One finds that a prescription strength fish oil slashed heart problems and heart-related deaths among people with high triglycerides, a type of fat in the blood, and other heart risks.

Doctors cheered the results and said they could suggest a new treatment option for hundreds of thousands of patients like these.

But another study in healthy people found no clear benefit from a lower dose of fish oil, an amount that's common in dietary supplements.

Fish oils, also called omega-3 fatty acids, are found in salmon, tuna and certain other fish. They reduce triglycerides and inflammation and may have other effects . There are different types, including EPA and DHA.

A separate test of vitamin D pills found they also did not lower heart or cancer risks.

Up to 10 per cent of US adults take fish oil . Even more take vitamin D , despite no major studies to support the many health claims made for it.

"Those who peddle it promote it as good for everything," but in this definitive test, vitamin D "showed a big nothing," said Dr James Stein, a heart specialist at the University of Wisconsin-Madison. He had no role in the studies or ties to the companies involved.

Results were revealed on Saturday at an American Heart Association conference in Chicago and published by the New England Journal of Medicine.

Australian Associated Press