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.

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

Several reviews have concluded that a moderate intake of these fats does not appear harmful (1).

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. However, there is growing disagreement on whether CLA is actually beneficial, and more definitive studies are needed (2. 3).

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 (4).

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 (5).

Meanwhile, most other fats tend to increase both LDL and HDL (6).

Indeed, many observational studies link trans fats to an increased risk of heart disease (5).

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 2001 study in over 80,000 women noted that those who consumed the most trans fats had a 40% higher risk of diabetes (7).

However, a more recent study found no relationship between trans fat intake and diabetes (8).

Several controlled studies examining trans fats and diabetes risk factors, such as insulin resistance and blood sugar levels, show inconsistent results (9).

A 2021 animal study concluded that trans fatty acids significantly increased the risk of metabolic diseases such as type 2 diabetes and fatty liver (10).

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 older studies indicate that trans fats increase inflammatory markers when replacing other nutrients in the diet — but another older study switched butter for margarine and found no difference (11, 12, 13).

More recent animal studies found a connection between trans fats and intestinal inflammation (10).

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 an older 4-week study in which trans fats replaced saturated fats, HDL (good) cholesterol dropped 21% and artery dilation was impaired by 29% (14).

More recently, a study of 111 subjects with coronary artery disease found a clear correlation between increased consumption of trans fatty acids with the severity of their arterial lesions (15).

Recent studies have examined trans fats’ effect on cancer, finding a connection between trans fat intake and increase of breast, prostate, and colorectal cancers (16, 17).

SUMMARY

Trans fats may damage the inner lining of your blood vessels and increase the risk of certain cancers.

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 have been 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 in the US, effective 2020 (18).

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. As of 2018, the use of trans fat in these products has been banned in the US.

Though trans fats have been banned in food in the US since 2020, it can be tricky to completely avoid traces of them.

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.

Thus, it is wise to reduce the amount of highly processed foods in your diet.

SUMMARY

The FDA has banned trans fats in food since 2020. Trace amounts may still be found, however, so the optimal option is to read labels carefully and avoid ultra-processed foods.

Most trans fats 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 even cancer, 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.