Dairy trans fats explained: what a review of 22 studies found, who paid for it, and what the WHO says

For two decades the message on trans fats has been simple: avoid them. A review publicized this week says one source deserves an exemption. The trans fats that occur naturally in milk, butter and cheese, it concludes, are not linked to heart disease or type 2 diabetes.

The review was published in March in the journal Nutrition Research and announced by the University of Reading in May. ScienceDaily carried the university’s account on October 7 under the headline that not all trans fats are bad.

Two facts belong beside that headline. The review was paid for by the dairy industry, and the World Health Organization (WHO) takes the opposite position. The evidence in between is real but limited, and much of it turns on how little trans fat dairy foods contain.

What trans fats are, and why dairy has them

Trans fats are a family of fats defined by the shape of their molecules. Chemists describe them as unsaturated fatty acids with at least one double bond in the trans configuration. The label describes the geometry of a chemical bond, which gives the molecule a straighter shape.

They reach the diet by two routes. Food manufacturers make them by partial hydrogenation, a process that turns liquid vegetable oil into a firmer fat. The other route is biological. Bacteria carry out a similar reaction in the rumens of cows and sheep. Rumen means the first and largest chamber of the stomach, where feed is fermented. The products end up in the animal’s milk and meat.

The industrial kind built the reputation. By 2006, a review in the New England Journal of Medicine could state that the intake of trans fat has been associated with coronary heart disease, sudden death from cardiac causes, and diabetes. Bans and reformulation followed in many countries.

That success changed the arithmetic. As early as 2010, Dutch researchers noted that ruminant fats are now the major source of trans fatty acids in most European countries, ruminants being cud-chewing animals such as cattle. The new review opens with the same observation: dairy foods are becoming one of the major dietary sources of trans fatty acids.

The amounts are small. Trans fatty acids make up 2% to 9% of the fat in milk, butter, cheese and beef, by the Dutch group’s figures.

What the dairy trans fat review found

The authors gathered two bodies of evidence. In all, researchers reviewed evidence from 22 studies involving thousands of participants in Europe, Canada and the United States.

Ten controlled trials. These compared the impact on blood lipids of regular dairy foods with dairy foods higher in trans fat. Lipids are blood fats such as cholesterol. The enriched dairy was produced by changing what the cows ate. Across the trials, participants consumed between 1.3 and 13.2 grams of trans fat per day.

The pooled results showed no difference for total cholesterol, for LDL cholesterol, the kind tied to artery disease, for blood fats called triglycerides, or for two proteins that carry cholesterol. The one exception was HDL cholesterol, often called the good kind. It was slightly lower with the enriched dairy, by 0.05 millimoles per liter, or about 2 mg/dL in US units, a difference at the edge of what the trials could detect.

Twelve long-term studies. These followed people for years, with some studies lasting more than two decades. They measured two dairy trans fats in participants’ blood, trans vaccenic acid and trans palmitoleic acid. Higher levels were not associated with increased risk of cardiovascular diseases, with death from them, or with type 2 diabetes. These studies were reviewed one by one and not pooled into a single estimate.

The authors conclude that eating trans fats from dairy was not linked to adverse effects on cardiometabolic health. Cardiometabolic means relating to the heart, the blood vessels and blood sugar.

One feature of the design is unusual. The authors state that they hypothesized from the outset that dairy trans fats would have no effect on biomarkers for or risk of cardiovascular diseases, biomarkers being blood measures linked to disease. The result matched what they set out expecting.

Ian Givens, a professor at Reading and one of the lead authors, put it in everyday terms. “This research should give people reassurance that dairy, eaten as part of a balanced diet, is not something to worry about for your heart,” Givens said.

Who funded the trans fat review

The version of the story that most readers saw does not say. ScienceDaily’s copy carries no funding statement.

The university’s own page does. This research was funded by the National Dairy Council, it says, referring to the American industry body. It adds that several authors work for or receive funding from dairy trade or industry organizations. The paper’s funding record also names the French dairy industry body known as CNIEL.

Industry money does not make a result wrong. It is a reason to check the result against work the industry did not pay for. The release calls the research the first of its kind, though earlier pooled analyses of the same question exist, and they did not all agree with it.

What earlier trans fat studies have found

A 2010 comparison. The same Dutch group, at a university in Amsterdam, pooled feeding studies of both kinds of trans fat. Their 39 studies provided results of 29 treatments with industrial trans fatty acids, six with ruminant ones and 17 with a related dairy fat. For each 1% of dietary calories, industrial trans fats raised the ratio of LDL to HDL cholesterol, a shift in the unfavorable direction, by 0.055 and ruminant trans fats raised it by 0.038. The two figures could not be told apart statistically. The authors concluded that all fatty acids with a double bond in the trans configuration raise the ratio of plasma LDL to HDL cholesterol.

