Low Carb or Low Fat? Perhaps the Question Was Slightly Off-Target

When people hear "heart-healthy diet," many immediately think of reducing something. Cutting carbs. Cutting fats. Cutting calories. Avoiding white rice and bread. Steering clear of oils. Giving up meat. Conversely, some believe that carbs are the real culprit and fats need not be feared.

For decades, discussions about diet and health have often been framed as a dichotomy of "low carb or low fat." In the diet market, straightforward labels like low-carb, ketogenic, low-fat, and calorie-restricted have captured people's attention. However, a large-scale long-term study in the U.S. has revealed a more understated yet far more crucial conclusion for real life.

The real question to protect the heart is not "Did you cut carbs?" or "Did you cut fats?"
It's "What are you eating, and how are you structuring your meals?"

The study featured by ScienceAlert followed about 200,000 men and women in the U.S. for nearly 30 years. It analyzed participants' dietary patterns to examine how low-carb and low-fat diets relate to coronary artery disease risk. The results showed that both low-carb and low-fat diets composed of healthy foods were associated with a reduced risk of heart disease. In contrast, diets high in refined carbohydrates, animal fats, and animal proteins were not necessarily heart-healthy, even if labeled "low-carb" or "low-fat."

In other words, being low-carb itself wasn't the winner, nor was being low-fat the correct answer. The winner was "food quality."

There Are Good Low-Carb and Bad Low-Carb Diets

When people think of low-carb diets, they often imagine meals that reduce bread, rice, noodles, and sugar. However, what you increase in place of carbs can significantly change the content of the diet.

For example, a low-carb diet focusing on vegetables, legumes, nuts, seeds, fish, olive oil, and avocados while reducing refined carbs and processed foods can be relatively high-quality. On the other hand, if reducing carbs leads to a diet heavy in processed meats, fatty meats, butter, cheese, fried foods, and processed snacks, then the same "low-carb" label can mean something entirely different.

The study found that healthy low-carb diets centered around plant-based foods and high-quality foods were associated with a reduced risk of coronary artery disease. In contrast, unhealthy low-carb diets focused on animal-based foods and refined foods were linked to increased risk.

This is an important message for those practicing carb restriction. The issue is not merely reducing carbs but what fills the plate afterward. Do you increase processed meats instead of white rice? Or do you increase beans and vegetables instead of pastries? The difference could be more significant than just calories or carb content.

"Low-Fat" Is Also Unhealthy If It Relies on Sugar and Refined Grains

The same applies to low-fat diets. There was a time when reducing fats was touted as a staple for heart disease prevention. However, focusing solely on reducing fats can lead to other pitfalls.

If reducing fats results in an increase in vegetables, fruits, whole grains, and legumes, the quality of the diet tends to improve. However, if low-fat claims lead to processed foods, sugary cereals, refined bread, sweetened beverages, and low-fat but high-sugar snacks, it can't be considered a healthy low-fat diet.

In this study, low-fat diets centered around high-quality carbohydrates were linked to positive outcomes, while low-fat diets high in refined carbohydrates and low-quality foods were associated with increased heart disease risk.

The important takeaway here is that it's not a simple matter of "fats are bad" or "carbs are bad." Carbohydrates found in whole grains and legumes are not the same as those in white bread and sugary treats. Fats found in nuts, olive oil, and fish are not the same as those in processed foods or excessive animal fats.

Even if the names of nutrients appear the same, the quality of the food, fiber content, micronutrients, degree of processing, and overall balance of the diet differ significantly.

Insights from Tracking Nearly 200,000 People Over 5 Million Person-Years

The strength of this study lies in its scale and duration. The subjects were participants in large U.S. cohorts, including the Health Professionals Follow-Up Study, Nurses’ Health Study, and Nurses’ Health Study II, totaling about 198,000 people. The follow-up period varied by study but generally spanned around 30 years.

Researchers evaluated participants' diets based on dietary questionnaires and classified low-carb and low-fat diets into several types. They considered not only the amounts of carbs or fats but also whether the diet was animal-based or plant-based, included high-quality carbohydrates like whole grains, or was high in refined carbohydrates.

