If you sit anywhere long enough - be it the banks of the Ganges or the saloon bar of the Dog and Duck - sooner or later you will hear someone say something interesting. It happened to me last week in the most improbable place imaginable: the public gallery of the European parliament at Strasbourg, where whole aeons can pass without anyone saying anything remotely interesting.

The remark I heard, or thought I heard, was: "Broccoli has lost 80% of its nutritional value." The implications of this were staggering. No other item of food so powerfully represents the dictates of healthy eating against pleasurable eating. "Eat your broccoli or else," is the most consistent mantra of the average mother-son relationship. In our household, we have staged - in my wife's absence - NSB (No Stupid Broccoli) weeks as a counter-attack.

It has even acquired political resonance. When George Bush Snr declared, as president, that he hated broccoli, he was furiously denounced by all the worthy mothers of America. But I can tell you that Al Gore lost heavily among male voters because he was, quite obviously, an enthusiastic broccoli eater. You can tell.

So was it possible? Had I misheard? If broccoli isn't good for you, what's it good for? I rang Tim Lang, professor of food policy at Thames Valley university, who said: "I wondered when someone was going to get on to this." He put me in touch with a man called David Thomas who said: "At last!"

Thomas is a Sussex-based researcher who did something simple, the essence of all major breakthroughs. He went to the British Library and found all the past editions of The Composition of Foods by McCance & Widdowson, the Wisden of British nutrition, which notes the content of all major foods. The book first came out in 1940 and by checking right back, he was able to plot some startling trends.

The problem is both broader and narrower than the Strasbourg broccoli remark suggests. Thomas's research refers specifically to trace elements, those obscure minerals which we all know we need because it says so on the side of cereal packets. He discovered that, since 1940, there had been some stunning declines of these in all fruit and vegetables: calcium is down 46%; sodium down 49%; copper down 75%. More specifically, carrots have lost 75% of their magnesium; broccoli has lost 75% of its calcium; and sodium has disappeared entirely from runner beans. There are less dramatic declines almost across the menu.

Most of us are vaguely aware that we need calcium for healthy teeth and bones: that's the sort of thing you hear from adverts. But copper? Apparently, it's a crucial guard against premature ageing in general, and ailments as varied as piles and emphysema in particular. Sodium? You can get too much of it, but it's essential to the working of the nerves and muscles. Magnesium, meanwhile, has all kinds of magical properties in warding off anything from kidney stones to PMT. And virtually all our intake comes from vegetables, fruit and nuts. That much is accepted science.

There are at least two possible explanations for these findings. Thomas himself thinks it is to do with the nature of modern agri-business, which has dumped huge quantities of fertiliser on the soil, increasing the content of the basic NPK growth elements at the expense of everything else. Professor Lang takes it further back: to plant breeding in general.

Every keen gardener will by now have received a load of seed catalogues, offering all kinds of newly developed vegetable varieties. They will have been specially bred to mature earlier, to resist disease, to last longer, to look better. The unglamorous business of trace elements is way down the priority list. And if that's true for ordinary gardeners, it's going to be 10 times more true at the industrial level, where our diet is controlled.

Thomas, it has to be said, is not a wholly unbiased observer. He is in the food supplements business, which the EU is now attempting to control in the interests of harmonisation. He thinks it's possible that somewhere in these statistics are the answers to the mysterious increase in a variety of diseases and problems - diabetes, asthma, allergies in general, leukaemia, eczema and so on. For all I know, this is drivel. But there are questions here that require answers.

Professor Lang thinks so: "I don't understand why nutritionists aren't looking at this. It's an issue that will come, I'm absolutely convinced."

In the meantime - and this is not going to go down well with at least one eight-year-old I know - the answer is not that we can give up broccoli. On the contrary, we have to eat four or five times as much to get the benefit that would have been available 60 years ago. Eat MORE broccoli or else!

matthewengel@ndirect.co.uk