MUNICH, Germany (AP) – The heart health benefits from fish like salmon and mackerel seem to be weakened when the fish are fed vegetable oil instead of fish oil, new research indicates.

So the answer might be to feed them more fish oil.

But that raises a different concern. Other studies have indicated fish oil increases the levels of pollutants in farm-raised salmon. That has encouraged some fish farmers to move to vegetable oil – which apparently decreases the heart benefits.

It’s yet another fish conundrum for consumers, like the debate about whether the mercury in some fish offsets their health benefits.

Still, many experts argue that for most adults, the benefits are probably greater than the concerns about pollutants linked to cancer. They note that many more people are at greater risk of cardiovascular disease than cancer.

Wild fatty fish such as salmon, tuna, mackerel, sardines and herring are rich in omega-3 fatty acids, the healthy fat that scientists believe raises the good HDL cholesterol, lowers unhealthy tryglicerides and slows the growth of plaque, protecting the heart from disease.

However, in modern fish farming, the fish are usually fed pellets that contain a mixture of natural fish oil and vegetable oil. And after a U.S. study earlier this year showed far higher levels of dioxins and other potentially cancer-causing pollutants in farm-raised salmon, some in the industry vowed to move more toward pellets with vegetable oil.

The latest study challenges that approach. This week at the annual meeting of the European Society of Cardiology, Norwegian scientists showed that people who ate salmon fed on pure vegetable oil or on 50 percent fish oil and 50 percent vegetable oil did not get any meaningful improvement in the relevant blood tests.

The study was small, involving 58 people with heart disease in Oslo, Norway, who were all taking heavy medication for their illness. The fish were farmed in northwest Norway, color-coded according to the pellets they were fed and shipped to a central kitchen in Oslo where they were served to the heart patients.

One-third of the people were fed salmon that had been given pellets of fish oil, another third got fish fed on a 50/50 mix of fish oil and vegetable oil from rapeseed and the last group got salmon raised on pure rapeseed oil pellets. Each volunteer ate 700 grams of the fish per week, or one fish meal per day, for six weeks.

The scientists, led by Dr. Harold Arnesen of Ulleval University Hospital in Norway, tested blood from the volunteers for concentrations of omega-3 fatty acids and changes in blood chemicals linked to heart disease.

“The composition of the food pellets was mirrored in the flesh of the salmon fillets and again mirrored in the serum fatty acids of the patients,” Arnesen told doctors.

Omega-3 levels increased substantially in the patients who ate salmon fed on fish oil, but not in the patients who ate salmon fed on mixed pellets or vegetable oil pellets.

The results were the same for improvements in chemical markers of inflammation, which is involved in building plaque in the arteries.

The most impressive difference was in triglycerides, which fell by 30 percent in the fish-oil group and not at all in the other patients. Everybody’s cholesterol dropped, but that was probably because they were eating fish instead of meat, which is high in saturated fat, the scientists concluded. Nobody lost weight during the study, which means the results could not have been due to differences in weight loss, Arnesen said.

“Only 2 percent of the market today is wild salmon. The farmed salmon market today is very close to 50/50 feed. It’s what we have in Norway and it’s more or less the same all over the world,” Arnesen said. “The findings underline the importance of tailoring the salmon with heart protective properties.”

Although experts believe that omega-3 rich fatty fish is good for the heart, the ideal amount to eat is not clear. The study indicates that if the group who ate the 50/50 salmon ate twice as much, they would likely gain the same benefit as those who ate the salmon fed with pure fish oil.

“If we are what we eat, then salmon are also what they eat,” said Alice Lichtenstein, a nutrition science professor at Tufts University in Boston who was not involved in the study. “This shows there are ways of breeding salmon that can increase the fatty acids.”

Lichtenstein has contended in the past that cardiovascular disease is a far bigger risk than the potential of getting cancer from eating fish tainted by pollutants.

The American Heart Association recommends adults eat fish, particularly fatty fish, at least twice a week. There is no equivalent European recommendation.