Tuesday, May 6, 2008

Defining fatty acids and their relationship to dietary fat

Fatty acids are the building blocks of fats. Chemically speaking, a fatty acid is a chain of carbon atoms with hydrogen atoms attached and a carbon-oxygenoxygen-hydrogen group (the unit that makes it an acid) at one end. All the fats in food are combinations of fatty acids. Nutritionists characterize fatty acids as saturated, monounsaturated, or polyunsaturated, depending on how many hydrogen atoms are attached to the carbon atoms in the chain. The more hydrogen atoms, the more saturated the fatty acid. Depending on which fatty acids predominate, a food fat is likewise characterized as saturated, monounsaturated, or polyunsaturated.
  • A saturated fat, such as butter, has mostly saturated fatty acids. Saturated fats are solid at room temperature and get harder when chilled.
  • A monounsaturated fat, such as olive oil, has mostly monounsaturated fatty acids. Monounsaturated fats are liquid at room temperature; they get thicker when chilled.
  • A polyunsaturated fat, such as corn oil, has mostly polyunsaturated fatty acids. Polyunsaturated fats are liquid at room temperature; they stay liquid when chilled.
So why is margarine, which is made from unsaturated fats such as corn and soybean oil, a solid? Because it’s been artificially saturated by food chemists who add hydrogen atoms to some of its unsaturated fatty acids. This process, known as hydrogenation, turns an oil, such as corn oil, into a solid fat that can be used in products such as margarines without leaking out all over the table. A fatty acid with extra hydrogen atoms is called a hydrogenated fatty acid. Another name for hydrogenated fatty acid is trans fatty acid. Trans fatty acids are not healthy for your heart. Because of those darned extra hydrogen atoms, they are, well, more saturated, and they act like —what else? — saturated fats, clogging arteries and raising the levels of cholesterol in your blood. To make it easier for you to control your trans fat intake, the Food and Drug Administration now requires a new line on the Nutritional Facts label that tells you exactly how many grams of trans fats are in any product you buy.

The same smart food chemists who invented hydrogenation have now come up with trans fat–free margarines and spreads, including some that are made with plant sterols and stanols.
Plant sterols are natural compounds found in oils in grains, fruits, and vegetables, including soybeans, while stanols are compounds created by adding hydrogen atoms to sterols from wood pulp and other plant sources. Sterols and stanols work like little sponges, sopping up cholesterol in your intestines before it can make its way into your bloodstream. As a result, your total cholesterol levels and your levels of low-density lipoproteins (otherwise known as LDLs or “bad cholesterol”) go down. In some studies, one to two 1-tablespoon servings a day of sterols and stanols can lower levels of bad cholesterol by 10 to 17 percent, with results showing up in as little as two weeks. Wow!

Fats are characterized according to their predominant fatty acids. For example, as you can plainly see in the table, nearly 25 percent of the fatty acids in corn oil are monounsaturated fatty acids. Nevertheless, because corn oil has more polyunsaturated fatty acid, corn oil is considered a polyunsaturated fatty acid. Note for math majors:

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