Are low-fat diets good or bad?
When it comes to heart disease, the amount of fat in your diet is less important than the type of fat.10 This is because the total amount of fat doesn't increase your LDL (bad) cholesterol; however, saturated fat (found in meat and dairy products) and trans fat (found in hydrogenated/partially hydrogenated oils) do. In the past, health advice was to cut down on fat. It was thought that people would find this easier to follow than trying to figure out the different types of fat. If you cut down on fat, you invariably lower your saturated fat intake, too. However, if you eat very little fat and fill up on carbohydrates (especially refined carbs), you can end up with high levels of triglycerides—another type of blood fat that raises your risk of heart disease—and lower levels of HDL (good) cholesterol.11, 12 These negative effects don't occur if the low-fat, high-carbohydrate diet is rich in fiber, part of a lifestyle that includes vigorous physical activity, or when the fat is mostly monounsaturated.13-16
The lesson learned is that telling people to cut the fat doesn't cut it. People need more specific advice about the types of fats and foods to include in their diet. The Dietary Guidelines for Americans recommend that you get at least 20% of calories from fat. If you go lower, it's hard to get enough of some nutrients, notably vitamin E and the essential polyunsaturated fat, alpha- linoleic acid (ALA).2 A diet with 20% of calories from fat can contain enough vitamin E and ALA if it includes foods that are good sources of these nutrients, such as certain liquid vegetable oils (soybean oil, corn oil).
Will a low-fat diet reduce my risk of heart disease?
The Women's Health Initiative Dietary Modification Trial took nearly 50,000 women aged 50 to 79 years and gave half extensive nutrition counseling to follow a low-fat diet (20% of calories from fat).17 Those who ate the low-fat diet reduced their LDL (bad) cholesterol levels as well as their blood pressure, but they did not lower their risk of heart disease, heart attack, or stroke.
The results of this study are not as surprising as they first appear when you realize that most dietary guidelines now emphasize the type of fat rather than the amount of fat. Looking at the results more closely, the women who ate a diet low in saturated or trans fat did reduce their risk of heart disease by a small but nonsignificant amount. Saturated fats (found in meat and dairy products) and trans fats (found in hydrogenated/partially hydrogenated oils) raise levels of LDL (bad) cholesterol. Women who included lots of fruits and vegetables also seemed to benefit.
The findings support the general consensus that your overall diet is more important than one component (the fat content or a single food). The low-fat diet in this study would not meet current recommendations for healthy eating that emphasize whole grains, fresh fruits and vegetables, and fish.
Are high-carb diets bad?
No. High-carb diets have been blamed for everything from the obesity epidemic to diabetes. As with fat, it seems that the amount of carbohydrate is less important than the type. Aim for carbohydrates that are whole grains and high in fiber. Whole grain and fiber are better for identifying good carbs than the old style of labeling them simple or complex. Another myth about high-carb diets is that they pack on the pounds. Of the 50,000 women enrolled in the Women's Health Initiative dietary modification trial, half were told to cut down on fat. Women who ate less fat and more carbs actually lost weight even though they weren't told to cut calories–these women were encouraged to eat whole grain foods.17 You may gain weight if you eat a lot of refined carbohydrates (sweet foods and drinks, white rice, white bread) even if your overall diet is not a high-carb diet.18 There is also no connection between the amount of carbohydrates you eat and your risk of diabetes.4