For decades, people trying to protect their hearts have been caught in a tug-of-war between low-carbohydrate and low-fat diets.

The answer may be simpler and far more practical, a new, large study suggests. It's not how much fat or carbs you eat, but where those nutrients come from.

The latest research finds that healthy versions of both low-carb and low-fat diets are linked to better cardiovascular and metabolic health and a lower risk of coronary heart disease (CHD).

Study shifts the spotlight to what's on the menu — not what's been left off.

On the other hand, unhealthy versions of either approach — those heavy on refined carbohydrates and animal-based fats — were associated with higher heart disease risk and unhealthier metabolic profiles.

“[I]t's not simply about cutting carbs or fat, but it's about the quality of foods people choose to construct those diets,” lead author, Zhiyuan Wu, PhD, a postdoctoral research fellow at Harvard T.H. Chan School of Public Health, said in a media release, adding, “Focusing only on nutrient compositions but not food quality may not lead to health benefits.”

The study is one of the most comprehensive to examine three factors: diet quality, blood-based metabolic markers and long-term heart disease outcomes.

Researchers analyzed data from nearly 200,000 U.S. adults enrolled in three major ongoing cohort studies: the Nurses' Health Study, Nurses' Health Study II and the Health Professionals Follow-up Study. Participants were followed for more than 30 years, amounting to over 5.2 million person-years of data. During that time, over 20,000 cases of coronary heart disease were recorded among those in the study.

The participants completed regular, detailed food-frequency questionnaires, allowing researchers to assess their long-term eating patterns. The investigators then developed scoring systems to distinguish healthy and unhealthy low-carbohydrates and low-fat diets.

Healthy versions emphasized plant-based foods, whole grains and unsaturated fats; unhealthy versions relied more heavily on refined or processed carbohydrates and animal-based fats and proteins.

The results were strikingly consistent. People who followed healthy versions of either low-carb or low-fat diets had a significantly lower risk of developing CHD. They also showed healthier metabolic profiles, including lower triglycerides levels, higher “good” HDL cholesterol and lower markers of inflammation. These are all key indicators of cardiovascular health.

By contrast, participants whose low-carb or low-fat diets were built around refined starches, sugars and animal fats faced a higher risk of heart disease. In other words, a low-carb diet based on vegetables, nuts and healthy oils looked very different, both biologically and clinically, from a low-carb diet centered on processed meats and refined foods.

Higher-quality diets favored heart health, regardless of whether diets were lower in carbohydrates or fat.

To further understand the effects of these differences, the researchers analyzed participants' cardiometabolic markers, examining blood levels of HDL cholesterol, for example, that reflect how the body processes food. These analyses supported the clinical findings, revealing that people eating higher-quality diets had favorable biomarker patterns, regardless of whether they were lower in carbohydrates or fat.

“These results suggest that healthy low-carbohydrate and low-fat diets may share common biological pathways that improve cardiovascular health,” said Wu. “Focusing on overall diet quality may offer flexibility for individuals to choose eating patterns that align with their preferences while still supporting heart health.”

The authors do note several limitations. Diets were self-reported, which can introduce measurement errors, and the participants, mostly health professionals, may not fully represent the general population. The findings also may not apply to extreme diets, such as very low-carbohydrate ketogenic plans.

That said, the take-away for folks remains clear: heart-healthy eating isn't about rigid rules or warring diet camps. And “higher quality” is not about cost. It's about choosing real, minimally processed foods, no matter what macronutrient label your diet carries. The low-carb versus low-fat argument may never fully disappear, but at least this study shifts the spotlight to what's on the menu — not what's been left off.

The study is pubished in the Journal of the American College of Cardiology.