A new study that tracked 5,248,916 person-years of follow-up has confirmed the best diet for your health [1].
This is such a relief after all the flip-flops.
We had the fat-free 90s. Then the Atkins explosion of the early 2000s. Now there's keto influencers and high-protein cheese puffs on grocery shelves. There have been decades of nutrition advice, billions in revenue raked in by a food industry who are 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 we aim for low fat? Or target low carbs?
And what if this whole debate has 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 shocking 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 data set 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 saw the same strong link between saturated fat intake and heart disease [6].
Ancel Key's research pointed to saturated fat — not just any fat — as the problematic element in the diet. But that nuance was easily lost. And 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 it recommended that we replace 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. It also 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 famed 7 Countries Study, a British researcher named John Yudkin was poring over the 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 looked at 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 going to be 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. And this was 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 we ended up with two competing pictures, each of which has gathered millions of adherents over the years. One says low fat is the key for heart health. The other says low carb.
And this debate has actually suited the food industry nicely. They've 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. We can see the same today with low carb. And more recently, the high-protein craze. But this approach is missing something profoundly important, as we'll see in a moment.
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. That's important. 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 study results, despite other data that has supported each of the two positions. 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 a person is anchoring their meals around things like 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.
We would expect the kind of health outcomes we'll see would be profoundly impacted by what precisely we're talking about when we use the labels "low fat" and "low carb." 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
And that brings us to the new study. Because it was specifically crafted to fill this gap in the literature.
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 [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? If we look at the overall numbers, leaving aside diet quality for the moment, here's what we see. 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 we consider diet quality, 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].
Before looking at how all this applies to us, let me just mention some quick caveats. First, this was an observational study. So it isn't able to tell us with confidence about causation.
It also didn't look at more extreme trendy diets like the keto diet, with exceptionally low carb intake.
Finally, it doesn't mean it never makes sense to think about the macronutrient content of our diet. It just suggests that, for most, this is unlikely to be the most helpful thing to concentrate on.
Takeaways
So what's the big lesson here? We need to stop focusing on "low fat" vs. "low carb." Macro quantity doesn't appear to be the key factor when it comes to heart health. Instead, we need to focus on food quality. Prioritize plant-based proteins like chickpeas, lentils, and beans, over animal sources, whole foods over processed. And unsaturated fats over saturated or trans-fats.
Both low-carb and low-fat dietary patterns can work — if we get the quality right.
This finding doesn't come as a shock to health researchers. Scientists like Dariush Mozaffarian have argued for years that we need to focus on eating healthful foods and avoiding unhealthy ones, not on avoiding specified nutrients like fat or carbohydrates [15].
He provides a striking example in an article he published in 2016. The top chart shows the effect on heart disease outcomes when a single macronutrient — lower fat — was the target. There's 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].
As I mentioned above, the food industry has been happy to supply what the trends focusing on macronutrients demand.
Yet a lesson of this new study is that labels like "low fat" or "low carb" aren't the thing to pay attention to. They don't signal healthy food. Junk food is still junk food, whether it's low fat, low carb, or high protein. If we're serious about health, we need to steer away from packaged, processed foods — no matter what trend they're tapping into with their labels.
Returning to fat for a moment, there's one aspect of this study that's likely to be controversial: The fact that they place animal-based food sources in the "unhealthy" category. This reflects a common understanding that animal products are a problem for heart health because of the saturated fat they contain. But you'll find plenty of people online who claim this is just a myth. So what's the true story when it comes to saturated fat and heart disease? The latest data may surprise you.
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/
