Researchers followed over 198,000 Americans for up to 30 years to see how low-carbohydrate and low-fat diets affect heart disease risk. The big surprise? It’s not about choosing low-carb or low-fat—it’s about choosing the healthy version of whichever diet you pick. Eating healthy low-carb foods (like vegetables and whole grains) or healthy low-fat foods both lowered heart disease risk, while unhealthy versions of either diet increased it. This suggests that the quality of what you eat matters far more than simply cutting carbs or fat.

The Quick Take

  • What they studied: Whether eating fewer carbohydrates or less fat helps prevent heart disease, and whether the type of foods you choose within those diets matters.
  • Who participated: Nearly 200,000 American health professionals (doctors, nurses, and other healthcare workers) tracked over 30 years. The group included 42,720 men and about 155,000 women of varying ages.
  • Key finding: Healthy versions of both low-carb and low-fat diets reduced heart disease risk by about 13-15%, while unhealthy versions increased risk by 12-14%. The quality of food choices mattered more than the diet type itself.
  • What it means for you: If you’re considering a low-carb or low-fat diet, focus on choosing whole, unprocessed foods rather than processed alternatives. Either approach can work for heart health if done the right way, but cutting corners with unhealthy processed foods defeats the purpose.

The Research Details

This was a long-term follow-up study called a cohort study, where researchers tracked the same people over many years. Scientists followed three groups of American health professionals from the mid-1980s through 2016-2019, asking them about their eating habits every few years using detailed food questionnaires. They recorded how many people developed heart disease during the study period.

The researchers created different scoring systems to measure how strictly people followed low-carb or low-fat diets, and importantly, they measured the quality of those diets. For example, a low-carb diet could emphasize vegetables (healthy) or processed meats (unhealthy). They also measured blood markers and metabolites (chemical substances in the body) to understand how different diets affected heart disease risk factors.

To validate their findings, they used a smaller group of 1,146 healthy participants to develop mathematical models that could predict heart disease risk based on blood metabolites and diet patterns.

This research approach is powerful because it follows real people over decades, showing what actually happens in everyday life rather than just what happens in a controlled lab setting. By measuring both diet quality and blood markers, researchers could understand not just whether diets worked, but how they worked—through what biological pathways they affected heart health.

This study is considered high-quality because: (1) it involved nearly 200,000 participants, providing very reliable results; (2) it followed people for 30 years, capturing long-term effects; (3) participants were health professionals likely to accurately report their diets; (4) researchers adjusted for many other factors that affect heart disease (smoking, exercise, age); and (5) findings were consistent across all three study groups. The main limitation is that participants self-reported their diets, which can have some inaccuracy.

What the Results Show

The study found that diet quality was the deciding factor in heart health outcomes. People following a healthy low-carb diet (emphasizing vegetables, whole grains, and plant-based proteins) had a 15% lower risk of heart disease compared to those eating an unhealthy low-carb diet. Similarly, those following a healthy low-fat diet had a 13% lower heart disease risk compared to unhealthy low-fat eaters.

Interestingly, an overall low-carb diet showed a slight increase in heart disease risk (5%), but this was entirely driven by people eating unhealthy low-carb foods. When researchers looked only at plant-based low-carb diets, the risk actually decreased by 6%. This shows that the source of carbohydrates matters tremendously.

The healthy versions of both diets also improved blood markers associated with heart health: they lowered triglycerides (a type of fat in blood), raised good cholesterol (HDL), and reduced inflammation markers. The unhealthy versions did the opposite, worsening these risk factors.

Beyond heart disease risk, researchers found that healthy diet patterns changed the body’s chemistry in favorable ways. People eating healthy low-carb or low-fat diets showed increased levels of a protective compound called 3-indolepropionic acid and decreased levels of valine (an amino acid linked to metabolic problems). These chemical changes suggest that healthy diets work through multiple biological pathways to protect the heart. Unhealthy diet patterns showed the opposite chemical signatures, indicating they may actively harm cardiovascular health.

This research builds on decades of debate about whether low-carb or low-fat diets are better. Previous studies often showed conflicting results because they didn’t account for diet quality. This study resolves that confusion by showing both approaches can work if done healthily. The findings align with other recent research suggesting that whole foods matter more than macronutrient ratios, but this is the largest and longest study to directly compare both diet types while measuring quality.

The main limitation is that participants reported their own diets through questionnaires, which can be inaccurate—people may forget what they ate or underreport unhealthy foods. The study also involved mostly health professionals, who may be more health-conscious than the general population, potentially making results less applicable to everyone. Additionally, the study is observational, meaning researchers observed what people ate and what happened, but couldn’t prove that diet changes directly caused the health differences—other lifestyle factors could play a role. Finally, the blood metabolite predictions (r = 0.21-0.38) showed moderate accuracy, suggesting these chemical markers are useful but not perfect predictors.

The Bottom Line

If you’re considering a low-carb or low-fat diet for heart health, choose the healthy version: (1) For low-carb: emphasize vegetables, nuts, seeds, and plant-based proteins while limiting processed meats and refined carbs. (2) For low-fat: focus on whole grains, legumes, fruits, and vegetables while avoiding processed low-fat foods high in sugar. (3) Either approach appears equally effective for heart health when done properly. (4) Avoid unhealthy versions of either diet, as they may increase heart disease risk. Confidence level: High—this is based on nearly 200,000 people followed for decades.

Anyone concerned about heart disease should care about these findings, particularly people with family history of heart disease, high cholesterol, or high blood pressure. People considering starting a low-carb or low-fat diet should use these results to guide their food choices. Healthcare providers should use this to counsel patients that diet type matters less than diet quality. People with existing heart disease should discuss any major diet changes with their doctor before starting.

The study shows associations over years and decades, not immediate changes. You might see improvements in blood markers (cholesterol, triglycerides, inflammation) within weeks to months of switching to a healthy diet pattern. However, the reduction in actual heart disease risk likely develops over years of consistent healthy eating. Don’t expect overnight results, but expect meaningful long-term benefits from sustained healthy eating patterns.

Want to Apply This Research?

  • Track diet quality score daily by logging: (1) servings of whole grains vs refined grains, (2) plant-based vs animal protein sources, (3) processed vs whole foods. Create a simple scoring system (e.g., 1 point for each healthy choice) to monitor whether you’re eating the healthy version of your chosen diet pattern.
  • If using a low-carb approach: replace processed low-carb snacks with vegetables, nuts, and seeds. If using low-fat: replace processed low-fat foods with whole grains and legumes. Set a weekly goal to increase one category of whole foods while decreasing one category of processed foods. Use the app to plan meals emphasizing whole foods within your chosen diet framework.
  • Monthly, review your diet quality score trend. Every 3 months, if possible, track relevant blood markers (cholesterol, triglycerides) through your healthcare provider. Use the app to identify patterns—which healthy food swaps are easiest for you to maintain? Which processed foods are hardest to avoid? Adjust your approach based on what’s sustainable for your lifestyle rather than trying to be perfect.

This research shows associations between diet patterns and heart disease risk but does not prove that changing your diet will prevent heart disease. Individual results vary based on genetics, overall lifestyle, and other health factors. Before making significant dietary changes, especially if you have existing heart disease, high cholesterol, diabetes, or take medications, consult with your healthcare provider or registered dietitian. This summary is for educational purposes and should not replace professional medical advice. The study was published in 2026 and reflects current scientific understanding, but medical knowledge evolves as new research emerges.