After shifting fads, time for a clear-eyed look at fat in our diets

After the low-fat, high-fat diet seesaw, experts say it's time for a clear reading of the science

With national headlines touting the message that low-fat diets are out and high-fat diets are in, people who once shunned butter are slathering it on toast, melting it in sautés and even plopping it into their coffee. And though the media has glorified saturated fat, it turns out that the main problem with the past decades' low-fat trend is that it has been misinterpreted.

"There is no real low-fat controversy," says Dr. David Heber, founding director of the UCLA Center for Human Nutrition. "The problem wasn't low fat; the problem was that when we lowered fat content, we increased carbohydrate and sugar content." Heber gives the classic example of SnackWells fat-free cookies, popular in the 1990s. With zero fat, SnackWells were considered more healthful than regular cookies, but they were only palatable because they contained high amounts of refined sugar and carbohydrates.

The low-fat fad emerged in the 1980s, following a growing body of research that linked a diet high in fats to increased cardiovascular disease risk. The U.S. Department of Agriculture's food pyramid reflected the wisdom of the time: The base showed bread, rice and cereal, which Americans were urged to eat plentifully; the tip of the pyramid depicted fats and oils to be used sparingly. Breakfasts of eggs and bacon were replaced with bowls of cereal; steak dinners were swapped for plates of pasta.

By the 2000s, mounting evidence showed that different fats had differing health outcomes. Saturated fat in particular was associated with increased cardiovascular disease risk, and unsaturated fats were shown to have a protective benefit.

Then, in 2014, a controversial meta-analysis published in the Annals of Internal Medicine was widely misconstrued to promote a diet high in saturated fat. Newsstands screamed, "Eat butter," and the saturated fat trend gained momentum. But in reality, the analysis showed not that saturated fat is good for us but that, when compared with diets low in fat but high in refined carbohydrates, a diet high in saturated fat is almost a wash in terms of cardiovascular disease risk.

Aside from misinterpretations of study results, the analysis itself had fundamental errors that prompted an international outcry from scientists. The authors had mixed up the results of one study they analyzed, omitted key studies from their analysis and failed to mention what people consumed as replacements when they ate less saturated fat. A corrected version of the paper was posted on the journal's website.

"When talking about diets, we always have to think about what the trade-off is," says Dr. Frank Hu, professor of nutrition and epidemiology at Harvard School of Public Health. "If you cut back on saturated fat, you're going to replace it with either unsaturated fat or carbohydrates. The type of replacement can have a major impact on health outcomes."

Diets low in fat and high in refined carbohydrates are associated with increased triglycerides and decreased levels of HDL ("good" cholesterol), factors that are linked to metabolic disorders and cardiovascular disease. Since the low-fat trend of the 1980s, rates of obesity and type 2 diabetes have skyrocketed, and heart disease is still the No. 1 cause of death in America.

Fats serve important functions in our diet, improving the taste and consistency of foods as well as helping foods maintain heat. Fats also serve vital functions in our bodies, providing efficient calorie storage and supplying building blocks for cell membranes. Different fats have different roles. For example, omega-3 fatty acids are essential for brain development, while omega-6 fatty acids contribute to cholesterol metabolism, cell signaling and skin function.

"It's important to emphasize that not all fats are created equal," says Hu. "You should swap unhealthy fats with healthy fats rather than cut back on dietary fat."

But teasing apart which fats are healthful can be confusing. In spite of the current butter mania, there is still evidence that a diet high in saturated fats (which include the wildly popular coconut oil) increases LDL or "bad" cholesterol, an important risk factor for heart disease. And though fats that contain omega-6 fatty acids (such as many vegetable oils, nuts and seeds) have recently come under fire for fears they may cause inflammation and thus increase the risk of heart disease, there is scant evidence that this is true, says Hu.

A recent meta-analysis Hu co-authored shows that people who swap 5% of the calories they consume from saturated fat sources, such as red meat and butter, with polyunsaturated fat sources containing omega-6 fatty acids lowered their risk of cardiovascular disease by 9% and their risk of death from cardiovascular disease by 13%.

Hu is a member of the 2015 Dietary Guidelines Advisory Committee, a scientific panel that gives recommendations to the USDA for new Dietary Guidelines for Americans every five years. Though the official guidelines won't be out until the end of the year, the committee recommends that people limit their intake of saturated fat to no more than 10% of their energy intake and eliminate trans fats from their diet.

The committee also recommends that the primary sources of dietary fat should come from polyunsaturated fats, such as canola and soybean oil, and monounsaturated fats, such as olive and avocado oil. And rather than replacing refined carbohydrates with saturated fat, people should replace them with healthful carbohydrates, including whole grains, legumes, vegetables and fruits.

Hu adds that, for optimal health, "the consumption of low-fat or nonfat products with high amounts of refined carbohydrates, refined grains or added sugars should be discouraged."

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