Trans fats

Trans fat is the name for unsaturated fatty acids with at least one double bond in the trans configuration, as the WHO defines it. The definition also counts conjugated linoleic acid, or CLA. It is one of the dietary fats, and it comes mainly from two places.

Industrial trans fat forms when liquid vegetable oils are partially hydrogenated into solid fats. Hydrogenated fats are rich in elaidic acid (trans-9 18:1) and trans-10 18:1.

Ruminant trans fat forms by bacterial action in the stomach of cows, sheep and goats, so it turns up in their milk, butter and meat. Its main fatty acid is vaccenic acid (trans-11 18:1), which the body converts in part to rumenic acid, a CLA isomer. The WHO notes that the individual isomers in the two kinds are largely the same, but present in different proportions.

Trans fat is the one fat that the WHO says has no known health benefits. In a 1990 trial of 59 people, a diet with 10% of energy as trans fat raised LDL (“bad”) cholesterol and lowered HDL (“good”) cholesterol. A saturated fat diet raised LDL but left HDL unchanged.

Since then, cohort studies (following groups over time) and meta-analyses (pooling many studies) have pointed the same way for the industrial kind. So have the laws that took industrial trans fat out of food.

Most readers do not know that the natural kind is a separate question. Trials that fed ruminant trans fat at the same dose as industrial trans fat found a similar rise in the LDL to HDL ratio. But cohorts link only the industrial kind to coronary heart disease, probably because people eat far less of the natural kind. The WHO’s limit of 1% of energy counts both.

Denmark has capped industrial trans fat since 2004. The US Food and Drug Administration ruled in 2015 that partially hydrogenated oils are not generally recognized as safe. The EU caps trans fat at 2 g per 100 g of fat. This page sets out each side at its real evidence level, with the rules as published.

Health effects of trans fat

• ★★★Worse cholesterol ratios than saturated fatIn 60 trials, replacing trans fat improved the total to HDL cholesterol ratio (a heart-risk marker) almost twice as much as replacing saturated fat. In a review of 39 studies, industrial trans fat, ruminant trans fat and CLA each raised the LDL to HDL ratio by a similar amount.
• ★★Linked to more heart diseaseA review for the World Health Organization found each 2% of energy from trans fat, in place of carbohydrate, went with 24% more coronary events. This figure comes from long-term observational studies.
• ★★Poorer blood vessel wideningIn 29 healthy volunteers, a 4-week diet with 9.2% of energy from trans fat cut HDL by 21% compared with a saturated fat diet. Arm artery widening was 4.4% after the trans fat diet and 6.2% after the saturated fat diet.
• ★★Stick margarine lowered LDL only 5% against butterIn 18 women and 18 men, compared with butter, stick margarine lowered LDL only 5%, and soybean oil lowered it 12%. HDL fell 3 to 6% against butter, most on stick margarine, which held the most trans fat.
• ★★Large amounts of natural trans fat raised LDLIn 38 healthy men, 10.2 g of ruminant trans fat (the naturally occurring kind) per 2,500 kcal raised LDL, but 4.2 g did not. The authors say moderate amounts were neutral.

Foods high in trans fat

Trans fat in our USDA ranking comes only from whole foods, in small natural amounts. Per serving, ground beef leads with 1.33 g in 4 oz (113 g). Semi-skimmed cow’s milk follows with 0.21 g in 1 cup (246 g), then a chicken drumstick with skin with 0.06 g.

Chia seeds, shrimp and eggs come next; the ranking shows their amounts. Per 100 g, ground beef leads (1.18 g), then chia seeds (0.14 g) and milk (0.09 g). Per 100 kcal, ground beef (0.46 g) and milk (0.17 g) lead.

The ranking holds whole foods only. It does not show margarine, shortening or packaged baked goods, so it is not a measure of industrial trans fat.

For scale, the WHO’s limit of 1% of energy is less than 2.2 g a day on 2,000 kcal. A 4 oz serving of ground beef at 1.33 g is about 60% of that figure. A cup of milk at 0.21 g is about 10% (our arithmetic).

The WHO’s limit counts ruminant trans fat. But its guideline says the recommendations do not preclude eating particular foods, while foods high in industrially produced trans fat should largely be avoided.

