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It's long been thought that a low-fat diet is essential for heart health and cancer prevention. But this notion has been recently tested, with surprising results.
A government-funded study — published in the February 2006 issue of JAMA, the Journal of the American Medical Association — showed that a low-fat diet didn't lower the risk of some cancers or heart disease for older women. This study, which followed almost 49,000 postmenopausal women over eight years, included two groups: One group of women was advised to eat less than 20 percent of their calories from fat and to eat more fruits, vegetables and whole grains; another group of women continued their usual eating habits. At the end of the study, researchers found no difference in the rate of breast cancer, colorectal cancer or cardiovascular disease between these two groups of women.
So does this mean that fat doesn't play a role in heart health or cancer prevention? Can you now dig into that chocolate cake or bag of chips knowing your long-term health won't be affected?
Not so fast, says Donald Hensrud, M.D., a preventive medicine and nutrition specialist at Mayo Clinic, Rochester, Minn. Here he helps clarify the role of low-fat diets in disease prevention and explains what these study results might mean to you.
What do you see as the major take-home message from this study?
For most people, a low-fat diet may not have a strong effect in decreasing their risk of breast and colon cancers or cardiovascular disease. Though this may seem surprising, these results actually support a growing body of evidence that says that the type of fat you eat may be more important to your long-term health than the amount of fat in your diet.
Does this mean that we might as well reach for the buttered popcorn and nachos?
No. You still need to be concerned about the types of fat you eat. Many desserts and snack foods, including buttered popcorn and nachos, include large amounts of saturated fat and trans fat — the harmful types of fat. These fats can increase your risk of heart disease by increasing your total and LDL ("bad") cholesterol.
Another consideration is calories. A gram of fat contains twice as many calories as a gram of carbohydrates or protein. So cutting down on high-fat foods can help you cut down on your daily calories and thus help you manage your weight.
This study asked women to cut back on dietary fats of all kinds. But isn't there strong evidence that some types of fats are actually beneficial?
This particular study didn't look at the types of fat in the women's diet, and the intake of all types of dietary fat decreased. But other studies have shown that reducing saturated and trans fats in the diet while increasing monounsaturated and polyunsaturated fats can lower your risk of heart disease by reducing the total and low-density lipoprotein (LDL) cholesterol levels in your blood.
One type of polyunsaturated fat, omega-3 fatty acids, may be especially beneficial to your heart. Omega-3s appear to decrease the risk of sudden death, especially in people at high risk of coronary artery disease. They may also help lower blood pressure and triglyceride levels along with other beneficial effects.
All studies have limitations. Which limitations do you see as most noteworthy with this study?
This study was primarily designed to test the hypothesis that a low-fat diet would decrease the risk of breast and colorectal cancers in postmenopausal women. The diet used in the trial was, therefore, aimed at limiting all types of fat in the diet, including the harmful fats (saturated and trans fats), as well as the healthier fats (mono- and polyunsaturated fats). But we now know that the type of fat — rather than the amount of fat — plays a bigger role in future health.
In addition to reducing their fat intake to 20 percent of their daily calories, the women were asked to increase their fruits and vegetables to five servings a day. Many of the women, however, didn't reach these diet goals. The increase in fruits and vegetables wasn't enough to show a difference in risk. And it may be that people needed to make greater changes. For the biggest health benefits, some studies suggest you need up to eight to 10 servings of fruits and vegetables in your diet every day. Other foods that may be particularly healthy — such as whole grains, fish and nuts — weren't specifically tested in this study.
Besides cutting back on harmful fats, what are the other components of a healthy diet?
I recommend a plant-based, high-fiber diet that is rich in fruits, vegetables, whole grains, healthy proteins (nuts, beans, low-fat dairy products) and foods that have ample "healthy fats" (fish, olive oil, canola oil).
What advice do you have for people wondering how these findings affect their diet?
Rather than focusing on total fat, look at how much saturated fat and trans fat you're eating. Then compare that with how many unsaturated fats are in your diet. A healthy diet includes more unsaturated fats and less saturated and trans fats and incorporates other healthy foods, such as fruits, vegetables and whole grains.
Here's where these different types of fat can show up in your diet:
- Saturated fat. Animal products (such as red meat, poultry, butter and whole milk) and coconut, palm and other tropical oils.
- Trans fat. Commercial baked goods (such as crackers, cookies and cakes), fried foods, snack foods, shortenings and some margarines.
- Monounsaturated fat. Olive, peanut and canola oils, avocados, and most nuts.
- Polyunsaturated fat. Vegetable oils, such as safflower, corn, sunflower, soy and cottonseed oils.
- Omega-3 fatty acids. Fatty, cold-water fish (such as salmon, mackerel and herring), flaxseeds, flax oil and walnuts.