What they are
For a general description of unsaturated fats, see the article on monounsaturated fat or polyunsaturated fat.
Unsaturated fats all have at least one pair of consecutive unsaturated carbons that are each only bonded to a single hydrogen atom, allowing the two carbons to form a double bond. An unsaturated fat can be classified as cis- or trans- depending on the locations of the missing hydrogen atoms on the double bonded carbons.
In a cis- unsaturated fat, the missing hydrogen atoms are all on the same side of the carbon backbone. Missing hydrogen atoms in the cis- position cause a bend in the carbon backbone of the fat at the point of the double-bonded carbons.
In a trans- unsaturated fat, the missing hydrogens are on opposite sides of the carbon backbone in at least one pair of double bonded carbons. Missing hydrogen atoms in the trans- position leave the carbon backbone straight. So, trans fats are unsaturated fats that are more structurally similar to saturated fats.
For a great example, oleic acid and elaidic acid are both monounsaturated fats with 18 carbon atoms, and a single pair of unsaturated carbons right in the middle of the carbon backbone. Oleic acid is a cis- unsaturated fat, meaning the missing hydrogen atoms are on the same side of the carbon backbone, which causes a bend in the carbon backbone of oleic acid. Elaidic acid is a trans- unsaturated fat, meaning the missing hydrogen atoms are on opposite sides of the carbon backbone, which leaves the carbon backbone straight. So, oleic acid – the most abundant fatty acid in olive oil – is a liquid at room temperature. Conversely, elaidic acid – the most abundant trans fat in hydrogenated vegetable oils – is a solid at room temperature.
What they do
Like all fats, trans fats provide energy and aid in the absorption of fat-soluble vitamins.
However, these effects are likely due to trans fats that were industrially produced through partial hydrogenation, like the aforementioned elaidic acid. Naturally occuring trans fats – most notably, conjugated linoleic acid and vaccenic acid – are also found in the meat and milk of ruminants like cows, sheep, and goats. Intake of ruminant-derived trans fats does not appear to be associated with increased risk of heart disease.
In 2015, the US Food and Drug Administration banned trans fats produced by the partial hydrogenation of vegetable oils, with the phase-out completed by 2020. A similar process was completed in Canada in 2020, Europe in 2021, and Australia in 2023. Most manufacturers of solid vegetable fats (like vegetable shortening and margarine) switched from partial hydrogenation to a different process called interesterification in order to achieve the same basic result – creating fat-based products that are solid at room temperature with (relatively) low levels of saturated fat.
Virtually all scientific and governmental agencies that generate nutrient intake targets recommend that trans fat intake – especially the intake of trans fats that were created by partial hydrogenation – should be kept as low as possible.
Likelihood of tracking completeness: Very high in the US and Canada, and low in many other places.
Nutrition labels in the US and Canada are required to list trans fat content. So, the vast majority of American and Canadian foods in the MacroFactor database should contain trans fat information, making it easy to accurately track your trans fat intake with consistent food logging.
However, in the EU, Australia, and many other countries, trans fat is a nutrient that is only reported on a voluntary basis. Many food manufacturers do not voluntarily list trans fat content on nutrition labels, so many branded products in the MacroFactor database from outside the US and Canada lack trans fat information. So, if you’d like to accurately track your trans fat intake, you’ll need to make a point of mostly tracking “common foods,” which come from research-grade databases that have full nutrient reporting.
For more on when you can track using branded foods versus common foods when you’re trying to accurately monitor your intake of particular nutrients, you should check out this article.
Likelihood of excessive intake: Previously high. Currently very low.
Industrially produced trans fats used to be fairly ubiquitous in the food supply (especially in pre-packaged foods) because they were cheap to produce and shelf-stable. However, as previously mentioned, many countries have recently banned the use of trans fats produced via partial hydrogenation. So, it would now take quite a bit of effort to consume large amounts of trans fats.
Low levels of naturally occurring trans fats can still be found in cow, sheep, and goat products (both meat and dairy), but these trans fats don’t appear to cause the same negative effects as trans fats produced via partial hydrogenation.
For more on nutrients with a greater likelihood of insufficient or excessive intake, you should check out this article.
Signs of excessive intake
There aren’t acute signs of excessive trans fat intake that would be obvious without lab tests for dyslipidemia.
Foods that still contain trans fats
Meat and dairy products from ruminants (cows, sheep, and goats) contain trans fats. Trans fats may also be present in some fried foods. When frying at high heat, some cis- unsaturated fats can be converted to trans fats. However, this risk can be significantly mitigated by frying at lower temperatures (below 200℃, or about 400℉), by not using the same batch of oil for longer than an hour, and by using oils enriched with antioxidants.
If you’d like to learn more about micronutrients generally, there’s a five-part series on the MacroFactor website you might enjoy.