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Which dietary fats are healthiest?

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Nearly a decade ago, I traveled to New York to meet with the Atkins diet folks to discuss a book project, but I was quickly shown the door after I broached the subject of saturated fat. Can’t we tweak the program a bit, I wondered aloud, to emphasize olive oil over artery-damaging fried pork rinds?

Fast-forward 10 years, and researchers are still debating whether saturated fat is devil or angel. A new re-analysis of a decades-old clinical trial involving 458 male heart patients found that those who were randomly assigned to eat a diet rich in polyunsaturated vegetable oils instead of saturated animal fats had a bigger drop in their cholesterol levels over a three-year period than those who stuck with their usual diet — but they also had more heart disease deaths.

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About 17 percent of the patients in the intervention group died from heart disease compared with about 11 percent of the patients in the control group. Because the study was fairly small, those differences barely reached the threshold for statistical significance. The National Institutes of Health researchers who conducted the analysis, published last Tuesday in the British Medical Journal, concluded that “these findings could have important implications for worldwide dietary advice to substitute polyunsaturated fats in general, for saturated fats.”

For years, the American Heart Association and other medical groups have recommended that most of the 25 to 35 percent of calories that we get from fat each day should come from polyunsaturated fatty acids (fish and safflower, corn, and other vegetable oils) and monounsaturated fatty acids (nuts, seeds, olive oil, avocados).

We’ve also been told to limit our total saturated fat intake to less than 7 to 10 percent of total calories. Trans fats — which everyone agrees are the most cholesterol-raising and heart-damaging — should be avoided as much as possible, and American food manufacturers have complied by taking partially hydrogenated oils, which contain trans fats, out of most processed foods.

But some nutrition researchers (including those who aren’t working for Atkins) contend that there aren’t any strong studies to show that saturated fats harm the heart while polyunsaturated and monounsaturated fats protect it. In an editorial with the study, Philip Calder, a nutritional immunologist from the University of Southampton in the United Kingdom, wrote that studies documenting the heart-protective benefits of replacing butter, cream, and lard with safflower and corn oil have frequently included other dietary changes — such as reducing harmful trans fats in margarine — which may have muddied the findings.

Others, though, believe the case is closed when it comes to the harms posed by saturated fats.

“I don’t think this new analysis study should be practice-changing and lead people to consume fewer polyunsaturated fats,” said Penn State University nutrition researcher Penny Kris-Etherton, who helped the American Heart Association develop its dietary fat recommendations.

Harvard School of Public Health nutrition chair Dr. Walter Willett agreed, pointing out that the study on which the new analysis is based was initiated in the late 1960s and was limited by the nutritional knowledge of that time. The study participants were provided with products that contained one type of polyunsaturated fat — an omega-6 fat found in safflower oil — rather than the full array of polyunsaturated fats including omega-3 fats found in fish oil and flaxseed oil; the heart-protective benefits of omega-3 fats have been well documented in more recent research.

“Very high intakes of omega-6 polyunsaturated fat without any omega-3 polyunsaturated fat is not desirable,” Willett said in an e-mail. “We do need adequate amounts of both.”

What’s also clear is that Americans often received bad advice to eat as little fat as possible when it was first discovered that reducing saturated fats could reduce cholesterol, and especially after researchers demonstrated that the practice could prevent heart attacks. Too many of us embraced fat-free cakes and cookies after swearing off cheddar cheese and steak — and often gained weight as a result of the excess sugar and calories.

“The controversy over saturated fat really comes from what do you replace it with?” Kris-Etherton said. “If you replace saturated fats with refined carbohydrates, then there may be no health benefits at all.”

Deborah Kotz can be reached at dkotz@globe.com.
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