There is a lot of disagreement in nutrition.

But one of the few things people actually agree on is the unhealthy nature of trans fats.

Fortunately, consumption of these horrible fats has gone down in recent years and decades.

But we are still eating way too much of them, which has various detrimental effects on health.

This article takes a detailed look at trans fats... what they are, why they're so bad for you and how to avoid them.

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

Unlike saturated fats, which have no double bonds, unsaturated fats have at least one double bond in their chemical structure.

This double bond can be either in the "cis" or "trans" configuration, which relates to the position of hydrogen atoms around the double bond.

Basically... "cis" means "same side," which is the most common structure. But trans fats have the hydrogen atoms on opposite sides, which can be a problem.

Unsaturated Fat Cis vs Trans

In fact, "trans" is latin for "on the opposite side," hence the name trans fat.

This chemical structure is believed to be responsible for numerous health issues.

Bottom Line: Trans fats are unsaturated fats with a specific chemical structure, where the hydrogen atoms are on opposite sides of the double bond.

Natural trans fats have been part of the human diet ever since we began eating the meat and dairy from ruminant animals (such as cattle, sheep and goats).

Also known as ruminant trans fats, they are completely natural, formed when bacteria in the animal's stomach digest grass.

These trans fats typically make up 2-5% of the fat in dairy products and 3-9% of the fat in beef and lamb (, ).

However, dairy and meat eaters to be concerned.

Several review studies have concluded that a moderate intake of ruminant trans fats does not appear to be harmful (, , ).

The most well-known ruminant trans fat is conjugated linoleic acid (CLA), which is believed to be beneficial by many and often consumed as a supplement (, , , ).

It is found in relatively large amounts in dairy fat from grass-fed cows, which is extremely healthy and linked to a reduced risk of heart disease (, ).

However... the same positive things can NOT be said about artificial trans fats, otherwise known as industrial trans fats or hydrogenated fats.

These fats are created by pumping hydrogen molecules into vegetable oils. This changes the chemical structure of the oil, turning it from a liquid into a solid ().

This process involves high pressure, hydrogen gas, a metal catalyst and is highly disgusting... the fact that anyone would consider them suitable for human consumption is baffling.

After they have been hydrogenated, the vegetable oils have a much longer shelf life and are solid at room temperature, with a consistency similar to saturated fats.

Although humans have been consuming natural (ruminant) trans fats for a very long time, the same is NOT true for artificial trans fats... which are seriously harmful.

Bottom Line: Natural trans fats are found in some animal products and are not harmful. Artificial trans fats are made by "hydrogenating" vegetable oils in a harsh chemical process.

In the past few decades, there have been numerous clinical trials studying trans fats.

In these clinical trials, people were fed trans fats (from hydrogenated vegetable oils) instead of other fats or carbohydrates.

The health effects were evaluated by looking at known risk factors for heart disease, like cholesterol or the lipoproteins that carry cholesterol around.

Replacing carbohydrates (1% of calories) with trans fats significantly increases LDL (the "bad") cholesterol, but does not increase HDL (the "good") cholesterol.

However, most other fats tend to increase both LDL and HDL cholesterol ().

Similarly, replacing other fats in the diet with trans fats significantly increases the total/HDL cholesterol ratio and negatively affects lipoproteins (ApoB/ApoA1 ratio), both important risk factors for heart disease ().

However... this goes beyond just risk factors, we also have many observational studies linking trans fats to an increased risk of heart disease itself (, , , ).

Bottom Line: Both observational studies and clinical trials have found 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 of over 80,000 women found that those who consumed the most trans fats had a 40% higher risk of diabetes ().

However, two other similar studies didn't find any relationship between trans fat intake and diabetes (, ).

Several controlled trials in humans have also looked at trans fats and important diabetes risk factors, such as insulin resistance and blood sugar levels.

Unfortunately, the results have been inconsistent... some studies appear to show harm, while others show no effect (, , , , ).

That being said, several animal studies have found found that large amounts of trans fats lead to negative effects on insulin and glucose function (, , , ).

