Observational study finds healthy diets associated with reduced coronary heart disease risk — Evidence Review

Low-carb and low-fat diets can both lower heart disease risk if they emphasize healthy, high-quality foods, according to a new long-term study published in the Journal of the American College of Cardiology from Harvard. The findings largely align with previous research showing that diet quality, not just macronutrient composition, is crucial for cardiovascular health.

• Related studies consistently indicate that plant-based, high-quality versions of both low-carb and low-fat diets are linked to reduced coronary heart disease (CHD) risk, while versions high in animal fats and refined carbohydrates are less beneficial or even harmful 1 12 14.
• Meta-analyses and systematic reviews have shown that low-carb and low-fat diets can both improve cardiovascular risk factors such as cholesterol and inflammation, but long-term outcomes depend on overall dietary patterns and food choices 2 5 11.
• Studies further suggest that the benefits of low-carb or low-fat diets are diminished or absent when diet quality is poor, reinforcing the importance of whole foods, healthy fats, and plant-based ingredients in heart health 1 12 13 14.

Study Overview and Key Findings

While low-carb and low-fat diets are popular for weight management, their long-term impact on heart health—especially the role of food quality within these diets—has been less clear. This large, multi-decade study addresses this gap by analyzing how different versions of these diets affect coronary heart disease (CHD) risk and metabolic health, using detailed dietary data and metabolomic analyses. The study stands out for its scope, following nearly 200,000 U.S. adults for over 30 years and distinguishing between healthy (plant-based, whole foods) and unhealthy (refined, animal-based) dietary patterns.

Property	Value
Study Year	2026
Organization	Harvard T.H. Chan School of Public Health
Journal Name	Journal of the American College of Cardiology
Authors	Zhiyuan Wu
Population	U.S. adults
Sample Size	198,473 participants
Methods	Observational Study
Outcome	Coronary heart disease risk, metabolomic indices
Results	Healthy diets linked to lower CHD risk and better biomarkers.

Literature Review: Related Studies

To place the new findings in context, we searched the Consensus database of over 200 million research papers using the following queries:

1. low carb diet coronary heart disease
2. low fat diet health biomarkers
3. dietary patterns cardiovascular risk comparison

Summary Table of Key Topics and Findings

Topic	Key Findings
How does diet quality influence the cardiovascular effects of low-carb and low-fat diets?	- Plant-based, high-quality diets (regardless of macronutrient emphasis) are associated with lower CHD and CVD risk 1 12 13 14. - Diets high in animal fat, refined carbs, or low in overall quality show less benefit or increased risk 1 3 14.
Do low-carb and low-fat diets differ in their impact on cardiovascular risk factors?	- Both low-carb and low-fat diets can improve risk factors such as weight, blood pressure, and lipid profiles in the short-term, but effects on LDL and HDL cholesterol may differ 2 5 11. - Most benefits on cardiovascular risk factors diminish over longer follow-up 11.
What is the role of specific dietary patterns (Mediterranean, DASH, vegetarian, etc.)?	- Mediterranean, DASH, and vegetarian diets are consistently linked to lower cardiovascular risk and improved biomarkers 7 12 14 15. - Multiple healthy dietary patterns (not just macronutrient-focused) can support heart health when built on whole foods and healthy fats 12 13 14.
Are the long-term health outcomes of low-carb and low-fat diets consistent across studies?	- Some observational studies suggest potential long-term harms from low-carb diets if diet quality is low, but no increased CVD risk when vegetable fats/proteins are emphasized 1 3. - The quality of food sources is a key modifier of long-term risk, more so than macronutrient ratio alone 1 3 12 13 14.

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How does diet quality influence the cardiovascular effects of low-carb and low-fat diets?

The new Harvard study emphasizes that the health effects of low-carb and low-fat diets depend heavily on the quality of foods chosen, a conclusion echoed in multiple previous studies. Both observational and interventional research finds that diets rich in plant-based foods, whole grains, and unsaturated fats reduce cardiovascular risk, regardless of whether they are low in carbohydrates or fat. Conversely, diets high in processed foods, refined carbohydrates, and animal fats are less likely to provide cardiovascular benefits and may increase risk.