Industrial trans fat is a matter of labels and laws more than USDA numbers. The WHO lists margarine, vegetable shortening, vanaspati ghee, fried foods and baked goods such as crackers, biscuits and pies. It says baked and fried street and restaurant foods often contain it.

The FDA calls partially hydrogenated oils the main dietary source of industrial trans fat. It adds that all refined edible oils hold some trans fat as an unintended byproduct, typically below 2 percent. Fully hydrogenated oils can also hold under 2 percent, and the fat of dairy and meat from ruminants typically about 3 percent.

The WHO puts the trans fat in partially hydrogenated oil at 25 to 45% on average. It says frying oil at high temperatures adds only a modest amount, up to 2 to 3%.

Two studies show what packaged foods held in the past and what blood tests have found more recently.

How trans fat works

Most of what is known comes from controlled feeding trials in people, plus cell and animal work. Four ideas cover it. They are the shape of the molecule and what it does to LDL, HDL and a lipid-transfer protein. Then come its effects on cells and arteries, and what has been seen in people.

The WHO’s guideline says the harm is believed to occur mainly through blood fats, with inflammation among other possible routes.

A trans double bond keeps the chain closer to straight

In a cis fat such as oleic acid, the double bond bends the chain. A trans bond bends it much less, so the molecule resembles a saturated fat. A 1997 review calls the artificial isomers structurally similar to saturated fats.

In model membranes, a lipid with elaidic acid (trans) melted at 31.5 °C. That is between the 7 °C of the oleic acid (cis) lipid and the 53 °C of the all-stearic acid (saturated) lipid. The authors link the smaller drop to the smaller bend in the chain. They speculate that this shape may play a role in the harm of dietary trans fat.

It raises LDL, lowers HDL and raises a transfer protein

In the 1990 trial, 10% of energy as trans fat raised LDL by 0.37 mmol/L against oleic acid. It cut HDL by 0.17 mmol/L. Saturated fat raised LDL by 0.47 mmol/L but left HDL unchanged. That is why the authors judged trans fat at least as harmful.

One link between the two changes is the cholesteryl ester transfer protein (CETP), which moves cholesteryl esters from HDL to lighter lipoproteins. In a 9-week study of three controlled diets in 55 volunteers, CETP activity averaged 114 after a high trans fat diet. Measured as a percent of a reference serum, it was 96 after linoleic acid and 97 after stearic acid.

In 27 men, activity was 23.95% on an elaidic acid-rich diet and 19.61% on an oleic acid diet. The rise was correlated with the fall in HDL cholesterol (r = −0.57). The authors say the higher activity may contribute to the lipid changes, not that it explains all of them.

It changes how cells and arteries behave

In the 29-volunteer trial on the cards above, replacing saturated fat with trans fat cut arm artery widening from 6.2% to 4.4%. A 2006 review of observational and experimental studies found trans fat to be pro-inflammatory and to harm blood vessel linings (endothelial dysfunction). It said the mechanisms are not well established, but may involve incorporation into cell membranes or effects on the PPAR and RXR pathways.

In macrophages, elaidic acid, but not vaccenic acid, reduced cholesterol efflux through ABCA1. In monkeys fed trans fat for 6 years, weight gain and belly fat rose even though calories were set to maintain weight. Both are on the cards above, as lab work.

What has been seen in people

The trials show the lipid effects, and no trial has tested trans fat against heart attacks. So the heart disease evidence comes from cohorts. The 2015 meta-analysis split them by source. Industrial trans fat went with more coronary heart disease and more deaths from it, and ruminant trans fat did not.

Putting the cards together: in feeding trials trans fat raised LDL, lowered HDL and worsened the total to HDL ratio. It also stiffened artery function in one crossover trial. Cohorts link industrial trans fat to coronary heart disease. The New York and Denmark studies saw fewer cardiovascular events after the limits, though they cannot prove the limits alone caused it.

The WHO graded the certainty that lower trans fat intake reduces cardiovascular disease as moderate. It graded the certainty that replacing it with polyunsaturated or monounsaturated fat lowers LDL as high. Ruminant trans fat is harder. The trials suggest it raises LDL at high doses, and the cohorts, which cover much lower intakes, find no clear link.

How much trans fat is too much?

The WHO advises keeping trans fat to 1% of daily energy or less. That is less than 2.2 g a day on a 2,000-calorie diet. The US National Academies and EFSA (the EU’s food safety agency) advise keeping it as low as possible.

