Susan Lang doesn't know for certain if her son's itchy skin and upset stomach were caused by eating food made from crops whose genes were altered in a lab.

But over the years, she believes she's been able to soothe the 8-year-old's eczema and digestive problems by eliminating genetically modified organisms from his diet.

"I know that when I feed this child better he does better, and feeding him better includes not feeding him GMOs," Lang said.

The Fair Oaks woman concedes, however, that her evidence is not scientific, saying she has "more than a hunch, but I don't have proof."

Lang learned about genetic engineering – the process of splicing plant or animal genes to create new characteristics – as she began altering her family's diet to help her son. In the process, she became concerned that consumers don't know enough about the technology that goes into producing a huge part of the American food supply. Eventually she became a volunteer for the Proposition 37 campaign.

The measure on Tuesday's California ballot asks voters if food companies should be required to label genetically engineered food. At the core of the debate is a seemingly simple question: Is it safe to eat?

Proposition 37 supporters offer little scientific evidence that genetically modified food is dangerous to human health. A recent French study that found rats developed tumors after months of eating genetically modified corn was quickly panned by the scientific community.

Supporters instead point out perceived deficiencies in most studies that exist, raise questions about the procedures for approving the food and argue that the biotechnology industry has undue influence on government regulators.

"Experts are still debating if foods modified with DNA from other plants, animals, bacteria and even viruses are safe," says a radio ad urging a "yes" vote on Proposition 37. "But while the debate goes on, we all have the right to make an informed choice."

Opponents are making the case that labeling the food implies health dangers that haven't been proved.

"As a doctor, it concerns me when families are given misleading health information," Dr. Sherry Franklin of San Diego says in a No on 37 ad.

The ad also points out that the American Medical Association has said there is "no scientific justification" for labeling genetically engineered food.

That is true – but incomplete. The association that represents the nation's doctors also calls for greater "availability of unbiased information and research activities on bioengineered foods." And it says there should be a different system for testing genetically engineered food before it hits store shelves. Right now, the testing process is voluntary; the medical association says it should be mandatory.

The voluntary testing system is a concern to Proposition 37 supporters. They say it puts too much control in the hands of companies that stand to profit from their biotech inventions.

Altered crops in many foods

Most corn, soybeans, canola and sugar beets grown in the United States are engineered to kill pests or withstand being sprayed with weed killers such as Round-Up. Those genetically engineered crops wind up in thousands of non-organic grocery products in the form of corn syrup, sugar, canola oil or soy-based emulsifiers. Some non-organic papaya, crook neck squash and corn on the cob is also genetically modified.