Since the mid-1990s, when corn and soybean varieties began being injected with genes from bacteria and other unrelated species, we've been paying participants in a food experiment with potentially unprecedented effects on human health, the environment and food security.

By 2005, the Agriculture Department says, the vast majority of U.S. soybean acres and 52 percent of corn acres were planted with genetically engineered seed.

The bounty of these acres is in our candy, crackers and chicken pot pies, in our pizza and pasta sauce, in our Coca Cola and Campbell's soups. Corn and soybeans are ubiquitous: tens of thousands of processed foods contain soy, and the typical consumer takes in 200 calories of high-fructose corn syrup per day. Alter the genomes of corn and soybeans, and you've altered the diet of most Americans.

Corn and soybeans are staples of animal feeds, so we're also modifying the diets of our beef cattle and milk cows, our pigs and chickens.

Yet lending our grocery dollars and stomachs to this venture gains us little.

The price of modified seed includes a technology fee that effectively siphons off the bulk of any additional revenue farmers might gain from reduced pest damage or decreased management costs.

Many hoped that genetically engineered crops would help the environment by cutting pesticide use. We should have known that growing crops engineered to tolerate herbicides could lead to more chemical use. A 2004 analysis funded by the Union of Concerned Scientists found that the introduction of engineered corn, soybeans and cotton caused a 122 million pound increase in pesticide use since 1996.

And because resistant crops have encouraged near constant use of one or two classes of herbicides, superweeds that withstand the chemicals have now emerged and will require ever more potent poisons to control.

Another hope was that gene tinkering would help end world hunger. But the dream of concocting drought-tolerant, insect-resistant, nutrient-dense supreme species ignores the reality of global markets already awash in food.

Hunger and malnutrition result from poverty, not a lack of food in the world.

It's unlikely that we're getting health benefits from eating these crops. Scientists are studying their possible effects. Among the findings: abnormal white and red blood cell counts and inflammation of the kidney in rats fed genetically engineered corn, accelerated growth of stomach and intestinal tissues of rats fed engineered potatoes, and immune responses in mice fed altered peas. The findings are controversial, but they should, at the very least, give us pause.

Meanwhile, pollen from genetically engineered crops is on the move. In a recent study by the Union of Concerned Scientists, 50 percent of nonengineered corn and soybean varieties tested by one laboratory contained DNA from engineered versions. Chasing down and eliminating this freeflowing DNA from our seed supply, should the need arise, will require Herculean effort.

The only clear reason why we're eating so much genetically modified food is that Monsanto, Dupont and Syngenta, which together control over 25 percent of global seed sales, want us to.

In the United States, Monsanto dominates many a menu. It owns half of the American corn seed market, and its modified traits are present in roughly 90 percent of soybean acres.

Monsanto is tossing salads too. In January 2005, it bought Seminis, supplier of 3,500 varieties of fruit and vegetable seed to 150 countries. Monsanto now controls more than 30 percent of the world's cucumber, hot pepper and bean seed sales, and more than 20 percent of onion, tomato and sweet pepper seed sales, according to the Action Group on Erosion, Technology and Concentration.

Now consider that Monsanto and its cohorts are free to undertake the genetic modification of any plant variety they own. The plant varieties they don't modify, they can remove from the market. With one-fourth of the total value of the worldwide commercial seed market already coming from engineered seeds, our choices for unmodified crops and foods are rapidly dwindling.

As we relinquish control over our food to the gene engineers, we must ask: Does Monsanto really know best?

Deborah Rich grows olive trees near Monterey, Calif., and writes about agriculture for the San Francisco Chronicle and other publications. She wrote this essay for the Land Institute's Prairie Writers Circle, Salina, Kan.