A 2009 HortScience paper showed scientific evidence on the decreased nutrition in commercially grown vegetables, fruits and grains in the last 50-100 years.

For example, protein concentration in wheat and barley decreased by up to 50% between 1938-1990. Newly developed corn varieties have declined protein, oil and some amino acids. Likewise, new wheat varieties have 22-39% less minerals.

This trend is caused by two forces.

 The first one is the environmental “dilution effect”, which occurs when yield increases are the result of fertilization, irrigation and other environmental means. This tends to decrease the concentration of nutrients in food.

The other force at work is genetic “dilution effect”, resulting from breeders developing high-yielding varieties without considering the nutrients in the crops.

It seems that the higher the yield, the less nutritioius the food.

If you are growing your own garden, you will get more nutritious vegetables by planting lower-yielding heirloom varieties. Also organic gardening will give us more nutritious homegrown food.

Also see this article.