Research about fat is confusing, and the internet is rife with conflicting recommendations.

Much of the confusion happens when people make generalizations about fat in the diet. Many diet books, media outlets and blogs talk about fats as though they were all the same.

In reality, dozens of fats are common in the diet, and each one has a different role in the body and effects on your health. Even within groups of fats like saturated, unsaturated and polyunsaturated, specific fats still have different roles.

This article will explain the differences between some of the main dietary fats and their health effects, both good and bad.

The key is to understand that each type of fat has its own unique effects on the body. Once you start thinking about fats more specifically, you’ll be better equipped to make healthy dietary choices.

Healthy vs. Unhealthy FatsShare on Pinterest

Decades ago, common sense was to eat fatty foods because it was the most efficient way to get energy. Fat contains more calories by weight than any other nutrient.

Over time, scientists began to understand that some fats are healthier than others. In the 1930s, Russian scientists found that feeding animals very high-cholesterol diets caused atherosclerosis (1).

This is a condition where plaque builds up in the arteries, narrowing them and increasing the risk of heart disease. Atherosclerosis is the most prominent cause of heart disease and stroke (1).

In the 1940s and ‘50s, heart disease in multiple countries decreased. Many attributed this phenomenon to wartime rationing in World War II. This fueled the belief that fat and cholesterol, which were high in the restricted foods, contributed to heart disease.

The Seven Countries Study, a large international study directed by American physiologist Ancel Keys and other international scientists, revealed several important risk factors for heart disease.

These included smoking, high blood pressure, weight gain, yo-yo dieting and blood cholesterol (2).

The Seven Countries Study contributed to the hypothesis that saturated fat increased blood cholesterol, predicting atherosclerosis and heart disease (3).

However, even decades ago Ancel Keys recognized not all fat is harmful. He was skeptical of the importance of dietary cholesterol and showed unsaturated fats reduce the risk of heart disease (4).

Unfortunately, his and other researchers’ science has been much misquoted by policymakers, nutritionists and journalists.

Black and white, extreme conclusions, such as “all saturated fat is bad” or “everyone should eat a low-fat diet,” are not helpful nor correct. This article will demystify the confusing literature on fat by looking at a combination of old and new research.

Summary Since the 1930s, scientists have suspected that fat and cholesterol could cause atherosclerosis, heart disease and stroke. However, later research has shown that judging all fats together — even all saturated fats — is an inaccurate oversimplification.

Cholesterol is made by the liver in humans and animals. For this reason, you only get it in your diet from animal products.

The main sources include egg yolks, animal liver, fish or fish oil, animal fats or oils such as butter, shellfish, meat, cheese and baked goods made with animal fat.

The liver adjusts the amount of cholesterol it makes depending on how much comes in from the diet. When you eat large amounts of cholesterol, the liver makes less.

Cholesterol you eat has a small effect on cholesterol levels in your blood. Even 50 years ago, Ancel Keys recognized this effect was trivial for most people.

“Attention to [dietary cholesterol] alone accomplishes little,” Keys said (5).

According to a large study that combined evidence from more than 350,000 adults, dietary cholesterol was not associated with heart attack or stroke (6).

However, a combination of several large studies found that up to 25% of people are more sensitive than average to dietary cholesterol. For these people, high amounts of dietary cholesterol increase both the “bad” LDL and the “good” HDL cholesterol (7).

Summary Dietary cholesterol does not change the risk of heart disease for most people, according to the largest studies available. However, for up to a quarter of the population, high dietary cholesterol increases “bad” LDL and “good” HDL cholesterol.

Saturated fat is different from unsaturated fat in that it has no chemical double bonds. This makes it more stable, so it is solid at room temperature.

Saturated fat is the subject of a great deal of controversy, and nutrition experts don’t always agree on how it affects health. There are several reasons why research on saturated fat can be confusing.

Not All Saturated Fats Are the Same

While people who give dietary advice often lump saturated fats together, there are many different kinds of saturated fats that have different effects on health. Labeling all saturated fats as “healthy” or “unhealthy” is an oversimplification.

One discriminating feature of fats is their length, meaning the number of carbon atoms they contain. Fats may be short (containing fewer than six carbons), medium (6–10 carbons), long (12–22 carbons) or very long (22 or more).

Your cells treat fats very differently depending on their chain length, which means fats of different lengths can have different effects on health.

A study of 16,000 European adults found that consuming very long-chain fatty acids (VLCFAs) was associated with a decreased risk of type 2 diabetes (8).

VLCFAs are found in nuts, including peanut oil and canola oil. The study also found that the long-chain fat arachidic acid, found in vegetable oils, was protective.

