New research is challenging widely held beliefs about the dietary benefits of unsaturated fats, showing that some types long considered healthy, such as corn and safflower oil, may actually harm people with heart problems.
The new results stem from a study published in 1978 that examined links between diet and heart disease. A re-examination of the original research, including some data not part of the previous analysis, found that eating fats identified as omega 6s was linked to higher death rates among research subjects, all men with a history of heart attacks.
"What we found didn't go along with the dietary advice that has been given out for the past half century," said Daisy Zamora, a nutrition epidemiologist with the University of North Carolina's Gillings School of Health in Chapel Hill, who was involved in the latest research.
The study was published earlier this month in the British Medical Journal.
Saturated fats, found in butter, red meats, cheeses and other animal-based products, have been known to build up in arteries, hamper blood flow and cause cardiovascular problems. In the 35 years since publication of the landmark Sydney Diet Heart Study, Americans have been urged to replace saturated fats with unsaturated fats derived from vegetables, nuts and fish.
Unsaturated fats are divided into two categories - omega 6 and omega 3.
Omega 6 fats come from a wide range of vegetable oils, such as safflower, sunflower and corn oil, and are used heavily by the food industry because they are widely available and inexpensive, said Elisabetta Politi, nutrition director at the Duke Diet and Fitness Center.
She said the new study findings should be considered as useful information by anyone considering day-to-day food choices, but added that she would need to see more research before advising people to stop eating omega 6 fats.
"I think it's better to focus on trying to include more omega 3s, such as salmon or flax seed, in your diet," Politi added.
The original study involved nearly 500 men age 30 to 59 with a previous history of heart disease. Half the group was instructed to use linoleic acid, found in safflower oil, to replace animal and dairy fats in their diets. The other half were not told to change their eating habits at all.
The study did not examine intake of omega 6 and omega 3 fats separately for either group.
According to Zamora, the amount of linoleic acid in American diets has tripled over the past century.
She agrees with Politi that without some additional studies "it would be a jump to say that this is what caused the rise in cardiovascular disease. Yet this is the conclusion we keep going back to."
Zamora said the team reanalyzing the heart-study findings confirmed that substituting saturated fats with omega 6 oil does in fact lower cholesterol levels.
"But our findings also suggest that, even though this happens, the risk of death from cardiovascular disease still increased as much as if they had kept on eating saturated fats," she said.
The new analysis showed that the group substituting omega 6 fats had a 17 percent higher risk of dying than the control group during the course of the study, which ran from 1966 to 1973.
Some foods rich in omega 6 fats:
- canola oil
- corn oil
- pine nuts
- safflower oil
- soybean oil
- sunflower seed oil
- Some foods rich in omega 3 fats:
- flax seed oil