I agree there's reason to believe the nutritional content of food has gone down. Not just because the soil is depleted but also because of the long-term storage of foods after harvesting, e.g. in inert gas, hard to believe that doesn't have an adverse effect.

http://observer.guardian.co.uk/food...1481443,00.html

I take fish oil & vitamin B, and rotate my way through various seeds (sesame, pumpkin, sunflower) and herbs & spices, fresh and dried, in my cooking (oregano, marjoram, sage, thyme, ginger root, garlic, cayenne, turmeric etc). I figure every little bit helps.

If you're in the northern hemisphere's summer, I'm sure you know that you can get a solid dose of vitamin D by getting some sunshine whenever it's out.
Canned sardines are great for calcium if you like them.