No amount is known to be needed. The WHO states that trans fat has no known health benefits, so there is no daily need to meet and no deficiency to watch for. The 1% figure is a ceiling, not a target. The FDA said in 2015 that it did not agree that a threshold below which partially hydrogenated oils are safe had been identified.

Source	Group	Value	Notes
WHO guideline 2023, recommendations 1 and 2	Adults and children (the WHO’s release says everyone aged 2 and older)	“reduce trans-fatty acid intake to 1% of total energy intake” (strong recommendation). “further reducing trans-fatty acid intake to less than 1% of total energy intake” (conditional recommendation).	Moderate certainty evidence for recommendation 1 and low for recommendation 2. The definition covers every fatty acid with a double bond in the trans configuration, from ruminant or industrial sources, including CLA. The WHO’s fact sheet (24 January 2024) puts 1% at “less than 2.2 g per day for a 2000-calorie diet”.
WHO guideline 2023, recommendation 3	Replacement nutrients	Replace trans fat with polyunsaturated fatty acids or monounsaturated fatty acids “primarily from plant sources” (conditional recommendation).	The guideline says replacing trans fat with saturated fat did not improve disease outcomes or blood lipids, so saturated fat is not a preferred replacement.
EFSA 2010, dietary reference values for fats	General population	“trans fatty acid intake should be as low as possible”	EFSA gives no number. The EU Commission’s Regulation (EU) 2019/649 records that EFSA concluded in 2009 that intake “should be as low as possible within the context of a nutritionally adequate diet” and again in a 2018 technical assistance that intakes “should be as low as possible”.
Institute of Medicine (National Academies) 2005	General population	“As low as possible while consuming a nutritionally adequate diet”	As listed in Health Canada’s table of macronutrient reference values. The FDA describes the recommendation as limiting trans fat as much as possible while recognizing that naturally occurring trans fat is unavoidable in ordinary, non-vegan diets without significant dietary adjustments.
FAO/WHO expert consultation 2010	Adults	“a mean population intake of TFA of less than 1%E”	The experts acknowledged that this recommendation “might need to be revised” because it does not fully take into account the distribution of intakes and the need to protect subgroups with very high intakes.

What do people actually eat? A review of surveys in 29 countries put total trans fat intake between 0.3% and 4.2% of energy, with seven countries above the WHO’s 1%. In 16 of 21 countries with data on sources, intake from animal sources was higher than from industrial sources.

Time trends from 20 countries showed large drops in industrial trans fat since 1995. Two authors work for Unilever, which markets margarines and dressings. A 2022 review reports that in EU member states trans fat provides on average less than 1% of energy. Intakes are marginally above recommendations mainly among young adults.

Forms of trans fat: industrial and natural

Trans fat reaches people in partially hydrogenated oils and in the fat of milk and meat from ruminants. It also comes in small amounts in refined and fried oils, and as CLA in supplements. The honest bottom line is that for blood fats the source matters less than the dose.

In the quantitative review, each 1% of energy raised the LDL to HDL ratio by 0.055 for industrial trans fat. For ruminant trans fat it was 0.038, and that difference was not statistically significant.

Where the sources differ is in how much people eat. The 2004 to 2006 fast-food survey found over 20 g of industrial trans fat in one menu in 17 of 18 countries. The ruminant intakes in the cohorts were 0.5 to 1.9 g a day.

Popular claims need a caveat. We found no authorized health claim for any trans fat and no trial of vaccenic acid supplements in people. For trans-palmitoleic acid we found only a small trial that raised the blood level without a health result. Studies of CLA used capsules, which are a different exposure from the CLA in dairy fat.