They also examined blood metabolites, lipids, and inflammation markers to explore the biological links between dietary patterns and cardiovascular risk. Healthy low-carb and low-fat diets were associated with more favorable cardiovascular risk profiles, such as higher HDL cholesterol, known as good cholesterol, and lower triglycerides and inflammation markers.

Coronary artery disease is a condition where the heart's blood vessels narrow or become blocked, leading to serious events like heart attacks. The finding that long-term dietary quality is related to this risk suggests that daily dietary choices can impact the body's metabolic environment.

However, It's Not Proven as a Cause

There are caveats to this study. As an observational study, it cannot definitively claim that specific diets directly reduced heart disease. The dietary content was based on self-reports, which could be subject to memory errors or reporting biases.

Additionally, many participants were healthcare professionals, who might have higher health awareness and better access to healthcare than the general population. Therefore, caution is needed when applying the results to all countries or groups.

Moreover, the low-carb and low-fat diets examined in this study differ from extreme ketogenic or extreme fat-restricted diets. It should be viewed as a comparison of people with relatively lower carbs or fats within real dietary patterns. Thus, extreme interpretations like "keto is safe," "you can consume unlimited fats," or "carbs don't matter" should be avoided.

Nevertheless, the direction indicated by this study is quite practical. When considering a healthy diet, focus first on food quality. This is more accessible for many people than precisely calculating carb or fat ratios.

On Social Media, Reactions Highlight "It's Just Common Sense"

This study has sparked various reactions on social media. Posts from ScienceAlert, JACC-related posts, and science and nutrition communities on Reddit have seen responses focusing on the importance of food quality over the long-standing "low-carb or low-fat" debate.

In Reddit's r/science, comments echoed the sentiment that "macronutrients are containers, and food quality is the content." It's possible to fill a low-carb container with processed foods or a low-fat container with sugar. Hence, the takeaway is to look at the content, not the label.

Another comment noted, "In the end, it comes back to traditional eating habits, namely whole foods, balance, and moderation." While the study is seen as new, its conclusion aligns more with the simple principle of eating less processed foods than with extreme dieting methods.

In the r/ScientificNutrition community, which consists of users knowledgeable about nutrition, there was an appreciation for the study's summary that emphasized "what you actually eat is more important than the macronutrients themselves." There was also interest in the methodology, such as how low-carb and low-fat were defined, which is a crucial point since definitions and comparisons can vary across nutrition studies.

On X, accounts related to heart disease and cardiology shared posts emphasizing that "it's not about low-carb or low-fat, but about food quality." JACC-related communications also highlighted the message to focus on food quality beyond the low-carb and low-fat debate.

Naturally, there is also backlash on social media. Some low-carb advocates argue that "low-fat policies increased obesity" and insist that low-carb is ultimately better. Conversely, some call for heightened caution against animal and saturated fats. Dietary methods often tie closely to personal success stories, making the discussions prone to sectarianism.

This is why this study serves more as a tool to reassess the debate from a slightly external perspective rather than declaring a winner. Whether reducing carbs or fats, the common priority should be the overall quality of the diet.

Think "Replacement" Rather Than "Restriction"

To apply this study to daily life, it's more straightforward to think about "what to replace" rather than "what to prohibit."

If you eat white bread, replace it with whole grain bread, oatmeal, brown rice, or mixed grains. If you drink sweetened beverages, switch to water, tea, or unsweetened drinks. If you frequently eat processed meats, combine fish, tofu, legumes, chicken, and nuts. When consuming fats, choose olive oil, nuts, fish, and avocados over fried foods and processed products.

Even when reducing carbs, aim to increase vegetables, legumes, nuts, and fish. When reducing fats, avoid turning to sugar and refined grains, and focus on whole grains, fruits, vegetables, and legumes. Whichever path you choose, moving towards high-quality foods is likely beneficial for the heart.

This approach is also practical for sustaining a diet long-term. Some people find it easier to cut back on rice and bread, while others find it easier to avoid fatty foods. Cultural, familial, work, dining environments, and preferences vary. Instead of uniformly deciding on low-carb or low-fat, it's more feasible to improve food quality within a range that the individual can maintain.