A disagreement noted in 2015. A large review in the BMJ recorded that a separate pooled analysis of 13 trials, by a group with the same lead author as the new review, had found no such relationship. The reasons for this discrepancy are unclear, the BMJ authors wrote.

A tightly controlled diet trial. The most direct test came from a US Department of Agriculture research center. It was a feeding trial in 106 healthy adults who ate each of four tightly controlled diets for 24 days. One diet supplied about 3% of calories as vaccenic acid, the main dairy trans fat. Another supplied the same amount of industrial trans fat.

Both raised LDL cholesterol compared with the control diet, by between 2% and 6% across the measures reported. Vaccenic acid raised LDL more than the industrial fat did, and it raised HDL too. The authors noted that earlier human intervention studies have been limited, underpowered, and not well controlled. Underpowered means too small to detect a modest effect.

Long-term health. The BMJ review also pooled studies that tracked illness. Industrial trans fats were associated with a 42% higher rate of coronary heart disease. Ruminant trans fats were not associated with it. The authors attributed the gap to quantity, saying the harm from trans fats overall was probably because of higher levels of intake of industrial trans fats than ruminant trans fats. One dairy fat even pointed the other way. Ruminant trans-palmitoleic acid was inversely associated with type 2 diabetes, meaning people with more of it in their blood had less diabetes.

Why the dose of dairy trans fat matters

Quantity explains part of the disagreement, though not all of it.

In the feeding trial, vaccenic acid at 3% of calories behaved like a trans fat. On a 2,000-calorie diet that is nearly 7 grams a day. In the long-term studies, where people ate ordinary amounts of dairy, no harm showed up. The BMJ authors drew the connection themselves, suggesting that the lack of association of ruminant derived trans fats with cardiovascular outcomes reflects how little of them people eat.

Simple arithmetic shows how little. The WHO’s limit for all trans fat, described below, is 2.2 grams a day. If trans fats are 2% to 9% of dairy fat, reaching that takes somewhere between 24 and 110 grams of milk fat.

So the new review and the independent studies may not be far apart on what happens at ordinary intakes. They part company on the general claim. Saying dairy trans fats are not linked to harm at the amounts people eat is a narrower claim than saying they are a different, safer kind of fat.

Quantity does not explain everything. The review’s trials went as high as 13.2 grams a day and still found no change in LDL cholesterol, where the feeding trial found one at about 7 grams. The designs differ. The feeding trial swapped purified fats into a controlled diet. The review’s trials compared ordinary dairy with dairy from cows on altered feed, so they tested two versions of a whole food and not trans fat against its absence.

What the WHO says about natural trans fats

The WHO does not make the distinction the review argues for. Its fact sheet, last updated in January 2024, is blunt. Both industrially produced and naturally occurring trans fat are equally harmful.

Its recommended limit covers all trans fat together: less than 1% of total energy intake, which is less than 2.2 g per day on a 2,000-calorie diet. The fact sheet adds that trans fat has no known health benefits. Among the steps it lists for individuals is to “reduce amount of meat and dairy foods from ruminant animals”.

The agency’s campaign for legal bans is aimed at the industrial kind only, which can be removed from the food supply by regulation.

The university’s release says the review could bring clarity to trans fat labeling policies that may cause confusion. The Department of Agriculture trial reached the opposite view on labeling. Its authors said their results are consistent with current nutrition labeling guidelines, under which vaccenic acid is counted as a trans fat.

Limits of the dairy trans fat review

Effects on disease. The trials measured cholesterol and other blood fats. None counted heart attacks or strokes.

Cause, in the long-term studies. A blood level of a dairy fat is a sign of how much dairy someone eats, and of the rest of their diet and life. The studies cannot separate the fat from the people who eat it.

A summary number for the long-term studies. They were described, not pooled.

Independence. The funder is a dairy body, and several authors are employed or funded by dairy organizations. The hypothesis was that there would be no effect.

Anything about industrial trans fats or saturated fat. The review concerns one minor component of dairy fat. It leaves the evidence against industrial trans fats untouched, and it does not address dairy’s saturated fat.

How dairy fits into one person’s diet depends on their wider health, which is assessed by their doctor.

A dairy-funded review found no sign of harm from the trans fats naturally present in dairy at the amounts studied, while a tightly controlled feeding trial of the main dairy trans fat and the World Health Organization treat them like any other trans fat.