Trans fats damage your health through multiple pathways at once: they raise your “bad” cholesterol, lower your “good” cholesterol, trigger inflammation, stiffen your blood vessels, and interfere with how your cells respond to insulin. For every 1% increase in daily calories from trans fats, coronary heart disease mortality rises by 12%. No other type of dietary fat packs this many risks into such a small amount.
What Trans Fats Do to Your Cholesterol
Most dietary fats affect cholesterol in one direction. Saturated fat, for instance, raises LDL (the kind that clogs arteries) but also nudges HDL (the protective kind) upward slightly. Trans fats are unique because they push cholesterol in the worst possible combination: LDL goes up while HDL goes down. This double shift widens the gap between harmful and protective cholesterol, accelerating the buildup of fatty plaques inside artery walls.
That plaque buildup is the foundation of coronary heart disease. Over time, narrowed arteries restrict blood flow to the heart, and unstable plaques can rupture, triggering a heart attack. The cholesterol effect alone would be enough to make trans fats dangerous, but it’s only part of the picture.
Inflammation Throughout the Body
Trans fats provoke a low-grade inflammatory response that affects your entire cardiovascular system. A study published in The American Journal of Clinical Nutrition found that trans fat intake is positively associated with higher levels of tumor necrosis factor receptors, markers tied to chronic inflammation. In women with higher body mass, each 1% increase in calories from trans fat was linked to measurably higher C-reactive protein, one of the most widely used blood markers for systemic inflammation.
Chronic inflammation is a driving force behind heart disease, stroke, and type 2 diabetes. When inflammation lingers in your blood vessels, it damages the inner lining, making it easier for cholesterol to penetrate the artery wall and form plaques. Trans fats essentially create the conditions that let their cholesterol effects do maximum harm.
Damage to Blood Vessel Walls
Your blood vessels are lined with a thin layer of cells called the endothelium. This lining controls blood pressure, prevents clotting, and regulates which substances pass into surrounding tissue. Trans fats impair this lining directly.
Research among 730 women found that greater trans fat intake was associated with significantly higher levels of circulating adhesion molecules and E-selectin, both established markers of endothelial dysfunction. When the endothelium stops working properly, blood vessels become stiffer, more prone to clotting, and less able to dilate when your body needs increased blood flow. This dysfunction is one of the earliest detectable steps on the path to heart disease, and it can develop long before cholesterol plaques become large enough to cause symptoms.
The likely mechanism involves trans fats physically embedding themselves into the membranes of endothelial cells and immune cells, disrupting signaling pathways that normally keep inflammation in check.
How Trans Fats Affect Insulin and Blood Sugar
Trans fats may also increase your risk of type 2 diabetes by interfering with insulin sensitivity. When trans fat molecules incorporate into cell membranes, they alter the membrane’s physical properties, making it more rigid. Cell membranes need to be fluid and flexible for insulin receptors to function properly. A stiffer membrane can blunt the cell’s response to insulin, meaning your body needs to produce more of it to move sugar out of your bloodstream.
Animal studies have shown that trans fats reduce fat cell membrane fluidity and impair insulin signaling. They also appear to suppress a key protein in fat tissue that helps regulate how your body stores and burns fuel. Over time, this combination of effects can push the body toward insulin resistance, the metabolic state that precedes type 2 diabetes.
Natural vs. Industrial Trans Fats
Not all trans fats come from a factory. Small amounts occur naturally in beef, lamb, and dairy products, produced by bacteria in the stomachs of ruminant animals. These ruminant trans fats have a slightly different molecular structure than the industrial version, which has led to speculation that they might be less harmful.
The evidence so far is thin. A USDA systematic review found limited support for any substantial biological difference between industrial and ruminant trans fats when ruminant trans fats are consumed at seven to ten times normal dietary levels. In practical terms, the amounts of ruminant trans fat people typically eat from meat and dairy are very small, so they contribute far less risk than the industrial kind. The concern has always centered on industrially produced trans fats, which historically showed up in large quantities in processed foods.
Where Trans Fats Come From
Industrial trans fats are created through partial hydrogenation, a process that bubbles hydrogen gas through liquid vegetable oil to make it more solid and shelf-stable. This process was once widespread because it gave foods like margarine, shortening, baked goods, and fried snacks a desirable texture and long shelf life. Partially hydrogenated oils were the dominant source of trans fats in the food supply for decades.
Fully hydrogenated oils, by contrast, contain very low levels of trans fat. The distinction matters because “hydrogenated” on a label doesn’t automatically mean trans fats are present. It’s the word “partially” that signals the problem.
The FDA Ban and Current Status
In June 2015, the FDA determined that partially hydrogenated oils are not Generally Recognized as Safe. Manufacturers were given until June 18, 2018, to stop adding them to foods, with extended deadlines through January 2020 for products already in distribution. For a small number of specific petitioned uses, the compliance deadline stretched to June 2019 for manufacturing and January 2021 for distribution.
The result is that partially hydrogenated oils have been largely eliminated from the U.S. food supply. Globally, the World Health Organization set a target for worldwide elimination of industrial trans fats by 2023. By the end of 2021, the number of countries with protective regulations had tripled over the previous year, and in early 2024, the WHO began formally recognizing countries that met elimination standards.
How to Spot Remaining Trans Fats
In the U.S., the major sources of artificial trans fat have been removed from the market. But if you’re buying imported foods, shopping in countries without bans, or eating older packaged products, checking labels still matters. The ingredient to watch for is “partially hydrogenated oil” of any type, whether soybean, cottonseed, or palm kernel. Even when a nutrition label reads 0 grams of trans fat, products were historically allowed to round down from amounts below 0.5 grams per serving, meaning small amounts could accumulate if you ate multiple servings.
Foods that were historically high in trans fats include store-bought pie crusts, microwave popcorn, refrigerated dough, non-dairy coffee creamers, and commercially fried foods. Most of these have been reformulated, but the categories are worth knowing if you’re evaluating unfamiliar products or eating in regions where trans fat regulations are still catching up.