with Paludrine and Imodium tablets.

This is the reverse culture shock that backpackers encounter when they return home. It comes as something of a surprise to discover that normal people don’t discuss their bowel movements over breakfast.

I’ve done a bit of travelling myself, and collected the scatalogical tales — such as the time in my gap year in northern Israel, when, on my first night with a charming host family, I reacted badly to the local cuisine. I was desperately tired, and thought maybe I could sleep it off. But sleeping meant relaxing my muscles. And relaxing meant disaster.

I moved to another family.

But it’s in South America that my dignity has most comprehensively imploded. I was in Chivay, a tiny village at 12,000ft in the Peruvian mountains. Chivay had one shop, one post office and an Irish pub. I went to the pub and ordered a “bacon burger”.

“Would you like it heated?” asked the Irish barman, José.

“No, I’d like it cooked.”

But José wafted the burger over the lukewarm coals just long enough to galvanise every dormant bacterium in Peru. Half an hour later, I was running for the loo. After two hours, I was afraid I might die. After four hours, I was afraid I might not. I eventually emerged five hours later, a wiser, older, significantly lighter man.

Then, just as one bodily function began to work properly, another went spectacularly wrong. I had started taking the drug Diamox to help me cope with South America’s rarified altitude (talking about what drugs you’re on — another backpacker habit). As well as giving you unpleasant pins and needles, this drug makes you pee incessantly.

Not a problem, had I not, fortified with powerful gastric inhibitors, boarded a loo-less night bus from Arequipa to Puno, on the shores of Lake Titicaca. Despite having taken nil by mouth for the entire day in preparation, by 3am I could bear it no longer. Even the film (Death Wish IX) couldn’t distract me from the constant pressure on my bladder.

I took myself off to a quiet seat at the back of the bus, and was trying to relieve myself into an empty bottle of Inca Kola when I looked up to see the bus conductor looking down on me.

I still can’t quite believe that he decided to punch my ticket and not me. I resolved to let my digestion make all my travel decisions for the remainder of the trip.