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Are fatty foods friend or foe?

New research from Harvard's School of Public Health offers a fresh perspective on eating fats and suggests replacing dietary fats, such as butter and oil, with more carbohydrates isn't the heart-healthiest choice.

In the 1970s, certain fats were portrayed as dietary bad guys as mounting research suggested they were linked to cholesterol, a risk factor for heart disease, New York City internist Dr. Alexandra Sowa told CBS News.

"So we started doing everything low-fat. Now we realize that there are two different types of fats, saturated fats that come from animal products -- dairy, meats -- and when we eat them in excess, they can be linked to cholesterol," said Sowa, who was not involved with the study. "There is another sort of fat, unsaturated fat, that we find in avocados, nuts, and certain oils that can actually be cardio-protective and good for your brain."

A few decades ago, when fats started getting a bad rap, people started replacing saturated fat with refined carbohydrates, but the switch-up did not lower their risk of heart disease, according to the Harvard scientists, whose research is published in the Journal of the American College of Cardiology. However, those who ditch saturated fats for more unsaturated fats or whole grains do lower their heart disease risk.

Many people fall back on carbohydrates, especially refined carbs, such as white bread, when they reduce saturated fat in their diets, senior author Dr. Frank Hu, a professor of nutrition and epidemiology, said in a press statement. He said that may in part explain findings from a controversial 2014 paper that called into question recommendations for limiting saturated fat for heart health, and led to headlines promoting the return of butter.

"Our research does not exonerate saturated fat," Hu said in the statement. "In terms of heart disease risk, saturated fat and refined carbohydrates appear to be similarly unhealthful."

Hu's study is the first prospective analysis to compare saturated fat with other types of fats and different types of carbohydrates in relation to heart disease risk. He and colleagues looked at diet and health information from participants in two long-running observational studies, the Nurses Health Study (84,628 women) and the Health Professional Follow-up Study (42,908 men), who were free of diabetes, cardiovascular disease, and cancer at baseline. Diet was assessed by food frequency questionnaires every four years. During follow-up, the researchers documented 7,667 cases of coronary heart disease.

The researchers also took into account cardiovascular disease risk factors such as age, body mass index (BMI), smoking, and physical activity.

They estimated that replacing 5 percent of calories from saturated fats with the equivalent number of calories from either polyunsaturated fats, monounsaturated fats, or carbohydrates from whole grains was associated with 25 percent, 15 percent, and 9 percent lower risk of heart disease, respectively. On the flip side, swapping 5 percent of saturated fat calories for the same amount of refined carbohydrates and sugars did not change heart disease risk.

The message: Cutting saturated fat won't do your heart any good if you just replace it with refined carbs and sugars.

There are two types of carbohydrates, simple and complex, said Sowa. Simple carbs are broken down very quickly by the body and when this happens, the pancreas secretes a large level of insulin and over time, it can lead to diabetes, weight gain, and fatty liver.

"The carbohydrates you want to focus on are complex carbohydrates that take longer for your body to break down," Sowa explained. "Those are in whole grains, lentils, some forms of potatoes, like sweet potatoes."

Sowa's advice to patients to stay healthy and keep weight off: Forget the low-fat, high-carb food fads of yore. "Generally, follow a low carbohydrate diet because people can stick to it and the protein does satiate you." Watching calorie intake and portion sizes are essential, too.

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    Mary Brophy Marcus covers health and wellness for CBSNews.com