In the United States, coronary heart disease kills half a million people every year [source: U.S. FDA]. It starts with the clogging of the arteries. When the heart's arteries get clogged, it's called atherosclerosis. Once you understand the traits of the arteries, it's easy to see why they'd get clogged by certain fats.
Arteries are smooth and flexible. They have to be able to let blood pass without difficulty, and they have to move easily with the heart's pumping motion. Smooth and flexible means the walls of the arteries are very soft, and it's easy for substances to stick to soft walls. Imagine pumping a thick gel through a rubber pipe. A lot of that gel is going to end up stuck to the walls of the pipe. That's kind of what happens when there's too much trans and saturated fat in the bloodstream: It sticks to the walls, narrowing the passageway that blood has to pass through. What happens next is even worse.
This narrowing tells the body that something is wrong, and the immune system releases chemicals to try to fix the problem. But these chemicals make the artery walls even stickier, and even more substances end up getting stuck there. Proteins and calcium in the blood join the fats that are clogging the arteries, and the reaction that joins these molecules together creates a material called plaque.
Plaque then becomes the culprit. It's usually hard on the outside and mushy on the inside, and it tends to crack. When it cracks, it exposes the soft, sticky inside, and as usual, sticky is bad. Platelets in the blood -- essentially, sticky pieces of blood cells that allow blood to clot when you get injured -- stick to the plaque, and blood clots in the arteries result.
At this point, the arteries are so narrow, blood has a hard time getting to and from the heart. The heart has to work overtime just to move small amounts of blood, and it ends up damaged from the exertion and lack of oxygen (oxygen is supplied by blood). This is when heart attacks occur.
The connection between fats and cholesterol is key to understanding why fats can lead to atherosclerosis. It's pretty simple: Cholesterol is essential to the function of cells. It's a fat, and it's a primary ingredient in cell membranes. Without cholesterol, our cells wouldn't be able to work.
But not all cholesterol is good. There are two types of cholesterol in our bodies, and both come from what we eat. LDL cholesterol (low-density lipoprotein, "bad cholesterol") helps plaque form in the heart's arteries. HDL cholesterol (high-density lipoprotein, "good cholesterol") makes it harder for plaque to form.
There are two main types of fats that increase LDL cholesterol and so increase the risk of heart disease. Saturated fats and trans fats are the types that get deposited in arteries. Until very recently, most fast food french fries were cooked in the worst of these fats, trans fats.
Ironically, trans fats in french fries are the result of the fast food industry trying to address concerns over the animal-based oil those potatoes used to be fried in. In switching from saturated beef tallow to a healthier polyunsaturated fat, the industry ended up using something worse.