Trans fats are harmful because they damage your cardiovascular system on multiple fronts: they raise your “bad” cholesterol, lower your “good” cholesterol, trigger widespread inflammation, and stiffen the lining of your blood vessels. Each 2% of your daily calories from trans fats is linked to a 23% higher risk of heart disease, according to a combined analysis of evidence presented to Congress by researchers at Harvard. That’s a steep increase from a small amount of fat.
How Trans Fats Damage Your Heart
Most dietary fats affect either your LDL (“bad”) cholesterol or your HDL (“good”) cholesterol. Trans fats do both, and in the wrong directions. They push LDL up and pull HDL down, creating the exact lipid profile that accelerates plaque buildup inside your arteries. Saturated fat, often considered the main dietary villain for heart health, at least leaves HDL alone. Trans fats are uniquely destructive in this regard.
The 23% increase in heart disease risk per 2% of calories sounds abstract, so here’s some context. Two percent of a 2,000-calorie diet is just 40 calories, or roughly 4.5 grams of fat. That’s the amount you’d find in a single serving of some commercially fried foods or packaged baked goods from the era before regulations tightened. A small daily exposure, sustained over years, meaningfully changes your odds of a heart attack or stroke.
Inflammation Throughout the Body
Beyond cholesterol, trans fats provoke a chronic, low-grade inflammatory response. A study of 730 women from the Nurses’ Health Study found that those who ate the most trans fat had C-reactive protein levels 73% higher than those who ate the least. C-reactive protein is one of the body’s primary markers of systemic inflammation, the kind linked not just to heart disease but to a cascade of chronic conditions.
The same study measured several other inflammatory signals. Levels of a molecule called E-selectin, which helps immune cells stick to blood vessel walls, were 20% higher in the top consumers. Interleukin-6, a signaling protein that drives inflammatory responses, was 17% higher. Molecules that indicate damage to the inner lining of blood vessels were 10% higher. These differences held up after researchers accounted for body weight, exercise, smoking, alcohol, and intake of other fats. The inflammation wasn’t explained away by other lifestyle factors.
This matters because inflamed blood vessels are more prone to developing the fatty plaques that cause heart attacks. Trans fats don’t just change your cholesterol numbers on a lab report. They actively irritate the tissue where heart disease begins.
Links to Insulin Resistance and Diabetes
Trans fats behave similarly to saturated fats in the body, but with potentially worse effects. Like saturated fat, they appear to reduce the ability of your cells to respond to insulin, the hormone that moves sugar out of your bloodstream. When cells become resistant to insulin, your pancreas has to produce more of it to keep blood sugar under control. Over time, this can progress to type 2 diabetes.
The evidence here is less definitive than for heart disease. Observational studies and some experiments support the connection, but inconsistencies across research make it difficult to pin down a precise risk increase. What is clear is that trans fats take on the harmful properties of saturated fats and, in several measurable ways, amplify them. Researchers have described trans fats as “more atherogenic” than saturated fats, meaning they’re even better at promoting the artery-clogging process.
Industrial Trans Fats vs. Natural Trans Fats
Not all trans fats are manufactured. Small amounts occur naturally in beef, lamb, and dairy products, produced by bacteria in the stomachs of these animals. A natural trans fat called conjugated linoleic acid shows up in butter and cheese.
The question of whether these natural trans fats carry the same risks has been studied, and the short answer is: the evidence doesn’t clearly separate them from the industrial kind. A systematic review from the USDA found limited evidence to support a meaningful biological difference between industrial and natural trans fats when it comes to cholesterol effects. The reason natural trans fats haven’t caused the same level of alarm is mostly about dose. People typically consume natural trans fats in very small quantities. At normal dietary levels, the exposure is low enough that it hasn’t been tied to the same dramatic health outcomes. When researchers tested natural trans fats at seven to ten times the amount people normally eat, the harmful effects looked similar.
Where Trans Fats Come From
Industrial trans fats are created through a process called partial hydrogenation, which pumps hydrogen gas into liquid vegetable oils to make them solid at room temperature. This gave food manufacturers a cheap, shelf-stable fat that made pie crusts flaky, margarine spreadable, and fried foods crispy with a longer shelf life. For decades, partially hydrogenated oils were a staple ingredient in packaged snacks, fast food, frozen meals, and commercial baked goods.
The problem is that partial hydrogenation reshapes the fat molecules into a configuration that your body handles poorly. The “trans” in trans fat refers to the arrangement of atoms across the molecule’s carbon chain. This shape makes the fat behave more like a rigid saturated fat than like the flexible unsaturated oil it started as, but with additional harmful properties that saturated fat doesn’t have.
How Regulations Changed the Food Supply
In 2015, the FDA made a landmark decision: partially hydrogenated oils were no longer “Generally Recognized as Safe.” This effectively meant manufacturers could no longer add them to foods. The deadline to stop production was June 2018, with products already made allowed to remain on shelves through early 2020 for most uses and into 2021 for a few specific applications.
This was a significant public health intervention. Before the ban, Americans consumed an estimated 4 to 5 grams of trans fat per day on average, with heavy consumers taking in far more. The combination of mandatory labeling (introduced in 2006) and the eventual ban drove reformulation across the food industry. Many companies switched to palm oil, fully hydrogenated oils blended with liquid oils, or other alternatives.
Why You Should Still Check Labels
Even with the ban in place, trans fats haven’t completely disappeared. U.S. labeling rules allow a product to declare “0 g” of trans fat if a serving contains less than 0.5 grams. That means a food with 0.4 grams per serving is labeled as zero. If you eat multiple servings, or eat several “zero trans fat” products in a day, the amount adds up. Partially hydrogenated oil on an ingredient list is the clearest red flag, but even fully hydrogenated oils can contain trace amounts of trans fats depending on processing.
Imported foods, some restaurant fryers, and products from smaller manufacturers may also still contain meaningful amounts. The safest approach is to scan ingredient lists rather than relying solely on the nutrition facts panel. If you see “partially hydrogenated” anything, the product contains trans fat regardless of what the label says about grams per serving.