Saturated fat and heart disease: what the trials show, why studies disagree and what guidelines say

For half a century the advice was simple: saturated fat clogs arteries, so eat less butter and fatty meat. Then, over the past fifteen years, a series of large analyses reported that people who ate the most saturated fat had no more heart disease than people who ate the least, and some headlines declared the old advice wrong.

Both the old advice and the new headlines draw on real studies, and the studies are less contradictory than they appear. The strongest evidence comes from randomized trials, summarized in a Cochrane review led by Lee Hooper at the University of East Anglia. It finds a modest benefit from cutting saturated fat. Whether a person sees that benefit seems to depend on what they eat in its place, which is also why the simpler studies found nothing.

What saturated fat is and where it is found

Fats are built from chains of carbon atoms. In saturated fats the chains carry as much hydrogen as they can hold, which makes them straight, tightly packed and usually solid at room temperature. The World Health Organization lists the main sources: saturated fatty acids can be found in fatty meat, dairy foods, and hard fats and oils such as butter, ghee, lard, palm oil and coconut oil.

The reason for concern is cholesterol. Eating more saturated fat raises the level in the blood of low-density lipoprotein, or LDL, the particle that carries cholesterol into artery walls. The Cochrane review opens from that point, stating that reducing saturated fat reduces serum cholesterol, and asks the question that matters to patients: whether it also reduces heart attacks, strokes and deaths.

What guidelines say about saturated fat

The WHO updated its guidance in 2023 and kept the limit it had before. Fat eaten by everyone aged two and over, it says, should be mainly unsaturated, with no more than 10% of total energy intake coming from saturated fatty acids. For someone eating 2,000 calories a day, that is about 22 grams.

The guideline also says what to eat instead. Saturated fat can be replaced, the WHO states, with other nutrients such as polyunsaturated fatty acids, monounsaturated fatty acids from plant sources, or carbohydrates from fiber-rich foods such as whole grains, vegetables, fruit and pulses.

The American Heart Association took a firmer line in a 2017 advisory. Its authors wrote: “we conclude strongly that lowering intake of saturated fat and replacing it with unsaturated fats, especially polyunsaturated fats, will lower the incidence of CVD”, meaning cardiovascular disease.

What the trials show: fewer cardiovascular events, no clear effect on deaths

The Cochrane team accepted only trials that randomly assigned adults to eat less saturated fat or to carry on as usual, that lasted at least two years, and that counted deaths or cardiovascular illness. They found 15 trials that qualified (16 comparisons, 56,675 participants). Some trials supplied all of the participants’ food and others relied on dietary advice.

Reducing dietary saturated fat reduced the risk of combined cardiovascular events by 17%. A combined event means any of a list that includes heart attack, stroke and cardiovascular death. About 8% of participants had one during the trials. The reviewers translated the result into a number: among people without existing heart disease, 56 would need to cut their saturated fat for about four years for one of them to avoid an event.

Two details strengthen the finding. Trials that achieved bigger cuts in saturated fat, and bigger falls in blood cholesterol, saw bigger reductions in events, which is the pattern expected if the fat is the cause. And the reviewers found no evidence of harmful effects of reducing saturated fat intakes.

The limits are just as clear. The review found little or no effect of reducing saturated fat on deaths from all causes or on deaths from cardiovascular disease. It found little or no effect on non-fatal heart attacks when those were counted separately. The benefit showed up only when all cardiovascular events were pooled, and the trials differed from one another more than chance would explain.

The reviewers judged the evidence for the main result to be of moderate quality and concluded that reducing saturated fat intake for at least two years causes a potentially important reduction in combined cardiovascular events.

Why cohort studies found no link between saturated fat and heart disease

The contrary headlines came from a different kind of study. Cohort studies ask large numbers of people what they eat and follow them for years.