Form	What it is	What studies show	Source
Partially hydrogenated oils (shortening, hard margarine, frying fat)	Vegetable or fish oils hydrogenated but not to complete saturation. Rich in elaidic acid (trans-9 18:1) and trans-10 18:1.	The WHO puts the trans fat in partially hydrogenated oil at 25 to 45% on average, and Stender and colleagues at up to 60% of the fatty acids. In a feeding trial, LDL cholesterol fell only 5% with stick margarine (20.1 g of trans fat per 100 g of fat), against 12% with soybean oil. The FDA ruled these oils not generally recognized as safe in 2015.	1, 2, 3, 4, 6
Fully hydrogenated oils	Hydrogenated to complete or near complete saturation (iodine value of 4 or less, in the FDA’s definition).	The FDA says they typically hold below 2 percent trans fat from incomplete hydrogenation, and left them outside its 2015 order.	1
Refined and fried oils	Non-hydrogenated vegetable oils, refined at high temperature or used for frying.	The FDA says refined oils typically hold below 2 percent trans fat as an unintentional byproduct. The WHO says frying oil at high temperatures adds a modest amount, up to 2 to 3%.	1, 2
Interesterified and fractionated fats	Replacement fats made by rearranging or separating fats rather than hydrogenating them.	A 2006 account by an employee of a fat producer says the industry moved from selective hydrogenation to fractionation and interesterification without significant changes in raw material cost. Trials of such fats are on our page on stearic acid.	5
Ruminant trans fat (vaccenic acid)	Trans-11 18:1, formed in the rumen. Found in milk, butter and meat. The body converts about 19% of it to rumenic acid.	The FDA puts trans fat at typically about 3 percent of the fat in ruminant dairy and meat, and Stender and colleagues at generally no more than 6%. LDL cholesterol rose at 10.2 to 12 g a day in two trials but not at 4.2 g per 2,500 kcal. Cohorts show no clear link with heart disease.	1, 3, 8, 9, 10, 11, 12
CLA (rumenic acid, cis-9,trans-11; and trans-10,cis-12)	Conjugated linoleic acids, with conjugated trans bonds. Rumenic acid is the natural ruminant isomer; some trials gave purified trans-10,cis-12.	In a quantitative review CLA raised the LDL to HDL ratio by 0.043 per 1% of energy. In one trial cis-9,trans-11 lowered the ratio and trans-10,cis-12 raised it. At a median 3.2 g a day, supplements gave a modest loss of body fat. The WHO’s trans fat definition includes CLA; the FDA’s 2015 order does not apply to it.	1, 7, 14, 15, 16
Trans-palmitoleic acid (trans-16:1 n-7)	A trans fatty acid in dairy fat, used as a blood marker of dairy fat intake.	Higher blood levels went with lower diabetes in a cohort (hazard ratios 0.41 and 0.38) and in a pooled analysis (risk ratio 0.58). A supplement trial in 13 adults raised the level but changed no glucose or lipid measure.	11, 17, 18

Here are the form-specific studies, strongest evidence first, 10 in all.

Is trans fat safe? Limits and who should watch it

No intake of industrial trans fat is known to be safe or needed. The limits below are limits on what foods may contain, and they differ by region. Quotation marks show the wording as published.

The studies that evaluated them are on the cards above. They are Denmark (Restrepo and Rieger), New York City (Angell, Brandt) and the 2004 to 2006 fast-food survey.

