In a comment on this post at Liberty Alone (a liberal Lib Dem blog, if you didn't know), someone called A C Baker pointed out in the comments the claim of the Soil Association that modern vegetables contain lower levels of vitamins and trace minerals than in the past.
This immediately set a few alarm bells ringing, because I was pretty sure that the King's Fund researches health policy issues rather than performing scientific research. A quick look at their website confirmed that scientific research is not what they do:
The King's Fund aims to improve health and health care by developing policy, people and services.
Bullshit detectors now sounding the alarms I followed the trail from A C Baker's original comment to its source. This turned out to be an article from the Soil Association's Information Centre, entitled What we can say - the quality and benefits of organic food. At first sight, it's a moderately impressive document with a list of references to the sources for each claim made. The section on vitamins reads:
Vitamins and minerals
No food has higher amounts of beneficial minerals, essential amino acids and vitamins than organic food.
The use of synthetic fertilisers, plant breeding, and longer delays between harvesting and consumption have led to reduced trace element and vitamin content in food.
The reference for this is, however, a bit different to the others. It reads
2. The King’s Fund, an independent medical charity.
This is not entirely helpful. You would have thought that, as a minimum, a document title would be required. But never mind; try searching the King's Fund Website. Enter "vitamins organic vegetables" - three documents returned, nothing relevant on any of them. Search the site through Google - no joy either.
A bit of more general Googling came up with a single study which be the basis for the claim. A scientist at the University of Texas called Donald Davis found apparent declines in nutrients in a range of plants over a 50 year period. He appears to be extremely cautious about his results though.
According to Davis, establishing meaningful changes in nutrient content over a 50-year time interval was a significant challenge. The researchers had to compensate for variations in moisture content that affect nutrient measurements, and could not rule out the possibility that changes in analytical techniques may have affected results for some nutrients.
So there may be a scientific basis for the claim, but it appears to be far from proven. Which might go some way to explaining why the referencing in the Soil Association's Information Centre is so vague.