On the web there is plenty of material like this one claiming that the vegetables we buy today are much less nutritious than vegetables from 40/70/100 years ago.
8 oranges today have the same nutrients as 1 orange 100 years ago
There is also plenty of websites reporting the relevant research, like Dirt Poor: Have Fruits and Vegetables Become Less Nutritious?.
The explanation generally given is that with our artificial selection we tend to focus on look, pest resistance or other characteristics in place of the the nutritional content. The other explanation is soil depletion.
Is this actually true? Are we actually losing nutrients or there is just some natural change (EG: less vitamin A but more zinc)?
There doesn't seem much research from what I have seen and it seems to be only focused on the "negative" side, so it's not easy to assess the real situation.