Trans fat was the fat you were told to cut from your plate, no exceptions.
For many people, that meant eyeing butter, cheese and whole milk with the same suspicion as fries cooked in old-style margarine.
A new review published in Nutrition Research in 2026 suggests the warning was too broad.
The researchers pooled 22 studies and found that the natural trans fats in dairy did not raise the risk of heart disease, stroke or type 2 diabetes.
Also read: These signs of bowel cancer can easily be mistaken for common stomach problems
The industrial kind, the one in partially hydrogenated oils, remains a different story.
Two kinds of trans fat
Trans fats come in two main forms. One is made in factories by hardening vegetable oils, a process called partial hydrogenation.
The other is made inside the stomachs of cows, sheep and goats, where microbes change the structure of fats in the animal's feed.
Both end up on the label as "trans fat," but their chemistry is not identical.
Also read: A Mayo Clinic AI may predict your pancreatic cancer risk up to 5 years before symptoms
The double bonds sit in different positions along the fatty acid chain, and the body appears to handle them differently.
What the review of 22 studies actually tested
The team, led by nutrition professor Ian Givens at the University of Reading, looked at 10 controlled feeding trials and 12 long-term population studies.
In the trials, people ate between 1.3 and 13.2 grams of dairy trans fat per day.
Their blood cholesterol and other lipid markers did not shift in a meaningful way compared with people on regular dairy.
Also read: WHO reveal official guidelines for Ebola and Marburg diseases
The 12 cohort studies tracked thousands of people in Europe, Canada and the United States for up to 20 years.
Higher dairy trans fat intake was not linked to heart disease, stroke, cardiovascular death or type 2 diabetes.
The review was funded by the National Dairy Council in the United States, and several co-authors work for dairy industry organizations.
The underlying trials and cohort data are still peer reviewed, but the funding source is worth knowing.
Also read: Marburg kills about half the people it infects. Two vaccines are now being tested
It is also worth noting that the World Health Organization still treats both kinds as harmful in its current fact sheet.
Its advice is to keep all trans fat below 1 percent of daily calories, roughly under 2.2 grams a day on a 2,000-calorie diet.
The industrial trans fats still worth avoiding
The FDA ruled in 2015 that partially hydrogenated oils, the main source of industrial trans fat, were no longer "generally recognized as safe."
Food companies had until June 2018 to pull them from most products, and a 2023 rule closed the remaining uses.
Also read: Inside the bat cave where scientists watched Marburg virus prepare to jump to humans
On the label, the clue is the phrase "partially hydrogenated oils."
If it appears in the ingredient list, the product carries industrial trans fat even when the nutrition panel rounds the figure down to zero per serving.
The categories most worth checking are packaged baked goods, microwave popcorn, non-dairy coffee creamers and some fried fast foods, especially products imported from countries without a ban.
What this means for your plate
For most Americans today, the industrial source has already shrunk.
The bigger practical question is dairy, and the new review suggests an ordinary serving of butter, cheese, yogurt or whole milk does not need to be rationed on trans-fat grounds alone.
Dairy is still a concentrated source of saturated fat and calories, and heart organizations continue to advise moderation for people with high cholesterol or existing heart disease.
Anyone making larger changes to their diet, or coming off long-standing dairy restrictions, should talk it through with their doctor.
The current FDA label advice still stands. Scan the ingredient list for partially hydrogenated oils before you put the box in the cart.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.