In 2010 Patty Siri-Tarino and Ronald Krauss at Children’s Hospital Oakland Research Institute pooled 21 such studies. During 5-23 y of follow-up of 347,747 subjects, 11,006 developed CHD or stroke, CHD being coronary heart disease. Those who ate the most saturated fat had no more cardiovascular disease than those who ate the least. A second pooling in The BMJ in 2015, led by Russell de Souza at McMaster University, agreed: saturated fat intake was not associated with all cause mortality, cardiovascular death, coronary disease, stroke or diabetes. The same analysis found that trans fats, the industrially hardened oils then common in baked and fried foods, were associated with more deaths and more heart disease.

A still larger study went further. The PURE study recorded the diets of more than 135,000 people in 18 countries and reported in The Lancet in 2017 that total fat and types of fat were not associated with cardiovascular disease, myocardial infarction, or cardiovascular disease mortality, whereas saturated fat had an inverse association with stroke. Myocardial infarction is the medical term for a heart attack. Its authors called for dietary guidelines to be reconsidered.

How can trials show a benefit from cutting saturated fat while cohorts show no harm from eating it? The likeliest answer is that nobody simply removes saturated fat from a diet. Calories that leave are replaced by something, and a study that compares high with low intake is silently comparing saturated fat with whatever the low group ate instead. In many diets, that is white bread, sugar and other refined carbohydrates.

Siri-Tarino’s team anticipated the point, writing that more data were needed on whether risks are influenced by the specific nutrients used to replace saturated fat. Researchers at Harvard then tested it. Yanping Li and colleagues followed 84,628 women and 42,908 men for up to three decades and modeled each kind of swap. Exchanging saturated fat for polyunsaturated fat or for whole grains went with less coronary heart disease. Exchanging it for refined starches and sugars showed no such benefit. Their conclusion was that unsaturated fats, especially PUFAs, and/or high-quality carbohydrates can be used to replace saturated fats to reduce CHD risk, PUFAs being polyunsaturated fats.

The American Heart Association read the trial record the same way. Trials that replaced saturated fat with polyunsaturated vegetable oil cut cardiovascular disease by about 30%, its advisory noted, while replacement of saturated fat with mostly refined carbohydrates and sugars is not associated with lower rates of CVD.

Does the food matter more than the saturated fat in it?

A group of researchers argues that the nutrient is the wrong unit altogether. In 2020 Arne Astrup of the University of Copenhagen and eleven co-authors wrote in the Journal of the American College of Cardiology that “the recommendation to limit dietary saturated fatty acid (SFA) intake has persisted despite mounting evidence to the contrary”.

Their case rests on particular foods. They wrote that whole-fat dairy, unprocessed meat, and dark chocolate are SFA-rich foods with a complex matrix that are not associated with increased risk of CVD. The matrix is everything else in a food, its protein, minerals and physical structure, which may change how its fat is absorbed and what it does. Cheese and butter, on this view, are not interchangeable simply because both are high in saturated fat.

Guideline bodies have not been persuaded to drop the limit. The WHO’s 2023 update came after both the PURE study and the Astrup paper and kept the 10% figure. The Cochrane reviewers, for their part, point to the trials, which tested diets and found fewer events.

Where the saturated fat trials fall short

The trials are the best evidence available and they are imperfect. People cannot be kept unaware of what they are eating, adherence to an assigned diet fades over years, and the comparison diets varied widely. The result for combined events is on the edge of statistical certainty, and the absence of an effect on deaths may mean that no effect exists or that the trials were too small and too short to see one.

The cohort studies carry the usual problems of diet research. Food questionnaires measure intake crudely, which tends to blur real associations, and people who eat a lot of saturated fat differ from those who eat little in income, smoking and much else.

How much saturated fat suits any one person depends on the rest of their diet and their cardiovascular risk, which is assessed by their doctor.

Cutting saturated fat lowers cardiovascular events modestly in trials when unsaturated fats or whole grains take its place, and it has not been shown to prevent deaths.