Whether a saturated fat has an even or odd number of carbons in its chain is also important.

The same study of 16,000 European adults found saturated fatty acids with an even number of carbons were associated with type 2 diabetes, while odd-length fats were associated with a lower risk of the disease (8).

Even-length saturated fats include stearate, found primarily in meat, cheese and baked goods.

They also include palmitate, which is named for palm oil, but also found in dairy, meat, cocoa butter and fully hydrogenated vegetable oils. Another even-length saturated fat, myristate, can be found in butter, coconut and palm oil.

Odd-length saturated fats, including heptadecanoate and pentadecanoate, come mostly from beef and dairy.

Because the health effects of saturated fats and the ways they are metabolized are so nuanced, it is not useful to think of them as collectively “good” or “bad.”

People Eat Foods, Not Individual Nutrients

While most nutrition studies look at effects of individual nutrients, even the same specific type of fat may have different effects depending on its source.

For example, the saturated fat palmitate from lard causes atherosclerosis in animals, but the same palmitate taken from tallow does not (9).

Moreover, reorganizing the way fats in lard are connected to one another to be more like tallow reverses palmitate’s harmful effects (9).

Though these differences are nuanced, the takeaway is that the specific food is more important than the type of fat it contains.

For example, an avocado contains the same amount of saturated fat as three slices of bacon.

Bacon increases the levels of “bad” LDL cholesterol (10).

However, eating about a half to 1.5 avocados daily actually reduces levels of “bad” LDL cholesterol, according to a study of 229 adults (11).

This is probably partly due to differences in the kinds of saturated fats in avocados and the way they’re structured. However, avocados also contain healthy plant compounds that may deliver other benefits.

When you’re deciding which fats to include in your diet, choosing a variety of healthy foods including vegetables, nuts, seeds and fish is more important than focusing on individual fatty acids.

Other Factors in Your Diet Change the Effects of Saturated Fat

When researchers look at associations between saturated fat and health, they often think of the saturated fat as coming from meat, cheese and other dairy.

In reality, 15% of saturated fat in the American diet comes from carb-heavy desserts including cakes, cookies, pastries and candies. Another 15% comes from “junk” foods such as burgers, fries, pizza and chips, and another 6% from dairy-based desserts (12).

When these junk foods and desserts are represented in research only by their saturated fat content, it becomes difficult to tell their health effects apart from those of other foods that also contain saturated fat.

For example, cheese contributes more saturated fat to the Western diet than any other single food. However, the largest study of cheese looked at its effects in 177,000 adults over the course of 5–15 years and found no link between cheese and early death (13).

Another large study following hundreds of thousands of adults for up to 25 years found consuming milk, cheese and yogurt did not increase heart disease, and even slightly reduced the risk of stroke (14).

Regarding meat, a study of more than 1.6 million adults found those who ate the highest amounts of processed meat had a roughly 20% higher risk of heart disease and death from any cause than those who ate the lowest amounts (10).

The study also found that those who ate the highest amounts of red meat had a 16% higher risk of dying from heart disease than those who ate the lowest amounts (10).

However, it’s important to note that people sometimes wrongfully attribute the effects of an unhealthy diet to saturated fats.

Diets high in saturated fat tend to be high in calories and can lead to weight gain, so it can be easy to blame saturated fats for effects that may actually have been caused by excess calories and weight gain.

For example, some studies have shown that heart disease is actually more closely linked to extra calories and weight gain than to saturated fat (15).

This is important because it means many foods high in saturated fat are safe as long as they are eaten in moderation in a diet that does not cause weight gain.

Summary Some saturated fats contribute to heart disease. However, calling all saturated fats bad is an oversimplification. In fact, when they come from dairy and vegetable sources, as well as certain meats, some saturated fats are healthy.

Trans fats are made industrially by “hydrogenating” vegetable oil in a process that involves bombarding it with hydrogen gas. This transforms the liquid unsaturated fats into solid or nearly solid saturated and trans fats.

The most common sources of trans fats include cakes, pies, frosting, creamy fillings, fried foods and cookies and biscuits made with shortening or margarine.

Oils that are “fully hydrogenated” become indistinguishable from saturated fats, and are treated as saturated fats by the body.

However, trans fats — at least the ones made from vegetable oils — are foreign to the body and contribute to atherosclerosis and heart disease (16).

A 39-month study of atherosclerosis in the heart arteries of 50 men showed the disease worsened faster in men who consumed more trans fats (17).

