SciFri 1.28.05

Trans fats are identified by the term "partially hydrogenated" on a food label. They're nasty, artificial fats, created to add to a food's shelf life.

Fat facts

By Deane Morrison

Published January 29, 2005

You're looking for a package of cookies and want to avoid the most unhealthful fats. The label contains the words "soybean oil." So you buy it. But can you be sure it was the right decision?

In recent years, you've probably heard plenty about "good" and "bad" cholesterol, not to mention trans fat, omega-3s, and all manner of oily things that we should or shouldn't put in our stomachs. It's not easy to sort out all the actors in the debate over fats, but it's worth a try.

First of all, we could not live without the "grease" of life. Fatty substances, or lipids, constitute the bulk of our cell membranes. They are the sheaths that insulate our nerves so they can carry electrical messages; the steroid hormones, including sex hormones; and, of course, the stored food reserves that also keep us warm, pad our derrieres, plump our lips, and send social signals about our health and age.

"Without fat to carry flavor compounds, our taste buds can't pick them up," says Dan Gallaher, a professor of food science and nutrition. "It's hard to get around that problem in fat-free food. The Holy Grail of the food industry is a low-fat cheddar cheese that tastes good."

The fats we consume tend to be delicious or to make other foods delicious, and there lies the rub. Fat-laden foods taste so good because many flavor compounds are fat soluble, meaning they dissolve in fat so you can taste them.

"Without fat to carry flavor compounds, our taste buds can't pick them up," says Dan Gallaher, a professor of food science and nutrition. "It's hard to get around that problem in fat-free food. The Holy Grail of the food industry is a low-fat cheddar cheese that tastes good."

What's up with hydrogen?

If it's solid at room temperature, it's a fat. If it's liquid, it's an oil. In general, the fats and oils in food are all fatty acids. Fatty acids are basically carbon atoms hitched to each other and to varying numbers of hydrogen atoms.

If a fatty acid--like stearic acid--has all the hydrogen atoms bonded to it that it can hold, it's called a saturated fat (it's saturated with hydrogen atoms).

Some fatty acids, like linolenic acid--found in flaxseed, fish, and canola oil--have room for more hydrogen atoms. They're called polyunsaturated fats because they have several places (thus the poly) where hydrogen could be added (thus they're "unsaturated" with hydrogen).

Monounsaturated oils, such as canola, olive, and peanut oils, contain just one carbon and hydrogen bond (thus the mono) and are considered healthy.

Then there are omega-3 fatty acids, referring to the fact that there's room for hydrogen atoms starting three carbon atoms in from the end of the fat molecule (omega is the last letter of the Greek alphabet). The other "omega" acids are the omega-6s, which have room for hydrogen atoms six carbon atoms from the end of the molecule.

Omega-3 fats are associated with good heart health, but modern diets include too little of them because people don't eat enough fatty fish. What's needed is a balance between omega-3 and omega-6 fatty acids, with the scales weighted slightly on the side of the omega-6s. Ideally, one should eat three to five omega-6s for each omega-3. But at this point in our history, we get too much omega-6, found in corn oil and so in corn-fed beef and products containing corn oil.

Trans fats are associated with poor heart health. They are artificially produced by the process of partial hydrogenation, in which hydrogen is added to unsaturated fats in order to prolong the shelf life of foods.

To avoid trans fat, the best thing to do is look for the term "partially hydrogenated" on a label. In the example at the start of this piece, "soybean oil" all by itself is a good ingredient, but "partially hydrogenated soybean oil" should be avoided. The most notorious sources of trans fat are products that mimic butter, cream, or milk: margarines and shortenings (except nonhydrogenated types), nondairy creamer, nondairy whipped cream substitutes, and some dry cocoa mixes. Also, foods fried in oil, especially oil that is changed infrequently, are loaded with trans fats. That includes much fast food, especially French fries.

A few notes on cholesterol. It's all one substance, and the "bad" LDL and "good" HDL refer to the carrier particles that transport cholesterol either into (bad) or away from (good) our arteries. But the effect of eating cholesterol isn't the same for everybody.

Trans fats are bad news, but even those reputed paragons of virtue, the polyunsaturated oils, can be problematic. Paul Addis, a retired professor of food science and nutrition, says that certain carbon atoms in polyunsaturated oils can be unstable and lead to the formation of toxic substances. If you take fish oil capsules, for example, he recommends cutting a capsule open and taking a whiff to make sure the oil hasn't spoiled.

"If it it smells like open ocean, it's OK," says Addis. "But if it has a strong fish odor, don't eat it."

Likewise, the villains in black hats--saturated fats--aren't necessarily all bad. Gallaher, working with colleague Craig Hassel, found that rats fed stearic acid (a saturated fatty acid) excreted more cholesterol in their droppings. Later, working with other colleagues, Gallaher found this happened because dietary stearic acid interfered with the absorption of dietary cholesterol in the rats' intestines.

A few notes on cholesterol. It's all one substance, and the "bad" LDL and "good" HDL refer to the carrier particles that transport cholesterol either into (bad) or away from (good) our arteries. But the effect of eating cholesterol isn't the same for everybody.

"Some people respond to increased cholesterol in their diet with an increase in blood cholesterol, and some don't," says Gallaher. It would be wonderful to have an easy way to tell who's who. But for now, he says, the best marker for predicting heart disease is to keep getting your cholesterol levels checked.

Of all the properties of fats and oils, the most unwelcome is their tendency to be fattening. In all three categories of food--fats, proteins, and carbohydrates--we derive energy from breaking bonds between carbon and hydrogen atoms. Fat packs about twice the energy, gram for gram, as either protein or carbs because it is so rich in hydrogen.

To avoid excess fat and the wrong kinds of fat, Gallaher advises several measures: Trim visible fat from meat and buy lean meats; if you must fry, change the cooking oil frequently and use oils with lots of omega-3s, like canola; avoid partially hydrogenated oils in food; and eat fatty fish, flaxseed and canola oil.

A couple of parting thoughts in defense of fats. Fats are vital because they allow us to absorb fat-soluble vitamins. Without fats, we would fall apart from lack of vitamins A, D, E, and K. Fats also signal youth and, in some cases, beauty. A well-rounded face with full lips tells the world we're young and healthy. Lotions help us retain some skin firmness because they contain oils, which seal in water. And as much as we struggle to lose weight, over the years, we mourn the loss of fat when it disappears from under our eyes and from our lips. Sometimes the losses can be quite striking. Have you seen Mick Jagger recently?