Not all fats are the same. Industrial trans fats have a very strong scientific consensus against them. The monounsaturated fats in extra-virgin olive oil are supported by some of the strongest cardioprotective evidence available. Saturated fats deserve attention, but more recent evidence shows that risk depends heavily on what replaces them. The “low-fat” narrative that dominated for forty years was wrong in its most simplified form. Here is the history, and where the evidence stands now.

For roughly forty years we lived in the era of fat as the enemy. “Low fat” and “0% fat” on a label became shorthand for healthy. Butter was the villain; margarine was the rescue. Entire generations stopped dressing their salads and chose skim yogurt because it was supposed to be “better for you.”

That story was wrong. Not completely, but wrong in the simplified form that spread through popular nutrition. It is worth understanding how we got there and what science actually says today.

The story begins with Ancel Keys, the same researcher who appears in the history of the Mediterranean diet. In the 1950s Keys proposed that dietary fat — especially saturated fat — contributed to cardiovascular disease by raising blood cholesterol. His Seven Countries Study appeared to support that hypothesis. From the 1970s onward, Western dietary guidelines were built in large part around a simple premise: less fat, healthier heart.

There is, however, a parallel story that emerged decades later. In 2016, research published in JAMA Internal Medicine reconstructed internal documents showing that the US sugar industry had funded Harvard researchers in the 1960s in ways that helped shift scientific attention away from carbohydrates and added sugars and toward fat as the principal dietary cause of coronary disease. Researcher John Yudkin, who had argued strongly for the role of sugar, was marginalized. The funding was not disclosed in the publications. The result was not that the science on fat became entirely false, but that the public scientific conversation was distorted for years.

None of this means fat is harmless by definition. It means the subject is more complicated — and the first step is to distinguish between different kinds of fat, because they do not behave the same way.

Saturated fats are found mainly in butter, cheese, red meat, coconut oil and palm oil. They are generally more solid at room temperature. For decades they were treated as the main dietary driver of higher LDL cholesterol — often called “bad cholesterol” — and therefore cardiovascular disease. More recent evidence is more nuanced. A 2020 Cochrane systematic review found that reducing saturated fat lowered the risk of cardiovascular events, particularly when saturated fats were replaced with polyunsaturated fats rather than refined carbohydrates. Replace saturated fat with refined starches or sugars, and the benefit largely disappears. More recent analyses have continued to debate how much saturated-fat reduction changes cardiovascular mortality in the general population. The scientific discussion is not closed, and the answer depends heavily on what replaces the saturated fat.

Industrial trans fats are much less ambiguous. The consensus is clear: they are harmful. They can be produced during partial hydrogenation of vegetable oils, a process that turns liquid oil into a more solid fat. They raise LDL cholesterol, lower HDL cholesterol and promote atherosclerotic risk. The European Union introduced legal limits on industrial trans fats in foods in 2021. The WHO recommends keeping intake below 1% of total daily energy. Small amounts of naturally occurring trans fats are also found in ruminant milk and meat, but they are consumed in much lower quantities and do not have the same evidence profile as industrial trans fats.

Monounsaturated fats are found in extra-virgin olive oil, avocado and nuts. Here the evidence is more consistent. PREDIMED, the large Mediterranean-diet trial discussed elsewhere on this site, found fewer major cardiovascular events in groups assigned to a Mediterranean diet supplemented with extra-virgin olive oil or nuts compared with a low-fat control group. The potential benefit is not only about oleic acid, the main monounsaturated fat in olive oil, but also the phenolic compounds present in good extra-virgin olive oil and largely reduced by refining. This is not about “fat” in the abstract; it is about which fat, in which food, in which dietary pattern.

Polyunsaturated fats fall broadly into omega-6 and omega-3 families. Omega-6 fats are common in many seed oils and nuts. Omega-3 fats are found especially in oily fish such as sardines, mackerel, salmon and anchovies, and in smaller or different forms in flaxseed and algae. Randomized trials and meta-analyses support cardiovascular benefits from omega-3 intake in certain contexts. The ratio between omega-6 and omega-3 has also attracted a great deal of research and debate. Observational studies can identify associations between different intake patterns and metabolic outcomes, but they cannot by themselves prove that a particular ratio causes disease.

A word on dietary cholesterol, because the consensus here has changed too. For decades people were told to limit eggs because they contain cholesterol. More recent guidance has moved away from treating dietary cholesterol as a simple one-to-one driver of blood cholesterol. The amount and type of saturated fat in the overall diet often matter more for LDL levels than dietary cholesterol alone. Eggs, eaten as part of a healthy dietary pattern, are not the problem they were once made out to be.

The synthesis is simple: “fat” is not one single nutritional category in terms of health effects. Industrial trans fats should be minimized. Saturated fats deserve attention, and what replaces them matters. The monounsaturated fats in extra-virgin olive oil sit inside a dietary pattern with strong cardiovascular evidence. Omega-3 fats from oily fish are well studied. And dietary cholesterol matters less than nutrition messaging once suggested.

Mediterranean eating has incorporated much of this for generations: extra-virgin olive oil as the main culinary fat, regular oily fish, nuts, relatively little red meat and little reliance on heavily processed fats. It is not a coincidence that this pattern has accumulated one of the strongest evidence bases in nutrition.