Heart Disease and Diabetes Risks Tied to Carbs, Not Fat, Study Finds

Two chocolate donuts
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Is the pendulum swinging back? In what seems contrary to mainstream dietary advice, a small new study shows that doubling the saturated fat in a person's diet does not drive up the levels of saturated fat in the blood.

Rather, the study found that it was the carbohydrates in people's diets that were linked with increased levels of a type of fatty acid linked to heart disease and type-2 diabetes. The results of the study, which followed 16 middle-aged, obese adults for 21 weeks, were published Nov. 21 in the journal PLOS ONE.

Saturated fats, largely from meat and dairy products, have been vilified for decades as a primary culprit in promoting heart disease. And most health authorities maintain this stance.

However, in recent years, scientists have seen the ill effects of completely replacing saturated fat with carbohydrates, particularly the simple carbs that are found so commonly in processed foods. A large analysis published in 2009 in the American Journal of Clinical Nutrition found that swapping saturated fats with carbs had no benefit in reducing people's risk of heart disease. But replacing those so-called bad fats with polyunsaturated fats — found in fish, olives and nuts — did.

"The unintended consequence of telling everyone to restrict fat was that people ate an even greater amount of carbohydrates," said Jeff Volek, senior author on the new study and a professor of human sciences at The Ohio State University. "This is a fact. It's not a stretch to make the connection between overconsumption of carbs and the obesity and diabetes epidemic."

The new study "challenges the conventional wisdom that has demonized saturated fat," Volek said, because it shows that saturated fats don't need to be replaced at all, neither with carbs nor polyunsaturated fats. [7 Foods You Can Overdose On]

The study, it should be noted, was funded by a grant from the Dairy Research Institute, the National Cattlemen's Beef Association, the Egg Nutrition Center, and the Robert C. and Veronica Atkins Foundation, although the researchers reported that the funders had no role in the study design or decision to publish the research.

For the analysis, researchers placed 16 participants on a tightly controlled diet of fats and carbs. The participants were on their own high-carb, low-fat diets before entering the study. For the first three weeks of the study, they doubled or tripled their saturated-fat intake, consuming 84 grams of saturated fats, and 47 grams of carbs per day. Researchers found no jump in the levels of saturated fat in the blood during this phase.

Then, every three weeks after this, the dieters decreased the fat and increased their carb intake, ending the study on a diet of 32 grams of saturated fat and 346 grams of carbs per day. The final phase modeled U.S. dietary recommendations for carbs and included whole grains.

"You can sort of think of this experiment as a dose-response study, where we exposed individuals to a range of dietary carb levels and monitored their fatty-acid levels to determine if they were accumulating saturated fatty acids and turning carbs into fat," Volek told Live Science.

The researchers found that as the amount of dietary fat was decreased, there were no changes in the levels of saturated fat in the participants' blood. But one kind of fatty acid, called palmitoleic acid, did progressively increase.

"Higher proportions of palmitoleic acid in blood or adipose tissue are consistently associated with a myriad of undesirable outcomes, such as obesity, …inflammation, metabolic syndrome, type 2 diabetes, coronary disease, heart failure, and incidence and aggressiveness of prostate cancer," the researchers wrote.

Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science & Policy at Tufts University, who was not involved in the research, described the work as a well-controlled interventional study confirming that dietary refined carbohydrate is the primary driver of circulating saturated fatty acids in the bloodstream.

"White bread, rice, cereals, potatoes, and sugars — not saturated fat — are the real culprits in our food supply," he Mozaffarian.

Dr. Walter Willett, the chair of the Department of Nutrition at Harvard School of Public Health, also thought this was a reasonably done study, but he added it is difficult to make conclusions about the risk of heart disease from a study so small and short.

"Basically in their study they are comparing two bad diets, and the adverse of carbohydrates is likely to be particularly serious in the obese and insulin resistant population that they studied," Willett said.

We know from many long-term studies, Willet added, that replacing saturated fats from red meat and dairy with vegetable fats high in polyunsaturated and monounsaturated fats will reduce risks of heart disease. This is characteristic of the Mediterranean diet.

Also, other studies have shown beneficial properties of dietary palmitoleic acid, and its role in health remains an open question.

Follow Christopher Wanjek @wanjek for daily tweets on health and science with a humorous edge. Wanjek is the author of "Food at Work" and "Bad Medicine." His column, Bad Medicine, appears regularly on LiveScience.

Christopher Wanjek
Live Science Contributor

Christopher Wanjek is a Live Science contributor and a health and science writer. He is the author of three science books: Spacefarers (2020), Food at Work (2005) and Bad Medicine (2003). His "Food at Work" book and project, concerning workers' health, safety and productivity, was commissioned by the U.N.'s International Labor Organization. For Live Science, Christopher covers public health, nutrition and biology, and he has written extensively for The Washington Post and Sky & Telescope among others, as well as for the NASA Goddard Space Flight Center, where he was a senior writer. Christopher holds a Master of Health degree from Harvard School of Public Health and a degree in journalism from Temple University.