Rule	Where	Wording as published	Notes
Danish Order no. 160, March 2003	Denmark	A maximum of 2 g of industrially produced trans fat per 100 g of fat (a description of the order in a 2006 Danish report, not the order’s own text).	In force from January 2004, and the first national limit of its kind, according to the WHO. Trans fat in foods such as French fries, microwave popcorn and bakery products was reduced or removed, according to the same Danish report.
New York City Board of Health	New York City	Required that artificial trans fat be phased out of restaurant food (a description by the city health department authors).	Adopted December 2006. By November 2008 the restriction was in full effect, and the authors estimate restaurant use of artificial trans fat for frying, baking, cooking or in spreads fell from 50% to under 2%.
FDA final determination on partially hydrogenated oils	United States	“there is no longer a consensus among qualified experts that partially hydrogenated oils (PHOs), which are the primary dietary source of industrially-produced trans fatty acids (IP-TFA) are generally recognized as safe (GRAS) for any use in human food”	Federal Register 80 FR 34650, 17 June 2015 (the FDA’s page says it was released on 16 June 2015). The order set the compliance date at “June 18, 2018”. The FDA later extended it: foods made with non-petitioned uses of PHOs before 18 June 2018 had until January 1, 2020, and the petitioned uses until June 18, 2019 for manufacturing and January 1, 2021 for distribution. The order does not apply to naturally occurring trans fat, to fully hydrogenated oils or to CLA. In 2023 the FDA issued a direct final rule to remove outdated references to PHOs.
Regulation (EU) 2019/649, Article 1	European Union	“The content of trans fat, other than trans fat naturally occurring in fat of animal origin, in food intended for the final consumer and food intended for supply to retail, shall not exceed 2 grams per 100 grams of fat.”	Adopted 24 April 2019 and published in the Official Journal on 25 April 2019. It entered into force on the twentieth day after publication, and “Food which does not comply with this Regulation may continue to be placed on the market until 1 April 2021.” Article 2 requires suppliers to other food businesses to pass on the amount where it exceeds 2 grams per 100 grams of fat.
Nutrition labeling, 21 CFR 101.9(c)(2)(ii)	United States	“If the serving contains less than 0.5 gram, the content, when declared, shall be expressed as zero.”	Trans fat is given in grams per serving “to the nearest 0.5 (1⁄2)-gram increment below 5 grams”. Declaration is not required for products with less than 0.5 gram of total fat in a serving if no claims are made about fat, fatty acid or cholesterol content. The FDA has required trans fat on the Nutrition Facts label since January 2006.
WHO REPLACE action package	Global	Best-practice policies: a “mandatory national limit of 2 grams of industrially produced trans fat per 100 grams of total fat in all foods” or a “mandatory national ban on the production or use of partially hydrogenated oils (a major source of trans fat) as an ingredient in all foods”.	REPLACE was released on 14 May 2018, with six actions: review, promote, legislate, assess, create awareness and enforce. The WHO’s plan for 2019 to 2023 lists eliminating industrially produced trans fat as a priority target, and its fact sheet (24 January 2024) says almost half of the world’s population is covered by best-practice policies.

The “0 g” rule deserves a note. In its 2015 order, the FDA said natural trans fat is only likely to reach 0.5 g per serving in products with partially hydrogenated oils.

Several servings of a food that rounds down to zero can still add up. Five servings that each hold just under 0.5 g would give almost 2.5 g, more than the WHO’s 2.2 g (our arithmetic).

• People with raised LDL cholesterol or heart disease. Everyone is advised to keep trans fat low, so there is no reason to eat more of it. The lipid trials in this page were in healthy volunteers and lasted 3 to 5 weeks. Ask a doctor or dietitian about the rest of your fat intake.
• People who live or eat where no rule exists. The WHO says action is needed in low- and middle-income countries, where controls on industrial trans fat are often weaker. It says baked and fried street and restaurant foods often contain it. The 2004 to 2006 survey found more than 20 g in one menu in 17 of 18 countries.
• Natural trans fat in dairy and meat. The WHO’s 1% limit includes it and the FDA’s 2015 order does not. The cohorts show no clear link with heart disease at usual intakes (0.92 and 0.93 times the risk). In a 38-man trial, 4.2 g per 2,500 kcal was neutral while 10.2 g raised LDL cholesterol. The WHO’s guideline says its recommendations do not preclude eating particular foods.
• CLA supplements. The WHO counts CLA in its definition of trans fat, and the FDA does not apply its 2015 order to CLA. In one trial the trans-10,cis-12 isomer raised the LDL to HDL ratio. Supplements at 3.2 g a day gave only a modest loss of body fat. We found no study that tested CLA supplements against heart disease.

How to eat less trans fat

• Check the ingredient list for “partially hydrogenated” oil. The FDA calls these oils the primary dietary source of industrial trans fat. The WHO suggests avoiding them in fried and baked foods when they are labeled.
• Do not rely on “0 g” alone. A serving can hold up to just under 0.5 g and still be labeled zero. So look for the ingredient and watch how many servings you eat.
• Cook with liquid oils. The WHO’s guideline advises replacing trans fat with polyunsaturated fats or monounsaturated fats from plants, and not with saturated fat. Be most careful with fried street and restaurant food where no rule applies.

History of trans fat

Partially hydrogenated oils entered the food supply in the early 20th century as a replacement for butter, according to the WHO. They became more popular in the 1950s through 1970s, after the negative health effects of saturated fats were discovered. The first controlled test of their effect on lipoproteins came in 1990. Mensink and Katan reported that trans isomers of oleic acid raised LDL and lowered HDL cholesterol in 59 people.

