The Skinny on Fats: What the Latest Research Says About What You Should Be Eating

Saturated, polyunsaturated, monounsaturated, and more: what it all means for your health.

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There are plenty of confusing topics in nutrition, but fats may take the cake. Are saturated fats like butter and animal fat terribly harmful? Should you worry about whether you're eating too much of one kind of polyunsaturated fat and not enough of another? What about olive oil? And shouldn't we be eating as little fat as possible, since so many of us are, well, fat? The distinctions are "enormously confusing unless you're a lipid biologist," says Marion Nestle, a professor of nutrition at New York University. There's a way to translate it all into relatively simple eating advice—more on that later. But first, here's the skinny on fat.

[Read: Why an All-Superfoods Diet Is a Mistake]

First, toss out the notion that the lower the fat content in your diet, the better. A certain amount of fat is essential to your body's functioning. And as you've probably heard, all fats are not alike in their effects on blood cholesterol levels, which can affect heart disease risk. Saturated fat, for example, generally increases levels of LDL, or "bad" cholesterol. But while this information was known when the surgeon general issued the first report on nutrition and health in 1988 and the National Academy of Sciences issued its own report in 1989, public health authorities felt that a message to reduce total fat would be best understood by the public. The thought was, says Nestle (who was managing editor of the 1988 report), that since saturated fats from meat and dairy products were the main sources of fat in the American diet, lowering total fat would automatically reduce consumption of saturated fat. That's certainly true, in theory.

But here's the rub in practice: "If you take out saturated fat, and you assume someone is in energy balance [i.e. their total calories haven't changed], what do you put in?" says Alice Lichtenstein, a nutritional biochemist and director of the cardiovascular nutrition laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston. As it turns out, what you substitute for saturated fat seems to be important. "People reduced fat and replaced that fat with simple carbohydrates and refined sugar," says Mimi Guarneri, cardiologist and medical director of the Scripps Center for Integrative Medicine in La Jolla, Calif.

That switch was aided by food companies, with their lines of "low-fat," yet sugar-filled products. A review published online in January in the American Journal of Clinical Nutrition said that research actually shows that subbing in a higher carbohydrate intake, especially refined carbs like white bread and sugary cereals, can actually "exacerbate" blood cholesterol problems, including elevated triglycerides and reduced HDL ("good") cholesterol levels. To improve cholesterol, people should focus on limiting those refined carbs and reducing excess body weight, the review said. So much for unlimited SnackWells.

By contrast, trading saturated fats for polyunsaturated fats—the omega-3 fatty acids found in certain fish and the omega-6 fatty acids in vegetable oils such as safflower and soybean oils—does seem to offer a heart benefit. An analysis of existing research published in March in PLoS Medicine found that consuming those polyunsaturated fats instead of saturated fats reduces the incidence of heart attacks and cardiac death. Based on the evidence, "polyunsaturated fats are the best...fat to be increasing in the diet," says Dariush Mozaffarian, an author of the study and an assistant professor at the Harvard School of Medicine and co-director of the program in cardiovascular epidemiology at Brigham and Women's Hospital.

[Read: Fish Oil Supplements, EPA, DHA and ALA: Does Your Omega-3 Source Matter?]

Ronald Krauss, senior scientist and director of atherosclerosis research at Children's Hospital Oakland Research Institute, and an author of the ACJN review, agrees. In fact, he says, it's unclear whether the benefits to cardiac health spring from removing the sat fat or adding the poly. But isn't saturated fat a "bad" fat? Possibly, but his research, including a meta-analysis also published in the ACJN, found that despite its effects on LDL cholesterol, there's no evidence dietary saturated fat alone is tied to the risk of coronary heart disease or cardiovascular disease. He also says that even in trials where saturated fats were swapped out in favor of polyunsaturated fats, there was not much benefit in reducing saturated fats below about 9 percent of calories. It may be that saturated fats aren't as evil as has been thought, but simply allow atherosclerosis to happen without conferring any protection; they raise both LDL and HDL cholesterols, leaving the ratio of the two little changed. By contrast, polyunsaturated fats "move the numbers in the right direction," says Krauss.