2 MYTH All fat is bad.

When studies published in the late 1940s found correlations between high-fat diets and high levels of cholesterol, experts reasoned that if you reduced the amount of total fats in your diet, your risk for heart disease would go down. By the 1980s, doctors, federal health experts, the food industry and the news media were reporting that a low-fat diet could benefit everyone, even though there was no solid evidence that doing so would prevent heart disease, obesity and other health issues.

As a result, said Vijaya Surampudi, an assistant professor of medicine at the University of California, Los Angeles, Center for Human Nutrition, many people—and food manufacturers—replaced calories from fat with calories from refined carbohydrates such as white flour and added sugar. (Remember Snackwell’s?)

“Instead of helping the country stay slim, the rates of overweight and obesity went up significantly,” she said.

In reality, not all fats are bad. While certain types, such as trans fats, can increase your risk for heart disease or stroke, healthy fats help reduce your risk. Examples include monounsaturated fats (found in olive oil, avocados, and certain nuts and seeds) and polyunsaturated fats (found in sunflower oil, walnuts, fish and flaxseed). Good fats are also important for supplying energy, producing important hormones, supporting cell function and aiding in the absorption of some nutrients.

If you see a product labeled “fatfree,” don’t automatically assume it is healthy, Dr. Surampudi said. Instead, prioritize products with simple ingredients and no added sugars.