A study that tracked 5,248,916 person-years of follow-up has confirmed what the best diet for heart health actually looks like — and the answer may upend decades of nutrition debate [1].
The nutrition conversation has seen repeated flip-flops. There was the fat-free 90s. Then the Atkins explosion of the early 2000s. Now there are keto influencers and high-protein cheese puffs on grocery shelves. Decades of nutrition advice and billions in revenue have been raked in by a food industry only too eager to cash in on these trends — and people are still confused.
Which diet is best when it comes to heart health? Should the focus be on low fat? Or low carbs?
New research suggests this whole debate may have been a distraction.
Table of Contents
The Back Story
The conversation about diet and heart health began decades ago in the 1950s. A dramatic shift was happening, and suddenly, everyone was talking about heart disease.
In England, two Oxford researchers carefully examined official records and documented a striking increase in deaths from heart attacks and strokes. They were killing adults in 1945 at a rate 15 times greater than in 1921 [2].
The same pattern was playing out in the U.S. Deaths from heart disease went from rare in 1900 to the leading cause of death by midcentury [3].
There was an urgent need to understand what was going on. One theory that emerged identified diet as a key culprit. Specifically, it was suspected that eating more saturated fat produced higher LDL cholesterol levels in the blood. And this high LDL cholesterol was the essential ingredient for plaque accumulation within blood vessel walls.
Researcher Ancel Keys put the theory on the map through a groundbreaking research project. He set out to find groups of people living in different places who had very different dietary patterns. In particular, he was interested in how much fat they ate. He ended up assembling a massive dataset of 16 cohorts of healthy men from select populations across 7 countries, including several in Europe, plus the U.S. and Japan. It was called the Seven Countries Study [4].
The first findings dropped in 1970. Heart disease was linked to blood cholesterol levels. And those in turn were linked to saturated fat in the diet [5].
Numerous studies continued to examine the data over the years as the cohorts aged. The relationship held. A 25-year follow-up found the same strong link between saturated fat intake and heart disease [6].
Ancel Keys's research pointed to saturated fat — not just any fat — as the problematic element in the diet. But that nuance was easily lost. Guidelines often recommended slashing fat intake of any kind. A statement from the American Heart Association in 1986, for example, pushed for a reduction of saturated fat and total fat, and recommended replacing those fat calories with complex carbs [7].
When the USDA released the flawed food pyramid in 1992, it proclaimed the same message. It guided consumers to minimize fats and graphically represented refined carbs as the foundation for a healthy diet [8].
This picture dominated for decades. The way to beat heart disease was to eat low fat.
But in the background, other researchers were quietly exploring an alternative story. At the same time Ancel Keys was launching his Seven Countries Study, a British researcher named John Yudkin was poring over statistics related to heart disease deaths.
Just like Keys, he spotted interesting links between the rates of heart disease and dietary patterns. But the thing that jumped out from the data for him wasn't fat or even saturated fat. It was sugar [9].
Yudkin's focus on carbs instead of fat as the driver of heart disease didn't gain much traction at the time.
But in the late 1980s, a landmark lecture began to change the conversation. It was delivered by Gerald Reaven, who was receiving the prestigious Banting Award for diabetes research. In it, he argued that insulin resistance was intimately linked to heart disease and related problems like high blood pressure [10].
And the driver of insulin resistance? In animal studies, it was a high-sugar diet [10].
By the year 2000, important observational evidence was coming in from human studies. A cohort of about 75,000 women had been followed for 10 years. Researchers examined the relationship between heart disease and glycemic load [11].
Glycemic load is a measure of how food impacts blood sugar. High-glycemic-load foods raise blood sugar more sharply. Foods that have a high glycemic load are carbohydrates — at the top of the list are things like white rice, white potatoes, white bread, and sugary drinks.
The results were stark. The diets with the highest glycemic loads were associated with double the risk of heart disease — even after taking into account other common risk factors [11].
As this research was happening in the background, a cardiologist named Robert Atkins was popularizing a new narrative inspired by it. His book Diet Revolution sold millions of copies. It painted carbs as the enemy when it came to heart health and weight loss, and advocated minimizing them in the diet. But fat consumption? No problem, according to Atkins [12].
So two competing pictures emerged, each gathering millions of adherents over the years. One says low fat is the key for heart health. The other says low carb.
This debate has actually suited the food industry nicely. They have been happy to play along with the trends focused on specific macronutrients. When low fat was the focus, they lined the shelves with low-fat products. The same pattern is visible today with low carb — and more recently, the high-protein craze. But this approach is missing something profoundly important.
It seems like it should be an easy matter to figure out which of these positions is correct. But as the actual data has come in, the picture has been messy. Results have been mixed.
For example, one massive randomized, controlled trial included nearly 50,000 participants with follow-up data over 8 years. The researchers were studying the impact of reducing total fat intake — total fat intake, not just saturated fat. The impact of reducing total fat intake turned out to be extremely modest. Heart disease risk didn't budge, though there was some improvement in risk factors [13].
On the other hand, a different trial put low-fat and low-carb diets head-to-head. Those on the low-carb diet lost more weight over a 6-month period, but there didn't appear to be any advantage in terms of heart disease risk metrics [14].
