The Food and Drug Administration says consumers shouldn't worry too much about levels of arsenic in rice — but should vary their diets just in case.

The agency released a study Friday of arsenic in 1,300 samples of rice and rice products, the largest study to date looking at the carcinogen's presence in that grain. Consumer groups have pressured the FDA to set a standard for the amount of arsenic that can be present in rice products.

The study shows varying levels, with the most arsenic in brown rice and the least in instant rice. Infant cereal and infant rice formulas are also at the low end of the spectrum.

The FDA says the amounts are so small that rice is safe to eat and there isn't any concern of immediate or short-term adverse health effects. But the agency said it is still studying the long-term effects of eating rice.

Rice is thought to have arsenic in higher levels than most other foods because it is grown in water on the ground, optimal conditions for the contaminant to be absorbed.

Arsenic is naturally present in water, air, food and soil in two forms, organic and inorganic. Organic arsenic passes through the body quickly and is essentially harmless. Inorganic arsenic — the type found in some pesticides and insecticides — can be toxic and may pose a cancer risk if consumed at high levels or over a long period.

The FDA is looking into how much organic and inorganic arsenic rice eaters are consuming and whether those levels are dangerous. The agency will conduct a risk assessment with the National Institutes of Health and the Environmental Protection Agency to further measure those effects.

The government, along with the public health community, has long encouraged consumers to vary their diets to minimize risk. Pediatricians, for example, have moved away from only recommending rice cereal as a baby's first solid food. There is evidence that other grains and even meats and fruits and vegetables can be just as healthful, says Dr. Stephen Daniels of Children's Hospital Colorado, the chairman of the nutrition committee of the American Academy of Pediatrics.

Daniels said the FDA results are "reassuring in many ways" and parents who have been giving their infants rice cereal should not be concerned.

Average levels of arsenic in the study ranged from 2.6 to 7.2 micrograms of inorganic arsenic per serving. Though the long-term effects are still unknown, that amount is tiny — a microgram is one-millionth of a gram.

Still, it is almost impossible to say how dangerous these levels are without a benchmark from the federal government. The advocacy group Consumer Reports, which is pushing for FDA to create standards, has used New Jersey's drinking water standard — a maximum of 5 micrograms in a liter of water — as an example of a benchmark because it is one of the strictest in the country. But the group acknowledges that it is difficult to compare standards for water and rice because one is a liquid and one is a solid and people drink more water than they eat rice.

The FDA study looked at rice from the United States, with some of the highest levels of arsenic found in rice grown in Southern states. It also looked at rice from Asia. The FDA said its study was not large enough to evaluate specific brands.

FDA toxicologist Suzanne C. Fitzpatrick said that because arsenic is naturally occurring it is going to be in food, and because rice is grown in water it will always have higher levels.

"It's not something that we can just pull off the market," she said.

The rice industry said Friday that it is working with the FDA and is encouraged by the results of the study. The industry has been conducting several of its own studies to try to figure out how to reduce arsenic levels, including investigating different ways to manage the water in which rice is grown and looking at processing and rinsing methods to see if there are ways to reduce arsenic levels.

Consumer groups said they also are pleased that the FDA is taking a hard look at arsenic in rice. Urvashi Rangan of Consumer Reports said the group hopes the FDA eventually sets specific guidelines for arsenic so growers will implement more steps to rid rice of the carcinogen.

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Dr. Steven Abrams of Texas Children's Hospital agreed that varying diet is the way to go. Rice "is a healthy food but it's not the only healthy food," he said.

Still, parents should not overreact and shy away from rice completely, he said. "We don't want to over-interpret the concerns so that we don't give kids the foods that they need," Abrams said.