Declining levels of nutrients in food

Research from the Biochemical Institute at the University of Texas, cites three kinds of evidence that points towards the decline in the nutrient value of fruits and vegetables in the US and UK over the last 50 to 100 years.

1. fertilized plants contained larger absolute amounts of minerals than the unfertilized plants, but these amounts were sufficiently diluted by the increased dry matter that all mineral concentrations declined, except for phosphorus, which is the common fertilizer.

2. apparent median declines of 5% to 40% or more in some minerals in groups of vegetables and perhaps fruits

3. plantings of low- and high-yield cultivars of broccoli and grains found consistently negative correlations between yield and concentrations of minerals and protein, a newly recognized genetic dilution effect.

In conclusion,
"Over three billion of the world’s population is malnourished in nutrient elements and vitamin,including in developed countries. Vegetables and fruits are among the richest sources of many nutrients. Thus, declining nutrient concentrations in horticultural products are most unwelcome. Past and ongoing efforts to increase yields, combined with apparent broad tradeoffs between yield and the concentrations of perhaps half of all essential nutrients, work against recent efforts to increase one or a few micronutrients in individual foods."

via US Food Policy