You may have heard a lot about trans fats.

These fats are notoriously unhealthy, but you may not know why.

Although intake has declined in recent years as awareness has increased and regulators have restricted their use, trans fats still pose a public health problem.

This article explains everything you need to know about trans fats.

Trans fats, or trans-fatty acids, are a form of unsaturated fat.

They come in both natural and artificial forms.

Natural, or ruminant, trans fats occur in the meat and dairy from ruminant animals, such as cattle, sheep, and goats. They form naturally when bacteria in these animals’ stomachs digest grass.

These types typically comprise 2–6% of the fat in dairy products and 3–9% of the fat in cuts of beef and lamb (1, 2).

However, dairy and meat eaters needn’t be concerned.

Several reviews have concluded that a moderate intake of these fats does not appear harmful (3, 4, 5).

The best-known ruminant trans fat is conjugated linoleic acid (CLA), which is found in dairy fat. It is believed to be beneficial and is marketed as a dietary supplement (6, 7, 8, 9).

However, artificial trans fats — otherwise known as industrial trans fats or partially hydrogenated fats — are hazardous to your health.

These fats occur when vegetable oils are chemically altered to stay solid at room temperature, which gives them a much longer shelf life (11).

SUMMARY Trans fats are found in two forms — natural, which occur in some animal products and aren’t considered harmful, and artificial, which are hydrogenated vegetable oils and have serious health consequences.

Artificial trans fats may increase your risk of heart disease.

In a series of clinical studies, people consuming trans fats instead of other fats or carbs experienced a significant increase in LDL (bad) cholesterol without a corresponding rise in HDL (good) cholesterol.

Meanwhile, most other fats tend to increase both LDL and HDL (12).

Similarly, replacing other dietary fats with trans fats significantly increases your ratio of total to HDL (good) cholesterol and negatively affects lipoproteins, both of which are important risk factors for heart disease (13).

Indeed, many observational studies link trans fats to an increased risk of heart disease (14, 15, 16, 17).

SUMMARY Both observational studies and clinical trials suggest that trans fats significantly increase your risk of heart disease.

The relationship between trans fats and diabetes risk is not completely clear.

A large study in over 80,000 women noted that those who consumed the most trans fats had a 40% higher risk of diabetes (18).

However, two similar studies found no relationship between trans fat intake and diabetes (19, 20).

Several controlled studies examining trans fats and diabetes risk factors, such as insulin resistance and blood sugar levels, show inconsistent results (21, 22, 23, 24, 25).

That said, animal research reveals that large amounts of trans fats harm insulin and glucose function (26, 27, 28, 29).

Notably, in a 6-year study in monkeys, a high-trans-fat diet (8% of calories) caused insulin resistance and elevated belly fat and fructosamine, a marker of high blood sugar (30).

SUMMARY Trans fats may drive insulin resistance and type 2 diabetes, but the results from human studies are mixed.

Excess inflammation is thought to be a primary cause of many chronic illnesses, such as heart disease, metabolic syndrome, diabetes, and arthritis.

Two studies indicate that trans fats increase inflammatory markers when replacing other nutrients in the diet — but another study switched butter for margarine and found no difference (31, 32, 33).

In observational studies, trans fats are linked to increased inflammatory markers, especially in people with excess body fat (34, 35).

SUMMARY Studies indicate that trans fats increase inflammation, especially in people with excess weight or obesity.

Trans fats are believed to damage the inner lining of your blood vessels, known as the endothelium.

In a 4-week study in which trans fats replaced saturated fats, HDL (good) cholesterol dropped 21% and artery dilation was impaired by 29% (36).

In another study, markers for endothelial dysfunction also increased under a trans-fat-heavy diet (37).

Still, very few studies have examined trans fats’ effect on cancer.

In a large-scale research effort called the Nurses’ Health Study, intake of trans fats before menopause was associated with an increased risk of breast cancer after menopause (38).

However, two reviews suggest that the cancer link is very weak (39).

Thus, more research is needed.

SUMMARY Trans fats may damage the inner lining of your blood vessels. Yet, their effect on cancer risk is less clear.

Partially hydrogenated vegetable oils are the largest source of trans fats in your diet because they’re cheap to manufacture and have a long shelf life.

While they’re found in a variety of processed foods, governments have recently moved to restrict trans fats.

In 2018, the Food and Drug Administration (FDA) banned the use of partially hydrogenated oil in most processed foods (40).

However, this ban hasn’t been fully implemented, so many processed foods still harbor trans fat.

Several other countries have taken similar steps to reduce the trans fat content of processed goods.

SUMMARY Processed food that contains partially hydrogenated vegetable oil is the richest source of trans fat in the modern diet, though regulators have lately begun to limit it.

It can be tricky to completely avoid trans fats.

In the United States, manufacturers can label their products “trans-fat-free” as long as there are fewer than 0.5 grams of these fats per serving.

Clearly, a few “trans-fat-free” cookies could quickly add up to harmful amounts.

To avoid trans fats, it’s important to read labels carefully. Don’t eat foods that have any partially hydrogenated items on the ingredients list.

At the same time, reading labels doesn’t always go far enough. Some processed foods, such as regular vegetable oils, harbor trans fats but fail to name them on the label or ingredients list.

One U.S. study of store-bought soybean and canola oils found that 0.56–4.2% of the fats were trans fats — without any indication on the packaging (44).

Thus, the best thing you can do is to reduce the amount of processed foods in your diet.

SUMMARY While reading labels is a helpful step to ensure you’re minimizing your trans-fat intake, the optimal option is to cut processed foods out of your routine entirely.

Most trans fats in the Western diet are hazardous to your health.

Although ruminant (natural) trans fats from animal products are considered safe in moderate amounts, artificial ones are strongly associated with health problems, including heart disease.

Artificial trans fats are likewise linked to long-term inflammation, insulin resistance, and type 2 diabetes, especially for people with obesity or excess weight.

Although the amount of trans fats in the modern diet has declined, the average intake is still a concern in many countries.