What's more likely -- death by an auto accident or death by french fries?

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.

When health concerns developed over the use of saturated animal oils in french fries, the fast food industry changed its cooking oil. They switched to a polyunsaturated vegetable oil. That was good. But in treating that unsaturated oil to make it better for frying, they ended up with trans fats. That was bad.

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Trans fats aren't as rampant as they were a few years ago, but their ability to accumulate over a lifetime still makes them dangerous.

­The term "saturated" doesn't refer to how densely fatty something is. It refers to the amount of hydrogen in a fat molecule.

A fat molecule is made up of carbon, hydrogen and oxygen. When all of the open slots in the molecule are filled with hydrogen ("hydrogenated"), that fat is hydrogen saturated, or just "saturated." Hydrogenation makes oil a solid at room temperature, as in margarine, and keeps it from going bad so quickly on store shelves. Whether a fat is "saturated" or "trans" refers mostly to the arrangement of those carbon and hydrogen atoms. The placement of hydrogen atoms in saturated fats makes the molecules bend; the placement in trans fats keep them straight. That's the big chemical difference. The health difference is much bigger.

Saturated fat molecules, found in foods like beef, butter and doughnuts, tend to increase bad cholesterol and decrease good cholesterol. Trans fats do the same, but worse. A study of 80,000 women showed that a 5 percent increase in daily calories from saturated fat can increase the risk of heart disease by 17 percent. That same study found that a 2 percent increase in trans fat can increase the risk of heart disease in women by more than 93 percent (in men, that number is closer to 25 percent) [source: Straight Dope]. Another study found that high intake of trans fat increases the risk of death by heart attack by 47 percent [source: AICR]. The USDA recommends limiting trans fat intake to 1 percent of your daily calorie intake. So if you eat 2,000 calories a day, you should only consume 20 calories of trans fat, which is about 2 grams. So check nutrition labels -- as of 2006, food manufacturers have to put the amount of trans fat on their packaging.

Now, lots of commercial foods contain trans fats. Why the focus on french fries, then? Until recently, here's why [source: U.S. FDA]:

Not long ago, eating a single serving of fast food french fries got you more than four times your daily healthy allotment of trans fats. These days, major fast food chains don't use trans fats because of health concerns. Unfortunately, fats can start building in the arteries from the preteen years, so we're not exactly out of the woods [source: WebMD].

Which brings us back to our initial question: Which is deadlier, french fries or car accidents? We're far more likely to die from overconsumption of trans fat. In the United States, people have a one-in-84 chance of dying in a car accident, and one-in-five chance of dying from heart disease. That's greater than the risk of dying from cancer, which is one-in-seven [source: NYT].

One the upside, Americans only have a 1-in-340,733 chance of dying in a fireworks accident. Fireworks displays are a fine replacement for sticks of butter [source: NYT].

For more information on heart disease, diet and trans fats, including how trans fats might increase infertility in women, check out the links on the next page.