In 1993 Willett and colleagues reported on 85,095 nurses. The top fifth of trans fat intake had 1.50 times the risk of coronary heart disease. By 1997 Ascherio and Willett estimated, conservatively, that 30,000 premature deaths a year in the US were attributable to trans fat. They wrote that consumption should be minimized and information on content made available to consumers.

The rules followed. Denmark issued its order in March 2003, and the limit took effect in January 2004. The FDA published its labeling rule on 11 July 2003 (68 FR 41434) and required trans fat on the Nutrition Facts label from January 2006. In December 2006 the New York City Board of Health required artificial trans fat to be phased out of restaurant food.

The FDA announced a tentative determination on partially hydrogenated oils on 8 November 2013, and the final one in June 2015. The WHO released REPLACE on 14 May 2018, and FDA compliance dates ran from 18 June 2018 to 1 January 2021.

The EU limit of 2 g per 100 g of fat was adopted on 24 April 2019. Food that did not comply was allowed on the market until 1 April 2021. In July 2023 the WHO issued its guideline on saturated and trans fatty acids.

Frequently asked questions about trans fat

Is trans fat banned?

Not everywhere, and the rules differ. In the US, the FDA ruled in 2015 that partially hydrogenated oils are not generally recognized as safe for any use in human food. Final compliance dates ran to January 1, 2021.

The EU limit is 2 g of industrial trans fat per 100 g of fat. Non-compliant food was allowed on the market until 1 April 2021. Denmark has had a 2 g limit since 2004. These rules do not cover the natural trans fat in dairy and meat.

The WHO says almost half of the world’s population is covered by best-practice policies, which leaves more than half without.

Is any amount of trans fat safe?

No amount is known to be safe or needed. The WHO recommends reducing intake to 1% of energy and suggests going lower. It puts 1% at less than 2.2 g a day on 2,000 calories.

EFSA and the US National Academies say as low as possible. In 2015 the FDA said it had not identified a threshold below which partially hydrogenated oils are safe.

Why can a label say 0 g trans fat when the food contains some?

In the US, trans fat is rounded to the nearest 0.5 g per serving. A serving with less than 0.5 g is declared as zero. Servings add up, so check the ingredient list for partially hydrogenated oil. The FDA said in 2015 that natural trans fat is only likely to reach 0.5 g per serving in products that contain these oils.

What foods contain trans fat?

The WHO lists margarine, vegetable shortening, vanaspati ghee, fried foods and baked goods such as crackers, biscuits and pies. It says baked and fried street and restaurant foods often contain industrial trans fat. Small natural amounts are in beef, milk and other ruminant foods. In our USDA data, ground beef has 1.33 g in 4 oz and semi-skimmed cow’s milk 0.21 g in a cup.

Is the natural trans fat in dairy and meat as bad as industrial trans fat?

At the same high dose, probably similar for blood fats. The quantitative review found the LDL to HDL ratio rose by 0.038 per 1% of energy for ruminant trans fat. For industrial trans fat it rose by 0.055, a difference that was not significant.

At usual intakes, which are far lower, the cohorts show no clear link with heart disease. Ruminant trans fat gave 0.93 times the risk, against 1.42 for industrial. The WHO’s 1% limit counts both kinds.

Does trans fat lower HDL cholesterol?

Yes, in trials. In 34 women and 25 men, 10% of energy as trans fat lowered HDL by 0.17 mmol/L against oleic acid. It raised LDL by 0.37 mmol/L.

Saturated fat raised LDL but left HDL unchanged. In another trial, HDL was 21% lower on a trans diet than on a saturated diet.

Is CLA a trans fat?

It has trans double bonds, so the WHO counts it in its definition of trans fat. The FDA’s label rule counts only trans bonds that are isolated, not conjugated, and its 2015 order does not apply to CLA. In a quantitative review, CLA raised the LDL to HDL ratio by 0.043 per 1% of energy. In one trial, the trans-10,cis-12 isomer raised the ratio while cis-9,trans-11 lowered it.

Is margarine full of trans fat?

It used to be, depending on the type. In a 1999 trial, stick margarine held 20.1 g of trans fat per 100 g of fat. Soft margarine held 7.4 g and semiliquid margarine under 0.5 g.

The US ruling on partially hydrogenated oils and the EU limit of 2 g per 100 g of fat now apply to margarine sold there. So check the ingredient list for partially hydrogenated oil, especially where no rule applies.