• Plant-based, high-quality versions of low-carb or low-fat diets are associated with reduced CHD risk 1 12 13 14.
• Diets high in refined grains and animal fats, even if low in carbs or fat, can increase CHD risk 1 3 14.
• Multiple studies highlight that the focus should shift from macronutrient percentages to overall dietary quality 12 13 14.
• The new study builds on this evidence with large-scale, long-term data and metabolomic analyses supporting the importance of food quality.

Do low-carb and low-fat diets differ in their impact on cardiovascular risk factors?

Short-term trials and meta-analyses have demonstrated that both low-carb and low-fat diets can yield improvements in weight, blood pressure, triglycerides, and some cholesterol measures. However, low-carb diets may increase LDL cholesterol, while low-fat diets are more likely to lower total and LDL cholesterol but may slightly reduce HDL and increase triglycerides. Over longer periods, the magnitude of these benefits diminishes, especially for weight and cardiovascular risk factors.

• Low-carb diets result in greater short-term weight loss and higher HDL cholesterol but may increase LDL cholesterol 2 5 11.
• Low-fat diets reduce total and LDL cholesterol but may lower HDL and increase triglycerides 10 11.
• Most improvements in cardiovascular risk factors with both diets are modest and decrease over 12 months 11.
• The new study’s finding that both diet types can protect heart health—when focusing on quality—aligns with this nuanced evidence.

What is the role of specific dietary patterns (Mediterranean, DASH, vegetarian, etc.)?

A large body of research supports the cardiovascular benefits of established dietary patterns like the Mediterranean, DASH, and vegetarian diets, which emphasize vegetables, whole grains, nuts, and healthy fats. These patterns are repeatedly associated with lower CHD and CVD risk, improved lipid profiles, and reduced inflammation. Importantly, these diets prioritize overall food quality rather than specific macronutrient targets.

• Mediterranean, DASH, Portfolio, Nordic, and vegetarian diets have all been shown to reduce CVD and CHD risk 7 12 14 15.
• High-quality dietary patterns are more important than single nutrients or macronutrient focus alone 12 13 14.
• The current study’s results are consistent with literature highlighting the benefits of diverse, high-quality eating patterns for heart health.
• Multiple healthy eating patterns can be adapted to individual preferences while supporting cardiovascular health 12 13.

Are the long-term health outcomes of low-carb and low-fat diets consistent across studies?

Long-term observational studies suggest that low-carb diets, when based on animal products and poor-quality foods, may be associated with increased all-cause mortality and potentially higher cardiovascular risk. However, when low-carb diets rely on vegetable fats and proteins, risk may be reduced. The evidence underscores that food quality is a critical factor in determining the long-term health effects of both low-carb and low-fat diets.

• Low-carb diets high in animal fat/protein are linked to higher all-cause mortality 3; plant-based versions lower risk 1.
• No significant association between low-carb diets and CVD risk when diet quality is high 1 3.
• The new study extends these findings, showing that healthy versions of both diet types are cardioprotective, while unhealthy versions are not.
• Quality of food sources outweighs the impact of macronutrient ratio on long-term cardiovascular risk 1 3 12 13 14.

Future Research Questions

While this study provides robust evidence that food quality within low-carb and low-fat diets is crucial for heart health, several important questions remain. Future research should clarify the effects of more extreme dietary patterns, explore mechanisms linking diet quality to metabolic health, and examine diverse populations and longer-term clinical outcomes.

Research Question	Relevance
Do very-low-carbohydrate or ketogenic diets with high food quality affect long-term cardiovascular risk?	The new study did not include extreme low-carb diets; understanding if high-quality ketogenic diets are equally protective is important for dietary recommendations 2 3 5.
What specific metabolic pathways mediate the heart health effects of diet quality?	Metabolomic analyses suggest shared pathways, but detailed mechanisms remain unclear and could inform targeted interventions 9 13.
How do low-carb and low-fat diets with high quality foods impact diverse populations outside health professionals?	The current cohort was mostly health professionals; broader studies could assess generalizability across socioeconomic, ethnic, and age groups 12 13 14.
Does diet quality modify the effect of low-fat or low-carb diets on clinical cardiovascular events?	While biomarker and CHD risk data are supportive, randomized trials with hard clinical endpoints are needed to confirm these relationships 4 11 14 15.
Which components of diet quality are most critical for cardiovascular protection in different dietary patterns?	Identifying the most beneficial foods or nutrients could refine dietary guidelines and personalize prevention strategies 9 12 14.