Is saturated fat a problem for our hearts, or not? It's a controversy people have been arguing over for decades. Voices on both sides of the question are easy to find.
The debate got a fresh jolt of energy recently when the HHS published new dietary guidelines. Though they keep the old guidance about limiting saturated fats to 10% of calories, there's a shift in the overall approach to fats.
The graphic and text make clear they are moving away from the previous guidelines' recommendations to avoid full-fat dairy and saturated fats in food like red meat.
Notice the prominent steak perched at the top left of this graphic from the new guidelines.
This article explains how we got here and what often lies behind the disagreement — and offers a nuanced picture of what the data really says.
Table of Contents
- Diet and Heart Disease
- The Saturated Fat Controversy
- Nuance, Risk, and the Bigger Picture
- References
Diet and Heart Disease
The basic question behind the controversy is this: How does diet relate to heart disease risk?
Back in the 1950s, researcher Ancel Keys felt confident he knew the answer. He was getting ready to gather the data needed to confirm his hypothesis in a groundbreaking study.
It was a time when the problem of heart disease was at the top of everyone's minds. It used to be a minor concern. That had changed in dramatic fashion.
In England, two Oxford researchers carefully examined official records and documented a shocking increase in deaths from heart disease driven by plaque buildup in the arteries — heart attacks and strokes. This condition was killing adults in 1945 at a rate 15 times greater than in 1921 [1].
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 [2].
When President Eisenhower had a heart attack in 1955, public attention became laser focused: What caused heart disease? And how could something be done about it? [3]
There was early intriguing evidence that elevated cholesterol in the blood was a key driver of plaque buildup in the arteries. And it appeared not all cholesterol was the same. Lower-density forms were the ones most closely linked to heart disease [4].
But how did diet connect?
Keys proposed that more saturated fat in the diet produced higher LDL cholesterol levels in the blood. It was a plausible idea, and there was some experimental evidence pointing in this direction. But more data was needed to confirm the link between diet, cholesterol, and heart disease.
Keys had an innovative approach. 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. His initial research turned up an intriguing clue: Cholesterol levels in the blood were closely associated with the share of total calories that came from fat [5].
But these observations came from a relatively small set of data, drawing on just a few places in Europe. He set his sights on a much larger project. 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 [6].
The first findings dropped in 1970. Keys' initial ideas were seemingly confirmed. Heart disease was linked to blood cholesterol levels. And those in turn were linked to saturated fat in the diet [7].
By 2004, according to one estimate, the Seven Countries Study had already been cited more than one million times.
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 [8].
This and other lines of evidence established what came to be called the diet-heart hypothesis, one of the most important and influential ideas in public health. The basic idea: Saturated fat intake raises LDL cholesterol. And that, in turn, increases heart disease risk.
The Saturated Fat Controversy
But this theory hasn't been without its critics.
The controversy was stoked anew in recent months with a new meta-analysis of studies on saturated fat and heart health outcomes. It looked at 9 trials with about 13,000 total participants. Researchers examined how reducing saturated fat intake might modify the risk for things like heart attacks and strokes. The meta-analysis found there were no statistically significant differences between those who lowered saturated fat in their diets and those who didn't [9].
Other meta-analyses say different things. An important review published by Cochrane in 2020 did find some benefit from saturated fat reduction — specifically, a 17% reduction in relative risk for combined cardiovascular events [10].
What's going on? Who should we believe?
The issue often comes down to this: Which studies get included in the analysis? Many of the studies on saturated fat and heart health are now fairly old. Look, for instance, at the list of included studies in the first meta-analysis. Many of them are from the 1960s, and the most recent one — from 2016 — is tiny [9].
There isn't a lot of new data. Instead, the differences between meta-analyses are usually a result of which of these older studies they include, and which they decide to leave out.
Zero in on that list and you'll see The Minnesota Coronary Survey. It's actually the largest study they draw on [9].
So it's an important piece of their analysis. Yet the Cochrane review doesn't include it. Some critics would claim the authors of analyses just include the studies that will give them the results they want.
But that's not the way Cochrane Reviews work. Authors lay out criteria for study inclusion that act like a filter, including some studies while excluding others. These criteria aren't arbitrary; they're built on common understandings of what makes for good quality data. And those rules apply to all Cochrane reviews, not just this one on saturated fat.
In the case of the Minnesota Coronary Survey, Cochrane doesn't include it for a very sensible reason: they focused only on studies that lasted at least 2 years [10]. The logic: it takes time for the effects of dietary interventions to show up.
