Study Reveals Diet Quality Matters More Than Low-Carb vs Low-Fat for Heart Health

If you’re trying to improve your heart health, you’ve likely been recommended a low-carb or a low-fat diet. But according to a new study in Journal of American College of Cardiology, limiting the amount of carbs and fat you eat may not be a silver bullet when it comes to reducing heart disease risk.

Harvard researchers analyzed diet survey data and health outcomes of some 200,000 health professionals for 30 years and measured their associated risk of developing heart disease. Researchers compared health professionals who ate a low carb diet and found heart disease risk could actually increase by 14 percent if the person regularly consumed unhealthy or refined carbs. However, if the person ate healthy carbs like whole grains, risk dropped by 15 percent.

This study suggests that health is “not simply about cutting carbs or fats,” says Zhiyuan Wu, postdoctoral research fellow at Harvard University’s department of nutrition and coauthor of the study. “Our central message is that diet quality matters more.”

Because this was an observational study, researchers can’t directly prove certain diets actually caused study participants to get heart disease. However, the fact that the study followed a large number of people for a long period of time is “impressive,” says Camilla Dalby Hansen, a researcher at the University of Southern Denmark, Odense who was not part of the study.

Researchers went a step further and collected biological data to help counteract potential self-reported bias from the health questionnaires, Wu says. The researchers took blood samples of more than 11,000 participants and were able to measure their metabolomics, the small molecules in cells and tissues, which corroborated their earlier findings.

“It’s a very complicated methodology that they succeeded with,” Hansen says. “What makes me very happy is the metabolomics addition. It’s super new and exciting.”

The body needs macronutrients—or carbs, proteins, and fats—to give us energy and support our overall health. And there’s no shortage of food options to get our macros in: Steak, fruits, oatmeal, fast food, ice cream, vegetables, and more.

But there are different qualities of the foods we eat. For example, in the study, Harvard researchers distinguished between low-quality, refined carbs and high-quality carbs like whole grains.

The study also differentiated the quality of fat sources. A low-quality, unhealthy fat like trans fat or saturated fat—found in foods like red meats, heavy cream, and butter—is not easily metabolized by the body. Unhealthy fats are often associated with higher risk for diseases like Type 2 diabetes and heart disease.

Contrarily, high-quality, healthy fats like avocados or nuts are easy for the body to break down to use for energy—and they also keep you full. “Fats are the most amazing macronutrients ever,” Hansen says. “They protect your brain health, your hormone function, your skin. They are also cardio-protective, [clearing] your arteries from the debris.”

Researchers created different categories of healthy and unhealthy diets when conducting the study. A healthy diet was defined as one that included more plant-based proteins and fats or high-quality proteins and fats, as well as high-quality carbs. Conversely, an unhealthy diet was defined as rich in low-quality refined carbs and animal proteins and fats, which were considered a low-quality fat source in the study.

The results clearly show the difference in fat and carb quality may reduce disease risk, even if participants ate fewer carbs or fats. Specifically, participants who ate high-quality carbs and more plant proteins and fats had a lower heart disease risk compared to those who ate more refined carbs and animal fats and proteins.

Furthermore, those who got their protein and fats sources from animals were more positively-correlated with having saturated fats and trans fats, which can also increase risk of cardiovascular disease.

Hansen believes one limitation of the study is how it simplifies the categories of “healthy and unhealthy” by ranking animal fats and proteins as unhealthy. “In my opinion, all animal proteins and all animal fats are not unhealthy,” she says.

Wu agrees that some low-fat animal proteins can be beneficial, such as yogurt. Still, other experts point to how this study underscores the extra benefits of whole grains and vegetable protein and fats compared to other types.

“That is not saying don't consume animal protein, but heavily [focus on] whole foods and plant-based” proteins and fats like legumes and beans, says Jennifer Sacheck, a provost professor at Brown University’s School of Public Health. “Eating heavier on the plant-based side has historically shown greater health benefits.”

The blood samples taken throughout the study also provided insight into participants’ actual health. Those with healthier versions of low-carb and low-fat diets had higher levels of good cholesterol and lower levels of triglycerides, which are produced when our body has reached a healthy caloric limit. Higher levels of good cholesterol can protect the body from heart disease by clearing out the bad cholesterol from our arteries.

The researchers also analyzed metabolites, or molecules leftover after our body metabolizes something. “If you say you're eating certain things, your metabolome is actually indicating that's the case,” Sacheck says.

One metabolite, hippuric acid, was more greatly associated with participants who followed the healthier low-carb and low-fat diets. Hippuric acid can be another indicator of a diet rich in fruits and vegetables. The findings validate the risk-modeling and suggest that healthier diets do lead to better biological outcomes.

Interestingly, Wu says that both those who followed a healthy low-carb diet and a healthy low-fat diet had overlapping signs of good health. To him, this underscores how we have more choice in the type of diet we want to follow—in this case, low-carb or low-fat—as long as we focus on eating whole foods and sources of high-quality fats and carbs.

“I think that provides more flexibility with your preference, while still providing more cardiovascular protections,” Wu says.