Forget about the cholesterol you eat, what matters is your liver—the fats you feed it determine whether its mood is helpful or homicidal.

A fat molecule looks a little like a jellyfish: a glycerol head with three fatty acids trailing off of it. The subtle details—the length of the chain, where and how much hydrogen is missing and how the chain bends—all matter to your liver. In naturally occurring fats, the fatty-acid chains can be straight or bent. When the chains are straight, the fats can pack in tightly and are solid at room temperature. Butter, red meat, dairy, cocoa—all the tasty things in life—deliver the straight-chain saturated fats that play pretty well with our livers, causing both good HDL and bad LDL cholesterol to increase. While overloading your diet with these fats increases your risk of a heart attack and stroke, in moderation these fats are pals. Besides, saturated fats like these—that pack together well—are the key to delicious baked goods, one of the true joys in life.

The bent chains of unsaturated fats, on the other hand, get in the way of one another, causing the fats to stay liquid at room temperature. Our livers adore fats with one bend—like olive or canola oil—and pump out good HDL cholesterol in gratitude. And we must eat fats with a few bends in the chain—like safflower, sunflower, corn, or fish oil—in order to survive. But these fats easily turn rancid and work poorly in baked goods.

In 1911 Crisco food chemists, seeking a cheaper and more-shelf-stable fat for industrial baking, came up with a clever idea: turn the bends in cheap unsaturated fats into kinks, creating trans fats. A kinked chain can pack in like a straight one, allowing tasty baked goods at a lower price. And as an added bonus, because kinked fats are unnatural, they take longer to spoil. Brilliant! But the problem is our livers hate these fats, and they protest by pumping out way less good HDL cholesterol than normal. Plus, each additional 2 percent of your calories from trans fats nearly doubles your risk of getting heart disease, which is why every reputable source agrees there is no "safe" amount of trans fat in your diet.

And now, as states (California), counties (King County), and cities (New York) are moving to ban artificial fats altogether, food chemists are scrambling to come up with alternatives. Some of these, such as interesterified or fully hydrogenated fats, are even bigger disasters than trans fats, since they anger both the liver and the pancreas. Other ideas, like breeding plants to produce better saturated fats, are right on track. But whatever chemists come up with, your best bet may be to just go back to butter. Clever chemistry can't beat nature. Trust your liver and stick with natural fats.