YOU learn a lot about your friends on holiday. Late on a hot evening, after the somethingth bottle, sex and death hold no taboos and you land up nattering about everything from childhood abuse to the most intimate details of your latest affair.

But there is one real no-go area. Everyone notices, but nobody mentions, that one of your pals disappears, after breakfast every day, to monopolise the bathroom for 45 minutes at least. Terrible constipation? Agonising piles? Early training by a ferocious nanny who put a tick or a cross in a book, according to whether you did or didn't Go?

It is not something you can ask about. Everybody does it (except of course the Queen), but the details of how often, how much, how hard, or what colour are less easy to enquire about than last night's orgasm. Recently, however, the fearless Mary Killen, in an article about food allergy in Harper's and Queen, let drop (I know, I know) the information that before her nutritionist's treatment she used to propel out a 'tennis-ball' twice a week (we hope, for her sake, not in a single lump), but now she can boast 'one or two Toulouse sausages a day'.

Lucky her will be the secret reaction of thousands who suffer from constipation, colitis, haemorrhoids, or the painful, stress-related 'irritable bowel syndrome'. One London GP told me that 'more than half' of his patients have some disorder in their excretion mechanism; at least a third of all the women he sees are badly constipated. (This might be a low figure, as often doctors just ask 'Are your bowels normal?' - which is a length-of-piece-of-string question.)

These problems afflict females most: constipation and piles are almost universal in late pregnancy. For both sexes, stress is also important: when food is gobbled down into a tense stomach, digestive juices do not work properly and the gut becomes blocked and toxic.

Emptying the bowels, though, goes far beyond the purely physical. It is a ritualistic, even an atavistic, activity. Men are especially, it seems, the ones for ritual enthronement.

George, a 35-year-old lawyer, admits that he just can't go in strange places, and has to make excuses to avoid the foreign holidays his girlfriend keeps suggesting - or suffer a fortnight's torment. Brian, a lecturer in his early 50s with a history of serious piles, also can't go except at home; his routine is so important that he re-schedules early classes to avoid leaving home before the deed is done.

My husband, when questioned, said: 'I wouldn't dream of having a crap at the office - surely nobody does?' (Well, well, the things you find out.) Two people cited it as the real reason why they consistently failed to give up smoking: the first fag of the day was what did the trick.

Fundamental blockages are more than just embarrassing or inconvenient, though: they kill. Cancer of the bowel, almost unknown in rural Third World countries, is now the most common fatal cancer in the US; in this country it takes joint first place with lung cancer. And research shows that it is directly linked to dietary habits that cause colonic blockage. So, without a normal, healthy throughput, you could die young and painfully.

Normal? Healthy? For all sufferers, graphic description of the Mary Killen sort can prove a great relief (I'll stop apologising) precisely because it's so difficult to make comparisons. Take my friend Kathy. An ebullient, pencil-thin American in her late thirties, she looks like a dream of success and confidence, though she lives on a knife-edge of tension. She and I have seen each other through thick and thin, and there is (as I thought) nothing we don't talk about. So I almost fell over with surprise when she told me that she goes to the loo about once every two weeks, and each time produces a few hard pellets like sheeps' droppings. Yes, she had been to her doctor. No, he wasn't very concerned: he had told her that as long as it was regularly every two weeks, that might be normal for her. Whatever you do, he said, don't get dependent on laxatives.

The doctor's reaction is shocking, but it is probably evidence not so much of stupidity and callousness as of one of those pendulum-swings to which medicine is subject. Only a few years ago, before dietary fibre became almost synonymous with moral fibre, the over-use of laxatives was a serious problem. To combat all this, doctors decided to relax Nanny's once-a-day rule by telling patients that whatever they did was probably all right.

Audrey Eyton, the high priestess of fibre, whose squillion-selling F-Plan Diet heralded a real change in our views, knows which side she's on: 'Come back matron, all is forgiven.' In her enjoyably frank chapter on the differences between developed and the developing countries, she reports that a rural African or Asian on a fibre-rich diet, will 'effortlessly - and daily - evacuate nearly one pound in weight of soft stool', while we miserable, bunged-up Westerners are lucky if we can rid ourselves of 'only a quarter of that weight in much firmer, harder stool . . . and only with difficulty'. It's enough to make you run, wincing, for the bran. Eyton describes one eminent medical researcher, when showing colour slides of the bowel movements of rural Africans, exclaiming 'Look - aren't they beautiful]'

Well, in the eye of the beholder. Still, modern research proves him right: he was looking at the stool of someone very unlikely to die of any of a handful of degenerative diseases. For this reason, as well as for our well-being and good-temper, we should all take the advice of the great Duke Ellington, who once did an advertisement for a 'herbal health preparation' (the word laxative was obviously too hot for 1940s America) with the slogan 'Leave all your troubles behind you'.

(Photograph omitted)