In a statement, Schwarzenegger noted that consuming trans fat is linked to coronary heart disease.
"Today we are taking a strong step toward creating a healthier future for California," he said.
Violations could result in fines of $25 to $1,000. Food items sold in their manufacturers' sealed packaging would be exempt.
New York City, Philadelphia, Seattle and Montgomery County, Md., have ordinances banning trans fats, but California is the first state to adopt such a law covering restaurants, said Amy Winterfeld, a health policy analyst for the National Conference of State Legislatures.
California and Oregon already had laws banning trans fats in meals served at schools, she added.
The legislation signed by Schwarzenegger will take effect Jan. 1, 2010, for oil, shortening and margarine used in spreads or for frying. Restaurants could continue using trans fats to deep-fry yeast dough and in cake batter until Jan. 1, 2011.
Trans fats occur naturally in small amounts in meat and dairy products. Most trans fats are created when vegetable oil is treated with hydrogen to create baked and fried goods with a longer shelf life.
Stephen Joseph, a Tiburon attorney who was a consultant to New York City in developing its ban, said trans fat is a larger health risk than saturated fat because it reduces so-called good cholesterol.
A 2006 review of trans fat studies by the New England Journal of Medicine concluded there was a strong connection between consumption of trans fats and heart disease. Studies also have linked trans fats to diabetes, obesity, infertility in women and some types of cancer.
Trans fats are like cigarettes, with no safe level of consumption, said Jeffrey Luther, a Long Beach doctor who is president of the California Academy of Family Physicians.
He said the California law, "when it finally takes effect, will be a tremendous benefit."
The California Restaurant Association opposed the bill. Spokesman Daniel Conway said the federal Food and Drug Administration rather than individual states should be developing regulations on trans fat use.
He said, however, that the association has no plans to challenge the law, in part because restaurants already are phasing out trans fats to satisfy customers. Several major fast-food chains have announced that they have eliminated trans fats from their menus or intend to so do in the near future.
"We're confident that California restaurants can meet the mandates of the bill," Conway said.
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