It’s been an article of faith in the medical community for more than 40 years that diets high in saturated fats put people at risk of heart disease. Most saturated fats boost blood concentrations of harmful, low-density-lipoprotein (LDL) cholesterol and lead to the buildup of plaque in arteries. So researchers at the Harvard School of Public Health were “surprised” by the results of a new study showing exactly the opposite effect in a group of postmenopausal women.
Dariush Mozaffarian and his colleagues examined the daily diets and coronary artery conditions of 235 U.S. women. The women’s average age was 66 at the beginning of the 3-year study, and all had some plaque buildup at that time.
The scientists took X-ray images at 10 places along each woman’s heart arteries at the start and end of the study. The women all provided comprehensive records of what foods they ate and how much, including details such as the kinds of oils used for frying and baking.
The researchers analyzed the women’s intake of various nutrients in relation to plaque buildup during the study, adjusting for factors such as age, education, smoking habits, and use of medication. The X-rays after 3 years showed that those women who had regularly eaten the highest amounts of saturated fats had the least amount of additional plague buildup in their arteries.
The women who ate more saturated fat also had a healthier balance of good and bad cholesterols, as well as more desirable blood concentrations of various kinds of fats.
The body needs a variety of dietary fats for energy, for proper functioning of the body, and for processing the fat-soluble vitamins A, D, E, and K. But excessive saturated fat can increase blood cholesterol, which circulates through the body in molecules called lipoproteins. When cholesterol builds up on the lining of blood vessels—a condition known as atherosclerosis—it constricts blood flow and can cause heart attacks and strokes.
Saturated fats come from meat and dairy products, as well as oils in some tropical plants such as coconut and palm trees.
No single answer
The Harvard researchers also examined the women’s carbohydrate intake in relation to plaque progression. The women with the highest amounts of carbohydrates in their diets over the 3 years had the most plaque buildup. This was especially evident among women who ate a lot of low-fiber carbohydrates and those who had less physical activity.
Mozaffarian says the results, published in the November American Journal of Clinical Nutrition, shouldn’t be construed as an endorsement of diets that advocate eating high amounts of fats and avoiding carbohydrates.
The outcome does support the advice of nutritionists who argue that people should be careful about what kind of carbohydrates they eat, he adds. “The women [in the study] who ate carbs in the form of whole-wheat grains did fine,” he says. “The problem was with women who ate highly-refined carbohydrates.”
An important distinction in this study, Mozaffarian cautions, is that the participants weren’t typical in a number of ways.
For one thing, all the women were postmenopausal. Moreover, three-fourths of them were overweight, and 40 percent were obese. A quarter had diabetes.
Other studies have shown that women generally experience changes in their lipid, or fat, levels as they age. Concentrations of good, high-density lipoprotein (HDL) cholesterol tend to decline after menopause, for example.
Most other dietary studies have linked saturated fat intake with high blood concentrations of bad LDL cholesterol. That effect wasn’t seen in this study, however.
Another significant trait of the group that may help explain the results is that most of the women consumed relatively low amounts of fat compared with those in typical U.S. diets. About 25 percent of this group’s calories came from fats, whereas the fat content of most U.S. diets may be 40 percent or more.
In the study, “the beneficial effects [of saturated fats] were the greatest in women with the lowest fat intake overall,” Mozaffarian points out.
He speculates that similar antiplaque benefits might also have occurred if the women had eaten more heart-friendly monosaturated fats, such as in canola and olive oils, in place of the saturated fats, so long as overall fat intake was low.
What interplay of factors affected plaque formation among the menopausal women in the study is a “paradox” that can’t be fully explained, lipid experts Robert Knopp and Barbara Retzlaff of the University of Washington in Seattle acknowledge in an editorial accompanying the published study.
Knopp says the answer probably lies in the specific combination of characteristics of the women in the group.
Most of them, he notes, were “sort of a model for metabolic syndrome”—a set of physiological conditions that puts a person at risk of diabetes and other diseases involving problems processing insulin. Furthermore, he points out, two-thirds of the women were taking hormone supplements.
Mozaffarian says the study highlights the complex way in which dietary fats interact, which is not well understood. “Fats are not just inert metabolic substances, they have wide-ranging metabolic effects in the body, and these effects are different for different kinds of fats,” he says.
Also, he says, “there’s greater appreciation today of the fact that the effects of dietary nutrients are different in different people.”
The researchers were “initially surprised by the findings, but not so much so” after considering possible explanations, Mozaffarian says.
Results of studies of saturated fat intake and coronary artery–disease risk have been inconsistent. And most of those studies have focused on men, while the effects of diet on heart disease may differ in men and women.
The volunteers in the Harvard dietary study were a subset of women from six cities who were enrolled in a randomized trial designed to evaluate whether hormone replacement therapy curbs atherosclerosis. The results, which showed that female sex hormones did not prevent coronary artery disease in the postmenopausal women, were “a great disappointment,” Knopp and Retzlaff note.