This increase in atherosclerosis increases the risk of heart attack. A study examined 209 people who had recently experienced heart attacks and found they had higher levels of trans fats in their fat cells compared to 179 adults who had not had heart attacks (18).

In the US, food labels are now required to list the amount of trans fats per serving. Unfortunately, companies are allowed to round down to zero if the amount per serving is less than 0.5 grams.

This is especially troublesome given that the serving size is not regulated, and companies may manipulate the serving size to be less than you would typically eat at one time in order to claim “0 grams trans fat per serving.”

To avoid this trap, take a look at the ingredients. If they list “partially hydrogenated,” then the food contains trans fats and should be used very sparingly.

While industrial or artificial trans fats are clearly harmful, dairy products and meat contain small amounts of naturally occurring trans fats. These natural trans fats are not associated with heart disease and may actually be beneficial (19).

Summary Industrial or artificial trans fats cause heart disease. Avoid them. Even if a food label claims it contains “0 grams of trans fats,” if its ingredients list says “partially hydrogenated” oil, that means it contains unhealthy industrial trans fats.

Unlike saturated fats, unsaturated fats have double chemical bonds that change how your body stores and uses them for energy.

Unsaturated fats are heart healthy, though some are more so than others. As with saturated fats, there are many different unsaturated fats. Their length and the number and position of double bonds influence their effects in the body.

Monounsaturated fats have one double bond, while polyunsaturated fats have two to six double bonds.

Monounsaturated Fats are Good

Monounsaturated fats are plentiful in olive and canola oils and avocados. They can also be found in tree nuts including almonds, walnuts, pecans, hazelnuts and cashews.

A study following 840,000 adults over a span of 4–30 years found that those who consumed the most monounsaturated fats had a 12% lower risk of death from heart disease compared to those who ate the least (20).

This benefit was strongest for oleic acid and olive oil, compared to other sources of monounsaturated fat.

Polyunsaturated Fats Are Even Better

Polyunsaturated fats are potentially even better than monounsaturated. In one study, replacing foods high in saturated fat with polyunsaturated fat sources reduced the risk of heart disease by 19% (21).

This works out to a 10% reduction in heart disease risk for every 5% of their daily calories people consumed from polyunsaturated instead of saturated fat.

Polyunsaturated fats are found primarily in vegetable and seed oils.

Omega-3 Fatty Acids Have Many Health Benefits

Omega-3 fatty acids, a specific type of polyunsaturated fat, are found in seafood, especially fatty fish like salmon, herring, bluefin tuna and albacore tuna.

One study in 45,000 adults used the amounts of omega-3 fatty acids in blood and fat tissue to estimate the amounts of omega-3s in the diet. It found that a high omega-3 intake was associated with a 10% lower risk of heart disease (22).

Not all studies have found the same benefits, and some people have concerns about eating fish because it can be a source of mercury, which is toxic if consumed in large enough amounts (23, 24).

The US Food and Drug Administration and Environmental Protection Agency have stated that two to three servings of fish weekly is the safe upper limit, though this depends on the type of fish (23).

They recommend against regularly eating fish with the highest levels of mercury, including large fish such as king mackerel, marlin, swordfish and bigeye tuna.

Albacore and yellowfin tuna have smaller amounts of mercury and are considered safe to eat up to once weekly, while salmon, trout and white fish are safe to eat 2–3 times a week.

Summary Olive oil, canola oil and seed oils are useful for cooking and are sources of heart-healthy monounsaturated and polyunsaturated fats. Tree nuts and fish are also sources of healthy polyunsaturated fats, including omega-3s.

The more you know about fats, the more equipped you’ll be to make healthy choices.

The key is to understand that every specific type of fat has unique effects on the body, and these effects can be good or bad.

For example, many studies lump all saturated fats together, while in reality there are many different kinds of saturated fats, each with different roles in the body.

Additionally, people don’t eat saturated fats in isolation — they choose foods with many different kinds of fats and other nutrients.

Even the same type of saturated fat can have different effects depending on how it is connected to other fats and what else is in the diet. For example, saturated fats in dairy, poultry and certain vegetable oils are neutral or even heart healthy.

Unsaturated fats are consistently heart healthy, while industrial trans fats are consistently harmful. In contrast, the small amounts of naturally occurring trans fats in dairy are harmless, as is cholesterol in eggs and other animal products.

Overall, choose good fats, including unsaturated fats and saturated fats from a variety of vegetables, nuts, seeds, fish and unprocessed meats. Avoid bad fats such as partially hydrogenated oils and saturated fats in processed meat.

Following these guidelines will help control your risk of heart disease and extend your life.