A study published in PLOS ONE shows that doubling or even nearly tripling saturated fat in the diet may not drive up total levels of saturated fat in the blood.
However, increasing levels of carbohydrates in the diet during the study promoted a steady increase in the blood of a fatty acid linked to an elevated risk for diabetes and heart disease.n the study, 16 adult participants were fed six three-week diets that progressively increased carbs while simultaneously reducing total fat and saturated fat, keeping calories and protein the same. All of the participants had metabolic syndrome, defined as the presence of at least three of five factors that increase the risk for heart disease and diabetes (excess belly fat, elevated blood pressure, low "good" cholesterol, insulin resistance or glucose intolerance, and high triglycerides). After getting them to a baseline reduced-carb diet for three weeks, researchers fed the participants the exact same diets, which changed every three weeks, for 18 weeks. The diets started with 47 g of carbs and 84 g of saturated fat each day, and ended with 346 carb g per day and 32 g daily of saturated fat. Each day's meals added up to 2,500 calories and included about 130 g of protein. The highest-carb level represented 55% of daily calories, which roughly matches the estimated daily percentage of energy provided by carbs in the American diet.
The researchers found that total saturated fat in the blood did not increase—and went down in most people—despite being increased in the diet when carbs were reduced. Compared to baseline, there were significant improvements in blood glucose, insulin, and blood pressure that were similar across diets. Palmitoleic acid, a fatty acid associated with unhealthy metabolism of carbohydrates that can promote disease, went down with low-carb intake and gradually increased as carbs were re-introduced to the study diet. "It's unusual for a marker to track so closely with carbohydrate intake, making this a unique and clinically significant finding. As you increase carbs, this marker predictably goes up," said Jeff Volek, a Professor of Human Sciences at The Ohio State University. When that marker increases, he said, it is a signal that an increasing proportion of carbs are being converted to fat instead of being burned as fuel. Reducing carbs and adding fat to the diet in a well-formulated way, on the other hand, ensures the body will promptly burn the saturated fat as fuel—so it won't be stored. The study does not address what happens to palmitoleic acid levels when high carbs are combined with a diet high in saturated fat. Volek sees this palmitoleic acid as a potential biomarker to signal when the body is converting carbs to fat, an early event that contributes to what he calls "metabolic mayhem."
"There is no magical carb level, no cookie-cutter approach to diet that works for everyone," he said. "There's a lot of interest in personalized nutrition, and using a dynamically changing biomarker could provide some index as to how the body is processing carbohydrates."