IT is the news that generations of schoolchildren have been waiting to hear - a scientific survey has discovered that eating your greens may not be that good for you after all.

Data gathered during The Composition of Foods, a comprehensive study of the content of all major foods, carried out since 1940, shows that the mineral values of fruit and vegetables have plummeted over the period.

Spinach, which cartoon hero Popeye takes for strength, was found to have 60 per cent less iron than it did 50 years ago.

And carrots, famous for helping you see in the dark, have lost 75 per cent of their magnesium, which helps prevent heart attacks, asthma and kidney stones.

The study blames modern farming methods for the falls in mineral content. Nutritionist David Thomas, one of those behind it, said the findings shocked him.

"Minerals have been recognised as being very important to our physiology, but the general public has no idea that there has been this dramatic decline in the levels of such elements in our food," he said.

Over the past 50 years, watercress has lost 93 per cent of its copper, broccoli 75 per cent of its calcium, and runner beans nearly all their sodium.

Fruits have not fared much better, with 67 per cent of iron being lost from oranges, and 62 per cent of sodium from avocados.

Mr Thomas said: "We are made up of these substances."

Stuart Goldie, manager of Middlesbrough's Nature's World, which grows its own organic vegetables, said the findings showed the disadvantages of using fertilisers.

"I think it's another good reason for looking at organic produce," he said. "It's obviously much less force-fed than a lot of produce you get, and we would argue that it tastes better."

Between June and December, Nature's World operates a vegetable subscription scheme where people can buy produce from its market garden. Preparations are being made to expand it, using New Deal volunteers to operate the scheme all year round.