The following correction was printed in the Guardian's Corrections and clarifications column, Friday February 3 2006





The mineral content of milk and popular meats has fallen significantly in the past 60 years, according to a new analysis of government records of the chemical composition of everyday food.

The research looked at government tables published in 1940, and again in 2002, in the nutritional bible, The Composition of Foods, to establish levels of important minerals in dairy products and meat before the second world war and today.

The research, which is contested by the food and farming industry, found a marked decline in nutritional value during the period. The analysis is published in this month's Food magazine by the consumer watchdog the Food Commission.

The levels of iron recorded in the average rump steak have dropped by 55%, while magnesium fell by 7%. Looking at 15 different meat items, the analysis found that the iron content had fallen on average by 47%. The iron content of milk had dropped by more than 60%, and by more than 50% for cream and eight different cheeses. Milk appears to have lost 2% of its calcium, and 21% of its magnesium too.

Most cheeses showed a fall in magnesium and calcium levels. According to the analysis, cheddar provides 9% less calcium today, 38% less magnesium and 47% less iron, while parmesan shows the steepest drop in nutrients, with magnesium levels down by 70% and iron all gone compared with its content in the years up to 1940.

The reseach was conducted by David Thomas, a chiropractor and nutritionist who prescribes and sells mineral supplements. He published an earlier historical analysis of the nutrient content of fruit and vegetables in 2000 which showed a similar decline in those foods. He attributes the loss of nutrients to intensive farming and industrial production.

Academics in the US and Denmark have also reported signficant changes in the nutritional profile of modern foods.

The Food Commission believes that changes in the methods of measuring the composition of food cannot account for the huge difference in nutrient content and has called for independent research on the effects of different farming methods.

"Minerals are easy to detect and measure and have been since the 19th century. It is almost impossible that methods have changed so much that it would explain the huge difference between these figures," the Food Commission's director, Dr Tim Lobstein, said. "One of the key arguments is that today's agriculture does not allow the soil to enrich itself, but depends on chemical fertilisers that don't replace the wide variety of nutrients plants and humans need."

Scientists at the University of Newcastle's agriculture school have looked at differences in the fat and vitamin composition of milk produced in different farming systems. "We know that the faster grass grows the more you dilute the uptake of trace elements," Gillian Butler, a researcher, said. Another explanation might be that in traditional farming, clover, which is higher in minerals than grass, also played a greater part in feeding animals.

The director of the Italian parmesan consortium, Leo Bertozzi, said the figures were a puzzle. "Our methods of making cheese have not changed, but milk in 1940 was not the same as milk today. Today cows yield five to six times as many litres a day, and their feed is different, with cereals and soya added to hay. But I find these figures surprising," he said.

The research has, however, been challenged by the food and farming industries which argue that the testing methods have changed. They also say that huge changes in the varieties grown and the ways in which food is transported and stored, make direct comparison difficult.

The Food Standards Agency, which publishes The Composition of Food, agrees that using the government tables to make historical comparisons is problemmatic. "Any differences over time could be due to a wide variety of factors, including variety and breed, animal husbandry, growth, storage conditions, preparation and cooking methods as well as diferences in analytical methodology," it said in a statement.

The Dairy Council said it believed that changes in farming practices and environmental factors would account for only a small reduction in mineral content. "It is more likely that the differences are due to improvements in analytical methods used to measure minerals in milk," its director, Judith Bryans, said.

The Meat and Livestock Commission also attributed the decline in nutrients to better testing methods. "What goes in is what comes out, and the only signficant shift in beef production has been from hay to silage. If these figures were true we'd expect to see a lot of anaemic cows wondering around," Mike Attenborough, an MLC technical expert, said.