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Articles March 2019 Issue

Why the Low-Fat Diet Failed

It turns out that some fats are healthy—and replacing fats with refined carbohydrates is not.

Image © Oleksandr Pupko | Getty Images

Manufacturers responded to advice to decrease fat intake by creating reduced-fat products like crackers and cookies that are high in unhealthful refined carbohydrates.

In the 1980s and 1990s, Americans were told that eating less fat would reduce risk for cardiovascular disease and obesity. Why didn’t it work? Essentially, reducing total fat led to intake of more refined carbohydrates and less healthy fats, and both of these changes had negative health impacts.

Evolving Guidelines: Dietary advice on fat intake has evolved over the years. Based on strong evidence that diets high in saturated fats and cholesterol were associated with high blood cholesterol levels and a greater risk of heart attack, the 1980 U.S. Dietary Guidelines recommended Americans “avoid too much fat, saturated fat, and cholesterol.” The 1990 Dietary Guidelines added risk for obesity to the list of reasons to cut back on fats. (Since a gram of fat has more than twice the calories of a gram of protein or carbohydrate, reducing fat intake is the fastest way to reduce calorie intake.) But, soon thereafter, data began to emerge that low-fat diets were not having the expected results. “The 2000 Dietary Guidelines Advisory Committee noted the adverse effects of low-fat diets, and for the first time since their inception changed the guidance from low fat to moderate fat,” says Alice H. Lichtenstein, DSc, a member of that committee who is now a senior scientist at Tufts’ HNRCA and executive editor of Tufts Health & Nutrition Letter. From 2000 on, the Dietary Guidelines for Americans (DGA) recommended consuming a diet moderate in total fat that keeps saturated fat intake to under 10 percent of calories. The 2015 DGA emphasizes consuming foods rich in polyunsaturated fat. Some scientists feel that even this moderate fat restriction goes too far. Dariush Mozaffarian, MD, DrPH, dean of Tufts’ Friedman School of Nutrition Science and Policy and editor-in-chief of Tufts Health & Nutrition Letter sites evidence that a higher proportion of calories from fat is not harmful for either cardiovascular disease (CVD) or obesity, and in fact can lower risk if healthy (poly- and mono-unsaturated) fats replace refined starches and sugars.

Image © Kate Headley

Unsaturated fats from foods like plant oils, fish, nuts, and avocados, are an important part of a healthy diet.

The Carb Conundrum: In response to the 1980 guidelines and resulting consumer demand, food manufacturers began marketing low- and reduced-fat products. “This led to a proliferation of reduced-fat products with poor nutritional value, such as low-fat breakfast cereals rich in starch and sugar, and low-fat versions of salty snacks, salad dressings, muffins, cookies, and other desserts,” says Mozaffarian. Evidence has now shown that replacing total fat or saturated fat with refined carbohydrate is not associated with reduced risk of cardiovascular disease. “Replacing saturated fat with refined carbohydrate reduces blood levels of total and LDL (bad) cholesterol but increases blood levels of triglycerides and reduces HDL (good) cholesterol,” says Lichtenstein. A small but well controlled feeding trial published in PLoS One in 2014 found that replacing dietary saturated fat with dietary carbohydrate did not reduce the blood levels of major saturated fatty acids. “The liver turns excess dietary starch and sugar into fat,” explains Mozaffarian, “in particular, saturated fat.” Replacing fats with carbohydrates, therefore, did little to improve factors that influence heart health.

But that doesn’t mean removing all carbs from our diets is the answer. We know that some carb-containing foods are associated with positive health impacts, and some are not. “Fruits, legumes, non-starchy vegetables, and minimally processed whole grains are protective. Refined carbs (white bread, white rice, crackers, cereals, bakery desserts, white potatoes, sweets, soda, and other foods rich in starch and sugar) are harmful—think of these as sides or treats, just like sweets,” says Mozaffarian. “We need to reduce refined starch and sugar, not all carbs.”

Not all Fats are Created Equal: “Cutting back on total dietary fat removed beneficial fats from our diets along with harmful fats,” says Lichtenstein. Unsaturated fats (especially polyunsaturated) can actually have a positive impact on health. The 2015 DGA sites “strong and consistent evidence” that replacing saturated fats with polyunsaturated is associated with lower blood levels of total and LDL-cholesterol, as well as lower risk of heart attack.

A recent American Heart Association advisory, written by Lichtenstein and colleagues and published in the journal Circulation, analyzed a number of studies examining what happens when people replace saturated fat with unsaturated fat. Several studies, including the Oslo Diet-Heart Study and studies conducted at the Veterans Administration center in Los Angeles, provided evidence that replacing saturated fat with polyunsaturated fat lowers the risk of heart attack, stroke, and angina. The authors of the advisory concluded that replacing foods high in saturated fat like butter, meat, and coconut oil with polyunsaturated fat-rich vegetable oils such as soybean and corn oils could lower rates of CVD by about 30 percent. Replacing foods high in saturated fat with monounsaturated-rich vegetable oils such as canola and olive oils was found to be associated with a 15 percent lower rate of CVD.

Some ways to replace foods high in saturated fats with foods high in polyunsaturated fat include using vegetable oils instead of butter, choosing fish over red meat, snacking on a handful of nuts instead of pretzels, and using vegetable oil-based salad dressings rather than cream- or cheese-based dressings.

“When making dietary choices, aim to increase foods rich in healthy fats, like nuts, plant oils, avocados, and fish,” says Mozaffarian, “and reduce foods high in refined starch and sugar.” It’s also important not to make food decisions on the basis of fat content alone. Most foods contain more than one kind of fat, often packaged with other macronutrients, micronutrients (vitamins and minerals), and, in the case of plant foods, phytochemicals and fiber. “Remember,” says Lichtenstein, “when it comes to good nutrition (including fats) it’s all about balance.”

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