But polyunsaturated fats aren't free of controversy: namely, whether people should be worried about a too-high ratio of omega-6 fatty acids to omega-3 fatty acids in the diet. Omega-3 fatty acids, found in albacore tuna, sardines, walnuts, and tofu, are linked to improvement in some measures of heart-disease risk; omega-6 fatty acids, found in eggs, certain fish, and safflower and soybean oils, may cause inflammation. At one point the ratio of omega-6s to omega-3s in the American diet was about 2:1; now experts say it might be as high as 40:1 or even 50:1. But there's no good evidence that they can actually cause clinical harm in humans, says Lichtenstein, who suggests people not fixate on the ratio.
What about olive oil, sesame oil, canola oil, and other oils high in monounsaturated fats? While they're a good replacement for saturated fats, they don't seem to have the same level of benefits as do polyunsaturated fats, says Mozaffarian. Most of the evidence of benefit has come from observations about the so-called Mediterranean Diet, but the health-conferring element in that eating pattern hasn't really been nailed down, he says. And if olive oil itself is beneficial, it may not be the monounsaturated fat it contains, but the phytochemicals, But again, Lichtenstein urges people not to get too hung up on that point. Instead, focus on unsaturated fats, period. If you want to cook some things with soybean oil, stir-fry veggies with sesame oil, and make salad dressing with olive oil, that's fine.
And that brings us to the easy guideline for consuming fat in the most healthful way possible: Think about foods, rather than chemistry, when you head to the supermarket. The fats in food are all mixtures, and you'll drive yourself crazy attempting to figure out exact proportions, notes Nestle. Besides, fussing over components ignores the way people actually eat. "I've never known anyone to sit down to a tub of saturated fat," says John La Puma, an internist and author of ChefMD's Big Book of Culinary Medicine: A Food Lover's Road Map to Losing Weight, Preventing Disease, and Getting Really Healthy. "People eat food, not saturated fat or trans fat or starch or sugar."
Lichtenstein agrees, saying we should focus on the pattern of eating, not any single dietary component. And research is fairly consistent on what constitutes a healthful eating pattern: a varied diet that's heavy on the fruits and vegetables, emphasizes whole grains rather than processed ones, focuses on fish and a limited amount of lean meats and lowfat dairy, and includes liquid oils for food preparation. Following that sort of pattern will naturally lead you to "good" sources of fat, and will reduce levels of the not-so-good—including most trans fats, which are still considered the worst of the bunch. (Hydrogenated oils "have nothing but adverse effects," says Krauss.)
And of course, remember that the quantity, not just the quality, of your food is important, says Nestle. Rather than having his patients count calories, La Puma recommends foods that will make them feel satisfied, gives them smaller plates, advises against skipping breakfast if it makes them ravenous later on, and suggests they try to sit down when they eat. "It reinforces the feeling of eating as something deliberate rather than just grazing," he says. But even healthy eaters should focus on portion control. "If you eat a lot of nuts, avocados, or olive oil, you'll get fat," says Guarneri. And once it's around your middle, it's definitely no longer "good" fat.
[Read: 'Diets' That Promote Health (and Always Have)]