So there have been underwhelming results from both sides, despite other data supporting each position. This mixed research picture has prevented any clear winner from emerging. The debate has continued. And the food industry has cashed in on this confusion.
The Hard Question
This probably shouldn't be a surprise. Because there is a serious problem that has often muddied the waters when it comes to studies investigating the linkages between these diets and heart health.
The problem is this: "Low fat" and "low carb" are broad descriptions that can be applied to very different kinds of eating patterns. A low-fat diet, for instance, could mean anchoring meals around chickpeas and vegetables. On the other hand, a steady diet of white bread, white rice, and fat-free cookies is also low fat.
A low-carb meal could mean salmon, avocado, and spinach. Or it could be eggs fried in butter with a heaping side of bacon.
Health outcomes would be profoundly impacted by what precisely the labels "low fat" and "low carb" actually describe. It's not just about the mix of macronutrients. The quality of those macronutrients matters, too. Past studies on diet and heart health often have not paid careful attention to this distinction — but it's obviously crucial.
The Study
That brings us to the 2026 study published in the Journal of the American College of Cardiology — one specifically crafted to fill this gap in the literature by accounting for both macronutrient quantity and quality in the same analysis.
The study authors examined data for around 200,000 people from 3 separate cohorts. These cohorts were tracked for a long time — around 30 years each. So this is a massive dataset of 5,248,916 person-years of follow-up [1].
Crucially, they used food questionnaires to distinguish diets not just in terms of macronutrient quantity, but also macronutrient quality. They divided diets into healthy and unhealthy versions of both low-fat and low-carb diets. Healthy meant, for example, plant-based food vs. animal products and whole grains vs. refined grains [1].
So which dietary pattern won — low fat or low carb? Looking at the overall numbers and leaving aside diet quality for the moment: those with the lowest carb diets had a 5% elevated risk of heart disease compared to those with the highest carb intakes. Those with the lowest fat diets had a 7% risk reduction compared to the highest fat intakes. Low fat is the winner here, but the effects are relatively modest [1].
But when diet quality is taken into account, the picture changes radically. Those who stuck closest to a healthy version of a low-carb diet had a 15% lower risk of heart disease compared to those farthest from this eating pattern. A high-quality low-fat diet showed nearly the same benefit, cutting risk by 13% [1].
And the unhealthy versions of both diets raised risks to a highly similar degree.
What's more, the healthy versions of both diets were linked to lower triglycerides, less inflammation, and other markers of better metabolic health [1].
A few important caveats are worth noting. First, this was an observational study — one that followed large populations over time rather than randomly assigning participants to different eating patterns — so it is not able to establish causation with confidence. It also did not examine more extreme dietary approaches like the keto diet, with exceptionally low carb intake. And it doesn't mean macronutrient content is never relevant — it just suggests that, for most people, it is unlikely to be the most important factor to concentrate on when evaluating diet quality.
Takeaways
The key lesson from this research: stop focusing on "low fat" vs. "low carb." Macronutrient quantity does not appear to be the primary driver of heart health outcomes. Instead, the evidence points to food quality as the factor that consistently separates the diets associated with better heart health from those that aren't. The research supports prioritizing plant-based proteins like chickpeas, lentils, and beans over animal sources, whole foods over processed options, and unsaturated fats over saturated or trans-fats.
Both low-carb and low-fat dietary patterns can support heart health — if food quality is right.
This finding doesn't come as a shock to health researchers. Scientists like Dariush Mozaffarian have argued for years that the focus should be on eating healthful foods and avoiding unhealthy ones, not on avoiding specified nutrients like fat or carbohydrates [15].
He provides a striking example in a 2016 article. The top chart shows the effect on heart disease outcomes when a single macronutrient — lower fat — was the target. There is basically no impact. The bottom chart shows a large study of the Mediterranean diet, which emphasized increasing certain healthy foods. Here, the impact was substantial [15].
The food industry has been happy to supply what the trends focusing on macronutrients demand. When low fat was the focus, shelves filled with low-fat products. The same is visible today with low carb, and now with the high-protein craze.
Yet a central lesson of this research is that labels like "low fat" or "low carb" don't signal healthy food. Junk food is still junk food, whether it's low fat, low carb, or high protein. For anyone serious about heart health, the evidence points away from packaged, processed foods — no matter what trend they're tapping into with their labels.
Returning to fat for a moment: one aspect of this study that may be controversial is the placement of animal-based food sources in the "unhealthy" category. This reflects a common understanding that animal products contribute to heart disease risk because of the saturated fat they contain. But the evidence here is genuinely debated, and the latest data may be worth examining further.
References
1. https://www.jacc.org/doi/10.1016/j.jacc.2025.12.038
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC503641/
3. https://pubmed.ncbi.nlm.nih.gov/24811552/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC9794145/
5. https://www.cabidigitallibrary.org/doi/full/10.5555/19711403775
6. https://pubmed.ncbi.nlm.nih.gov/7644455/
7. https://pubmed.ncbi.nlm.nih.gov/3779925/
8. https://www.britannica.com/science/food-pyramid
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC11257042/
11. https://pubmed.ncbi.nlm.nih.gov/10837285/
12. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)16986-9/abstract
13. https://pubmed.ncbi.nlm.nih.gov/16467234/
