Saturated fat isn't as bad as you think...

After years of assertions that saturated fatty acids are bad for coronary health and that unsaturated fats are beneficial, a study by Dariush Mozaffarian involving a group of post-menopausal women, calls this idea into question.

An editorial in the American Journal of Clinical Nutrition by Knopp and Retzlaff shows just how complex the relationship between diet and lipoprotein physiology really is.

According to Knopp and Retzlaff, one of the earliest and most convincing studies to show that unsaturated fats reduced cholesterol levels and heart disease was the Finnish Mental Hospital Study conducted between 1959 and 1971.

In this the typical institutional diet, high in saturated fats, was replaced for some of the patients by an equally high fat diet, but one in which the saturated dairy fats were replaced with soybean oil and soft margarine and polyunsaturated fats were used in cooking.

Each diet was provided for 6 years and then the alternate diet was provided for the next 6 years. In men and women, the incidence of coronary heart disease was reduced by 50% and 65%, respectively, after consumption of the polyunsaturated diet.

In their editorial, Knopp and Retzlaff explore the reasons why Mozaffarian's findings could be different from the Finnish study. They also cite other scientific papers which have explored different aspects of the relationships between dietary fats and carbohydrates, HDL and LDL cholesterol, triacylglycerols, insulin sensitivity and the failure of female sex hormones to prevent coronary artery disease.

Dariush Mozaffarian from the Channing Laboratory at Harvard, US and his colleagues Eric Rimm and David Herrington looked at the associations between dietary macronutrients and the progress of coronary atherosclerosis in post-menopausal women.

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 women's mean total fat intake made up 25% of their energy intake. During follow up, a higher saturated fat intake was associated with a smaller decline in minimal coronary diameter and less progression of coronary stenosis.

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.

Polyunsaturated fat intake was positively associated with disease progression when replacing other fats but not when replacing carbohydrate or protein. Monounsaturated and total fats were not associated with progression.

Mozaffarian says the results, published in the 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. Most of them were taking hormone supplements.

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.

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.

Reference
Mozaffarian D, Rimm EB, Herrington DM. (2004). Dietary fats, carbohydrate, and progression of coronary atherosclerosis in postmenopausal women. American Journal of Clinical Nutrition, 80, 1175-1184