But the average time participants in the Minnesota Coronary Survey spent on the intervention diet was just over a year [11].
Even if it had lasted 2 years, the study has other serious methodological problems. The bulk of participants were young, with the largest group being under 30. No benefit from lowering LDL cholesterol would be expected at this age — particularly in such a short timeframe [11].
And then the lower-saturated-fat diet had a fatal flaw: it included margarine. The margarines of the time contained plenty of trans fats — which are even worse for the heart than saturated fats [11].
Plus, participants came and went from the hospital and were only on the intervention diet while institutionalized [11].
Garbage in, garbage out. Other meta-analyses that include the Minnesota study will not show a link between saturated fat intake and cardiovascular disease.
Cochrane Reviews, which only include high-quality data, show the link clearly.
What does the observational evidence show when we step away from randomized controlled trials?
One major study included more than 120,000 people in two large cohorts, followed for around 30 years [12].
Replacing saturated fats with unsaturated fats was associated with a significantly lower risk of death. Specifically, replacing 5% of energy from saturated fats with polyunsaturated fats was associated with a 27% reduction in total mortality, and with monounsaturated fats, a 13% reduction [12].
A key piece of evidence ties it together: there is good evidence that saturated fat raises LDL cholesterol levels, which is a critical risk factor for heart disease.
There's a consistent pattern across over 200 studies: higher LDL-cholesterol in the blood correlates with a higher rate of heart disease [13].
The link is so strong that the study authors concluded the evidence shows clearly that LDL causes heart disease [13].
But what about otherwise-healthy individuals?
A common claim online is that people at a healthy weight, without diabetes, insulin resistance, or blood pressure problems, don't need to worry about blood cholesterol levels.
The PESA study addresses this directly. It found that arterial blockages can develop when LDL-C is above 50–60 mg/dL, even when all other risk factors such as insulin resistance are absent [14].
"These data reinforce the idea that desirable LDL-C concentrations are probably much lower than those currently recommended, and suggest that atherosclerosis in both men and women develops above an LDL-C threshold concentration of approximately 50–60 mg/dL."
Nuance, Risk, and the Bigger Picture
The link between saturated fat and heart disease is strongly supported by several types of evidence.
There is, however, important nuance. The impact of reducing saturated fat may be more pronounced in some populations than others. A study published at the end of last year found that, for individuals at low risk, reducing saturated fat intake had little benefit over 5 years. But for those at high risk, cutting intake yielded important reductions in the risk of death and heart attacks [15].
Reducing saturated fat intake is not a magic key. It's one element of an overall strategy to reduce heart disease risk.
Looking again at the Cochrane review, many areas showed no benefit:
"We found little or no effect of reducing saturated fat on all‐cause mortality (RR 0.96; 95% CI 0.90 to 1.03; 11 trials, 55,858 participants) or cardiovascular mortality
(RR 0.95; 95% CI 0.80 to 1.12, 10 trials, 53,421 participants)… There was little or no effect of reducing saturated fats on non‐fatal myocardial infarction (RR 0.97, 95% CI 0.87 to 1.07) or CHD mortality (RR 0.97, 95% CI 0.82 to 1.16), but effects on stroke and CHD events were unclear."
And the benefit that appeared — that 17% reduction in cardiovascular events — was modest [10].
Saturated fat intake isn't the only factor to pay attention to, nor is it the most important. The evidence points to a broader set of modifiable factors:
- Boost fiber intake (if no IBS or IBD)
- Get plenty of exercise
- Avoid alcohol and smoking
- Keep blood pressure under control
- Maintain a healthy weight
- Manage stress
The greatest benefit in terms of heart health comes from addressing all these areas together, rather than focusing on any single one.
References
1. https://pmc.ncbi.nlm.nih.gov/articles/PMC503641
2. https://pubmed.ncbi.nlm.nih.gov/24811552
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC9794145
4. https://www.ahajournals.org/doi/epdf/10.1161/01.CIR.5.1.119
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC441692
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC9794145
7. https://www.cabidigitallibrary.org/doi/full/10.5555/19711403775
8. https://pubmed.ncbi.nlm.nih.gov/7644455
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC12095860
10. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD011737.pub3/full
11. https://www.ahajournals.org/doi/epdf/10.1161/01.ATV.9.1.129
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC5123772
13. https://pubmed.ncbi.nlm.nih.gov/28444290
14. https://www.sciencedirect.com/science/article/pii/S0735109721051159
