However, once the children returned to eating conventionally grown foods, concentrations of these pesticide metabolites quickly climbed as high as 263 parts per billion, says the study, which was published Feb. 21.

Organic crops are grown without the chemical pesticides and fertilizers that are common in intensive agriculture.

There was a “dramatic and immediate protective effect” against the pesticides while consuming organically grown foods, said Chensheng Lu, an assistant professor at the Rollins School of Public Health at Emory University.

These findings, along with the results of another study published in Britain earlier in March, have fueled the debate about the benefits of organically grown food versus conventional, mass-produced foods. According to the new British analysis of government nutrition data on meat and dairy products from the 1930s and 2002, the mineral content of milk, cheese and beef declined as much as 70 percent in that period.

“These declines are alarming,” Ian Tokelove, spokesman for The Food Commission that published the results of the study, told Tierramerica. The commission is a British non-governmental organization advocating healthier, safer food.

The research found that parmesan cheese had 70 percent less magnesium and calcium; beef steaks contained 55 percent less iron; chicken had 31 percent less calcium; and 69 percent less iron, while milk also showed a large drop in iron, along with a 21 percent decline in magnesium. Copper, an important trace mineral (an essential nutrient that is consumed in tiny quantities), also declined 60 percent in meats and 90 percent in dairy products.

Although controversial, a number of other studies have also found differences between conventionally produced foods and foods grown organically or under more natural conditions. Organic fruits and vegetables had significantly higher levels of cancer-fighting antioxidants, according to a 2003 study in the Journal of Agricultural and Food Chemistry. The organic plants produced these chemical compounds to help fight insects and competing plants, researchers said.

A 2001 report by Britain’s Soil Association looked at 400 nutritional research studies and came to similar conclusions: Foods grown organically had more minerals and vitamins.

“Modern plant breeding for quick growth and high yields could also be affecting the nutritional quality,” says Katherine Tucker, director of the nutritional epidemiology program at Tufts University in Boston.

Farmers in other parts of the world should not adopt the intensive farming practices of North America or Europe, says Ken Warren, a spokesman with The Land Institute, which is based in Kansas.

“It’s an unsustainable system that relies heavily on chemical fertilizers ... to keep yields high and produces ‘hollow food,’” he told Tierramerica.

“Hollow food” contains insufficient nutrition and is suspected in playing a role in the rapid rise in obesity, as people may be eating more to get the nutrition they need, he explained.

Crops take minerals, trace elements and other things from the soil every year. Modern agriculture only puts back into the land some chemical fertilizers, which do not replace all that has been lost, Mr. Warren noted.

Moreover, herbicides and insecticides kill microorganisms in the soil that play an important role in maintaining soil fertility and helping plants grow.

Pesticide residues in modern agriculture are another cause for concern. A 2003 University of Washington study found that children eating organic fruits and vegetables had concentrations of pesticide six times less than children eating conventional produce.

The Land Institute advocates what it calls “natural systems agriculture.” This involves the use of perennial crops in polycultures: planting several different crops together as has been practiced in traditional gardens and farm plots in many parts of the world.

“Farmers in other parts of the world should learn from American agriculture’s mistakes,” Mr. Warren maintained. “Looking to nature is a better model for farming.”