His research finds that recently grown crops have shown decreases of up to 38% in protein, magnesium, calcium, vitamin C, phosphorus, iron, zinc and riboflavin when compared with produce from past decades.

What accounts for this negative trend? Like any other competitive industry, farmers’ attempts to drive up profits have led them to use new techniques to increase production, Davis said. The faster-grown fruits don’t have as much time to develop the nutrients.

“Farmers get paid by the weight of a crop, not by amount of nutrients,” Davis said. He called this the “dilution effect”: As fruits and vegetables grown in the United States become larger and more plentiful, they provide fewer vitamins and minerals.

“It’s a simple inverse relationship: The higher the yield, the lower the nutrients,” he said. Today’s jumbo-sized produce contains more “dry matter” than anything else, which dilutes mineral concentrations. In other words, when it comes to growing food, less is more. Chemical fertilizers and pesticides may help speed the market-readiness of produce, but slower-growing crops have more time to absorb nutrients from both the sun and the soil.

“Lots of agricultural scientists don’t know about this, and the public doesn’t know about this,” he said. However, scientific papers have cited one of the first reports of this effect, a 1981 study by W.M. Jarrell and R.B. Beverly in Advances in Agronomy, more than 180 times since its publication, “suggesting that the effect is widely regarded as common knowledge.”

Davis does note that historical data can sometimes be misleading, if not altogether inaccurate. Take early measurements of iron in foods: because scientists failed to sufficiently remove clinging soil, iron levels appeared unusually high in certain vegetables like spinach. Then again, good historical data provides the only real-world evidence of changes in foods over time, and such data does exist — one farm in Hertfordshire, England, for example, has archived its wheat samples since 1843.

Modern monoculture farming practices have also led to soil-mineral depletion, which, in turn, affects the nutrient content of crops.

In addition, there is a “genetic dillution effect,” in which selective breeding to increase crop yield has led to declines in protein, minerals and amino acids.. Because nearly 90% of dry matter is carbohydrates, “when breeders select for high yield, they are, in effect, selecting mostly for high carbohydrate with no assurance that dozens of other nutrients and thousands of phytochemicals will all increase in proportion to yield.”

Wheat Also Being Examined

Davis is currently researching the dilution effect in 14 varieties of wheat. His findings already suggest that, once again, the larger the yield of wheat, the lower the nutrients.

Jeff Cronin, at the Center for Science in the Public Interest, said scientists and the USDA often overlook farming practices.

“Breeding plants to improve crop yield at the expense of all other things seems to be the problem, as well as depleting soil and not rotating crops properly,” he said.

While Davis is not pleased about the decreasing levels of nutrients in produce, he still encourages people to eat plenty of fruits and vegetables.

“Even though amounts of nutrients have declined, fruits and vegetables are still the richest source of protective nutrients, much better than eating highly refined foods such as white flour, sugars and fatty foods,” he said.

This research suggests that local community farming initiatives and backyard vegetable gardens using organic gardening methods may produce fruits and vegetables higher in nutrients while enriching the soil for future crop production.