In the Low-Carb vs Low-Fat Debate,
Study Suggests Food Quality Matters
More for Heart Health

This section is compiled by Frank M. Painter, D.C.
Send all comments or additions to:    Frankp@chiro.org

FROM:   JAMA 2026 (Feb 27) [EPUB] ~ FULL TEXT

OPEN ACCESS

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Samantha Anderer

Staff Writer,
Medical News, JAMA.

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What’s healthier: a low-carbohydrate diet or a low-fat diet?

Both have been touted for their supposed benefits, but a new study adds to evidence that focusing on macronutrients — carbs, fats, and proteins — may not be the best approach for improving cardiovascular health. The findings, published in the Journal of the American College of Cardiology, suggest that diet quality matters more than composition when it comes to reducing coronary heart disease risk.

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The Backstory

In recent decades, the pendulum of popular diets has swung from low fat to low carb, and debate continues about which approach is better for weight loss and overall health.

Low-fat diets arose first, based on the idea that eating less fat — which has more calories per gram than carbohydrates and protein — would lower caloric intake and prevent obesity. Additionally, studies correlating high-fat diets with high blood cholesterol levels, dating back to the mid-20th century, suggested that low-fat diets might help prevent heart disease. By the 1990s, fat-free foods were a regular part of US diets, and the food pyramid was advising all fats to be used “sparingly.” More recently, nutrition guidelines have recognized the nuanced role of fats, differentiating between types and promoting moderate intake from healthy sources.

Meanwhile, cardiologist Robert Atkins, MD, published a book in 1972 promoting a low-carbohydrate weight-loss strategy. The idea was that restricting carb intake would force the body to burn fat for fuel. Known as the Atkins diet, the nutrition plan peaked in the early 2000s as studies showed it could reduce weight and improve cardiovascular risk factors. Although the popularity of the Atkins diet eventually waned, newer iterations are built on similar low-carb principles.

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Why This Matters

Although research has shown that the quality of the diet is important for cardiovascular health, it’s less clear whether this is more essential than the ratio of carbs, fat, and protein a person eats, according to the new study’s authors. They noted that past studies of low-fat and low-carb diets for reducing coronary heart disease risk have had mixed findings, perhaps because both high- and low-quality foods can make up either of these dietary patterns. Plus, few studies have explored the effects of these dietary patterns over long periods. With this study, the researchers aimed to clarify how low-carb and low-fat intakes within healthy and unhealthy diets may influence heart health over decades and gain insight into the underlying biological mechanisms.

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The Methods

The researchers examined data from around 200,000 people in 3 cohort studies with more than 30 years of follow-up: the Nurses’ Health Study, the Nurses’ Health Study II, and the Health Professionals Follow-Up Study.

After enrollment, participants received mail questionnaires every 2 to 4 years that inquired about diet, lifestyle factors, medication use, and chronic disease diagnoses.

Based on responses to food frequency questionnaires, the researchers indexed “healthy” and “unhealthy” low-carb and low-fat diets. They defined unhealthy diets as those favoring animal proteins and fats, potatoes, refined grains, and added sugars. Healthy diets were defined as those inclined toward vegetable proteins and fats, nonstarchy vegetables, whole fruits, legumes, and whole grains.

The main analysis assessed how low-carb intake, low-fat intake, and food quality were associated with incident coronary heart disease, which included self-reported coronary evascularization, nonfatal myocardial infarction, and coronary heart disease death. The researchers used health records to confirm heart attacks and used family notifications, postal authorities, and national death records to confirm deaths.

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The Findings

About 20,000 cases of coronary heart disease occurred over the follow-up period. The healthy versions of both low-carb and low-fat diets were associated with similarly lower relative risks of coronary heart disease, whereas the unhealthy versions were linked with similarly higher risks. Healthy diets were also linked with improved lipid and inflammatory biomarkers, including lower triglyceride, higher high-density lipoprotein cholesterol, and lower high-sensitivity C-reactive protein levels.

An analysis of metabolites in plasma from a subset of participants supported the findings. Whether eating patterns were low carb or low fat, diets with healthy macronutrient sources were associated with favorable metabolomic profiles, such as higher levels of indolepropionic acid and lower levels of valine, whereas the opposite was true for unhealthy versions.

“We combine the dietary data, metabolomic data, as well as clinical risk biomarker data and disease outcome data in this study, which allowed us to directly link what people eat and how the human body responds to this diet and the risk of developing coronary heart disease,” said Zhiyuan Wu, PhD, a postdoctoral research fellow at Harvard T.H. Chan School of Public Health and lead author of the study.

“I think the most important finding is that both the healthy low-carb and low-fat diets are associated with similarly reduced risk of heart disease, even when they emphasize different composition of macronutrients,” Wu continued. “Our findings suggest the diet quality and food sources matters more than simply cutting fat or carbs.”

Dariush Mozaffarian, MD, DrPH, a cardiologist and director of the Food is Medicine Institute at Tufts University who was not involved in the study, said that “by itself, this study doesn’t prove cause and effect, but it’s consistent with a wealth of other literature and evidence over the last 20 years that we need to be shifting from a nutrient focus to a food focus.”

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The Bottom Line

“Promoting an overall healthy eating pattern rather than strict macronutrients restriction should be a central strategy for the primary prevention of heart disease,” Wu said. “I think focusing on that pattern may be easier and offer flexibility for individuals to choose eating patterns that align with their preference and also their cultures while still supporting heart health.”

Moreover, Qi Sun, DSc, MD, senior author of the study and an associate professor in the departments of nutrition and epidemiology at Harvard, said that the healthy foods the study focused on “are the same foods emphasized by many other healthy dietary patterns: the Mediterranean diet, the DASH diet, the healthy plant-based eating index,” among others.

The authors cautioned that the study population was made up of health professionals, so the results might not be widely generalizable. They also noted that their research didn’t assess more extreme dietary patterns, such as the ketogenic diet, where carbohydrates typically make up about 5% of total energy intake. Mozaffarian added that focusing on macronutrients may still be worthwhile for certain conditions or for individuals working on building muscle mass. But for most people, it may make more sense to concentrate on a well-rounded diet of whole foods.

“This study confirms what we’ve learned over the last decade or so, that a healthy diet can’t be defined by a macronutrient formula,” Mozaffarian said. “We obsessed over low-fat diets and then we obsessed over low-carb diets, and now people are obsessing over high-protein diets,” he added, alluding to grocery store shelves lined with high-protein cheese puffs and pastries, the latest ploy to rebrand highly processed foods as healthy.

“I think doctors need to understand that it’s not about the macros,” he said. “It’s about the food choices.”

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Conflict of Interest Disclosures:

Dr Mozaffarian reported receiving research funding from the National Institutes of Health, The Rockefeller Foundation, National Association of Chain Drug Stores Foundation, Kaiser Permanente Fund at East Bay Community Foundation, and Google Health; serving as scientific advisor for Branch Venture Partners, Elysium Health, Instacart Health, January Inc., Nourish, and WndrHLTH; providing scientific consulting for Amazon Health and Google Health; holding equity in HumanCo; and receiving chapter royalties from UpToDate. Dr Wu reported receiving support from the American Diabetes Association. No other disclosures were reported.

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