Most notable was a 6 year study on monkeys which found that a high trans fat diet (8% of calories) caused insulin resistance, abdominal obesity (belly fat) and elevated fructosamine, a marker of high blood sugar ().

Bottom Line: It is possible that trans fats cause insulin resistance and drive type II diabetes, but the results from human studies are mixed.

Excess inflammation is believed to be among the leading drivers of many chronic, Western diseases.

This includes heart disease, metabolic syndrome, diabetes, arthritis and numerous others.

There have been three clinical trials investigating the relationship between trans fats and inflammation.

Two found that trans fats increase inflammatory markers such as IL-6 and TNF alpha when replacing other nutrients in the diet (, ).

The third study replaced butter with margarine and found no difference ().

In observational studies, trans fats are linked to increased inflammatory markers, including C-Reactive Protein, especially in people who have a lot of body fat (, ).

From looking at the evidence, it seems pretty clear that trans fats are an important driver of ... which can potentially lead to all sorts of problems.

Bottom Line: Clinical trials and observational studies both indicate that trans fats increase inflammation, especially in people who are overweight or obese.

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

When saturated fats were replaced with trans fats in a 4 week study, HDL cholesterol was lowered by 21% and the ability of arteries to dilate was impaired by 29% ().

Markers for endothelial dysfunction were also increased when trans fats replaced carbohydrates and monounsaturated fats ().

Unfortunately, very few studies have looked at the association between trans fats and cancer.

In the Nurses' Health Study, intake of trans fats before menopause was associated with increased risk of breast cancer after menopause ().

However two review studies have concluded that the cancer link is very weak. No compelling associations have been observed so far ().

Bottom Line: Trans fats can damage the inner lining of your blood vessels, causing a condition known as endothelial dysfunction. The effect on cancer risk is less clear.

Hydrogenated vegetable oils (the biggest source of trans fats) are cheap and have a long shelf life.

For this reason, they are found in all sorts of modern processed foods.

Fortunately, because the governments and health organizations around the world have been cracking down on trans fats, consumption has gone down in recent years.

In 2003, the average US adult consumed 4.6 grams of artificial trans fats per day. This has now been reduced to 1.3 grams per day (, ).

In Europe, the Mediterranean countries were found to have the lowest intakes of trans fats. This may partly explain their low risk of cardiovascular disease (, ).

The FDA only recently decided to remove the GRAS (Generally Recognized as Safe) status for trans fats, even though these studies have been out for many, many years.

However... even though artificial trans fat consumption is lower than before, it is still way too high and should be reduced to zero.

Bottom Line: Consumption of trans fats has gone down significantly in recent years. However, current intake is still high enough to cause harm.

Big improvements have been made in recent years, although trans fats are still present in many processed foods.

In the US, manufacturers can label their products "trans fat free" as long as there is less than 0.5 grams of trans fats per serving.

You can see how a few a "trans fat free" cookies could quickly add up to harmful amounts.

To make sure you're avoiding trans fats, read labels. Don't eat foods that have the words "hydrogenated" or "partially hydrogenated" on the ingredients list.

Unfortunately, reading labels isn't enough in all cases. Some processed foods (like regular vegetable oils) can contain trans fats, without any indication on the label or ingredients list.

One US study that analyzed store-bought soybean and canola oils found that 0.56% to 4.2% of the fats were trans fats, without any indication on the packaging ().

In order to avoid trans fats, the best thing you can do is eliminate processed foods from your diet.

Choose real butter instead of margarine, and olive oil or coconut oil instead of harmful vegetable oils... and make time for home-cooked meals instead of fast food.

Ruminant (natural) trans fats from animal products are safe.

But industrialized (artificial) trans fats from processed foods are downright toxic.

Studies have strongly linked artificial trans fats to cardiovascular problems, including heart disease.

Consumption is also associated with long-term inflammation, insulin resistance and type II diabetes risk, especially for people who are overweight or obese.

Although the amount of trans fats in the modern diet has gone down, the average intake is still dangerously high.

Unfortunately, the labels on junk foods and processed vegetable oils cannot always be trusted. Many "trans fat free" products still contain trans fats.

You have been warned.