Be Cautious of "Healthy-Looking Processed Foods"

Another question this study raises is not to be misled by food labels.

Low-carb, low-fat, high-protein, gluten-free, plant-based, calorie-off. These words may seem healthy at first glance. However, they don't necessarily indicate high food quality. Some low-carb foods are highly processed and high in saturated fats and salt. Some low-fat foods are high in sugar and additives. Even plant-based foods can be unhealthy if they are processed with refined oils, sugar, and salt.

The reaction on social media that "it ultimately comes back to whole foods" likely reflects the intuitive understanding of many people. Focus on vegetables, fruits, legumes, whole grains, nuts, fish, and fermented foods with low processing. While perfection isn't necessary, structuring the majority of your diet with such foods is more likely to result in a heart-friendly diet than merely chasing nutrient numbers.

Applying This to Japanese Diets

Although this study targeted Americans, there are many perspectives applicable to Japanese dietary habits.

For example, there's no need to completely vilify staple foods. Adjust the amount of white rice while incorporating brown rice, mixed grains, oatmeal, whole grain bread, and soba. Increase vegetables, seaweed, mushrooms, tofu, natto, and fish. Reduce the frequency of fried foods, processed meats, pastries, sweetened beverages, and snacks. Instead of just avoiding fats, choose quality ones like fish, nuts, and olive oil.

Japanese cuisine easily incorporates fish, soy products, seaweed, vegetables, and fermented foods, but it tends to be centered around white rice and can be high in salt. Therefore, it's important not to assume that "any Japanese food is healthy," but to reassess the amount of refined staples, salt, frequency of processed foods, and quality of vegetables and protein sources.

For those wanting to go low-carb, instead of just increasing meat in place of rice, combine fish, tofu, eggs, vegetables, mushrooms, seaweed, and nuts. For those opting for low-fat, don't just reduce oils but also avoid leaning towards sugar and white wheat products. In either style, the common point is to return the focus of the table to "less processed foods."

From Diet Debates to the Reality of the Table

The debate over low-carb or low-fat is easy to grasp. It's because you can pinpoint a single enemy. Carbs are bad, fats are bad, calories are bad, meat is bad, rice is bad. Such definitive statements spread more easily as information.

However, the body doesn't operate on a single nutrient alone. Diet is a combination of foods, a lifestyle, a culture, and an action that can be sustained. Merely isolating the ratios of carbs or fats doesn't determine whether a diet is heart-healthy.

What this large-scale study has shown is the need to look at the "content" rather than the "name" of the diet. Whether low-carb or low-fat, it can be structured healthily. Conversely, even if a diet carries either label, if it's composed of unhealthy foods, it doesn't necessarily protect the heart.

As reflected in social media reactions, this might be a statement so obvious it's almost anticlimactic. Yet, in a diet market that continually generates new labels, reaffirming this obvious truth holds value.

A heart-healthy diet is more about the accumulation of daily choices than extreme abstinence. It's not just about what to reduce, but what to increase. Not just about which nutrients to avoid, but what foods to fill the table with.

Low carb or low fat.
Before asking that question, consider first whether it's a "high-quality diet."
That might be the most practical first step to protecting the heart.

Source URL

ScienceAlert. An article introducing the importance of food quality over low-carb or low-fat.
https://www.sciencealert.com/giant-study-reveals-the-secret-to-heart-health-isnt-low-carb-or-low-fat

Research published in the Journal of the American College of Cardiology. Investigated the relationship between the quality of low-carb and low-fat diets, metabolic markers, and coronary artery disease risk.
https://www.jacc.org/doi/abs/10.1016/j.jacc.2025.12.038

Press release from the American College of Cardiology. Overview of the study, tracking of nearly 200,000 people over 30 years, and comments from authors and experts on the importance of food quality.
https://www.acc.org/about-acc/press-releases/2026/02/11/18/16/healthy-versions-of-low-carb-and-low-fat-diets-linked-to-better-cardiovascular-and-metabolic-health

EurekAlert / Harvard T.H. Chan School of Public Health release. Details on study subjects, main results, approximately 15% lower coronary artery disease risk, and the observational nature of the study.
https://www.eurekalert.org/news-releases/1115888
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