Before rushing to make salmon a staple of a heart-healthy diet, consumers should check the origin of their fish supply, a new study recommends.

It turns out where the salmon comes from — and what they are fed — determine whether the health risks will outweigh the benefits, according to the study recently published in the Journal of Nutrition.

Researchers found that the contaminant levels in farmed salmon from certain regions of the world increased the risk of cancer enough to outweigh the heart health benefits of salmon.

The toxin levels were so high in some farmed salmon from Europe that people should only eat a single serving once every five months, the study found.

"That's pretty horrendous," said David Carpenter, lead author of the study and director of the Institute for Health and the Environment at the State University of New York at Albany's School of Public Health.

While the toxin levels in wild salmon weren't high enough to exceed the health benefits, the same wasn't true for farmed salmon, which are raised on a diet of fish oil.

The level of contaminants in fish oil — often derived from local fish — vary depending on the region of the world.

"What [the salmon] are fed turns out to be a huge part of the story," said Steven Schwager, an author of the report and researcher at Cornell University.

Farmed salmon from South America had the lowest level of pollutants followed by farmed salmon from North America. Salmon from Europe had the highest level of pollutants, according to the study.

"We think it's because that area's been industrialized much longer," Carpenter said.

Prompted by other studies indicating that fish oil increases the levels of toxins in farm-raised salmon, some fish farmers in recent years have switched to using vegetable oil pellets.

But a study last year found the heart health benefits from fish like salmon were weakened when they were fed vegetable oil instead of fish oil.

To determine whether the heart health benefits of farmed salmon were worth the risk, researchers used advisories developed by the U.S. Environmental Protection Agency for cancer effects and the fish consumption advisory issued by the American Heart Association.

"In farmed salmon, the cancer risk dominated the health benefits," Carpenter said.

That's not a call for people to shun farmed salmon, however.

Salmon and other fatty fishes like mackerel and sardines are rich in omega-3 fatty acids, the healthy fat that scientists say raises the "good" HDL cholesterol and lowers the "bad" tryglicerides.

The AHA recommends people eat fish — particularly fatty fishes — at least twice a week.

"None of us argue that benefits are real. But the dirty little secret is that there are risks," Schwager said.

Even taking into account the risks, however, the benefits of salmon are worthwhile for some groups, including older people who may be recovering from coronary problems, Schwager said.

"But for young people worried about a lifetime accumulation of pollutants, the risks far outweigh the benefits," he said.

Recent studies from Scotland have reported that feeding salmon vegetable oils except in the final stages of farming resulted in salmon with significantly lower levels of contaminants but with most of the omega-3 fatty acids obtained from the standard diet.

"We're not opposed to farmed salmon, just how it's farmed. The industry can reduce the level of toxins by changing how they feed [the salmon]," Carpenter said.