DEAR DOCTOR K:
I’ve read in your column about “good fats” and “bad fats.” I’ve also heard that recent studies challenge which fats are “bad.” Can you shed a little light on this issue?
I don’t blame you for being confused. One problem with medical studies is that they don’t always agree. That’s why we often need a lot of them to determine the “truth.”
Let’s start at the beginning. For years, you probably heard that all fats were bad for you and carbohydrates (“carbs”) were good. That was nonsense. We need both fats and carbs in our diet.
We need to eat mainly “good fats,” and to minimize the intake of “bad fats.” Which are which? For many decades, hundreds of scientific studies have found that eating lots of saturated fats is unhealthy. Saturated fats are found mainly in meat and whole-fat dairy foods.
Now, a recent study that got a lot of coverage in the media claimed to find that saturated fats were not unhealthy, after all. Don’t believe it! What that study showed was that if you reduce saturated fats in your diet (which is supposed to be healthy) but replace the saturated fat with refined carbohydrates, your health won’t benefit. That’s because the refined carbs are as bad for you as the saturated fats, or worse. If you replace saturated fats with good fats, however, your health will benefit.
In the past 25 years, largely from research done by my colleagues here at Harvard, another kind of fat also has been fingered as “bad”: trans fats. Trans fats are found in margarine, particularly hard stick margarine; commercially baked goods like cookies and crackers; and in many fried foods. There is no safe level of trans fats, and you should eat as little of them as possible. To do so, avoid packaged foods where the package label says they contain “partially hydrogenated” oils, and look for the amount of trans fats on the Nutrition Facts panel.
In the U.S., trans fats may soon be a thing of the past. Several cities have banned the use of them in restaurant cooking. The Food and Drug Administration (FDA) has labeled them as “no longer generally recognized as safe,” and moved to eliminate them from the commercial food supply over the coming years.
Now to the good fats: polyunsaturated and monounsaturated fats. Good fats can help lower LDL (bad) cholesterol and prevent heart disease.
There are two types of polyunsaturated fats: omega-3 fatty acids and omega-6 fatty acids. Omega-3s come mainly from fish, especially fatty fish like salmon and sardines. But you can also find them in flaxseeds, walnuts and canola oil. Omega-6 fatty acids are present in safflower, soybean, sunflower, walnut and corn oils.
Good sources of monounsaturated fats are olive oil, peanut oil, canola oil, avocados and most nuts.
So, in my opinion, the science continues to show that the good fats — polyunsaturated and monounsaturated fats — are good for you, and the bad fats — saturated and trans saturated fats — are bad for you.