Saturated fats have been the whipping boy of heart disease and diabetes for more than half a century, but a new study found that carbohydrates, not fats, are the true culprit.
Research at Ohio State University showed that doubling — or almost tripling — saturated fat in the diet does not increase the total levels of saturated fat in the blood. However, increasing the amounts of carbohydrates in the diet steadily increased blood levels of a fatty acid linked to an elevated risk for heart disease and diabetes.
The outcome of the study "challenges the conventional wisdom that has demonized saturated fat and extends our knowledge of why dietary saturated fat doesn’t correlate with disease," said senior author Jeff Volek, a professor of human sciences.
"It’s unusual for a marker to track so closely with carbohydrate intake, making this a unique and clinically significant finding," Volek said. "As you increase carbs, this marker predictably goes up."
Volek studied adults with metabolic syndrome, meaning they had at least three of the five factors that increase the risk for heart disease and diabetes (high blood pressure, too much belly fat, low "good" cholesterol, insulin resistance, and high triglycerides).
In the study, participants were fed six three-week diets that varied the amount of fats and carbohydrates in the diet, while keeping protein and total calories the same.
Researchers had speculated that when the amount of saturated fats was increased, the total amount of saturated fats in the blood would also increase. Instead, fat in the blood did not increase and even went down, even when participants ate almost three times the amount of fat.
As fats increased in the diet, the amount of palmitoleic acid, a fatty acid associated with the unhealthy metabolism of carbohydrates, also went down. But as carbohydrates were reintroduced into the diet, the amount of palmitoleic acid increased.
When the amounts of palmitoleic acid increase, Volek says it's a sign that more carbs are being converted to fat instead of being burned as fuel.
Reducing carbs and adding fat to the diet, he says, makes sure the body will quickly burn the saturated fat as fuel and not store it as fat.
"When you consume a very low-carb diet your body preferentially burns saturated fat," he said. "We had people eat two times more saturated fat than they had been eating before entering the study, yet when we measured saturated fat in their blood, it went down in the majority of people. Other traditional risk markers improved, as well.
"There is widespread misunderstanding about saturated fat," Volek said. "In population studies, there’s clearly no association of dietary saturated fat and heart disease, yet dietary guidelines continue to advocate restriction of saturated fat. That's not scientific and not smart."
"But studies measuring saturated fat in the blood and risk for heart disease show there is an association," he continued. "Having a lot of saturated fat in your body is not a good thing. The question is, what causes people to store more saturated fat in their blood, or membranes, or tissues?
"People believe 'you are what you eat,' but in reality, you are what you save from what you eat," he said. "The point is you don’t necessarily save the saturated fat that you eat. And the primary regulator of what you save in terms of fat is the carbohydrate in your diet. Since more than half of Americans show some signs of carb intolerance, it makes more sense to focus on carb restriction than fat restriction."
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