The Broadbalk Wheat Experiment is one of the longest running scientific studies in history.
Since the year 1843, the scientists have grown different strains of wheat and analyzed various factors, including nutrient composition.
From 1843 until about 1960, the nutrients in wheat didn’t change much.
However, from the year 1960, which coincides with the introduction of modern wheat, the nutrient content starts trending downwards.
Concentrations of Zinc, Copper, Iron and Magnesium were 19-28% lower in the years 1968-2005, compared to 1845-1967 (6).
At the same time, there was no evidence that the soil had changed. So it is clearly something about the nature of modern wheat that makes it less nutritious than the older varieties.
Another study that also compared different strains of wheat found that the older varieties contained significantly more Selenium (7).
Given how incredibly widespread wheat consumption really is, it is easy to see how this may have contributed to nutrient deficiencies