An essential fatty acid is one that your body needs but cannot assemble from other fats in your diet. For proper nutrition, you must include them in your diet. 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-oxygen-oxygen-hydrogen group (the unit that makes it an acid) at one end.

In 2002, the Institute of Medicine (IOM) published the first daily recommendations for two essential fatty acids, alpha-linolenic acid and linolenic acid. The former is an omega-3 fatty acid that’s found in fish oils, milk, and some veggie oils. The latter is an omega-6 fatty acid (ditto), found in safflower and corn oil. IOM recommends that

  • Women get 12 grams linolenic acid and 1.1 grams alpha-linolenic acid per day

  • Men get 17 grams linolenic acid and 1.6 grams alpha-linolenic acid per day

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.

Margarine, which is made from unsaturated fats such as corn and soybean oil is solid because it’s been artificially saturated. Chemists add hydrogen atoms to some of its unsaturated fatty acids—through a process, known as hydrogenation. 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. They contribute to clogged arteries and raised cholesterol levels in your blood.

The following table shows the kinds of fatty acids found in some common dietary fats and oils. Fats are characterized according to their predominant fatty acids. For example, 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: Some of the totals in the table don’t add up to 100 percent because these fats and oils also contain other kinds of fatty acids in amounts so small that they don’t affect the basic character of the fat.

What Fatty Acids Are in That Fat or Oil?
Fat or Oil Saturated Fatty Acid (%) Monounsaturated Fatty Acid (%) Polyunsaturated Fatty Acid (%) Kind of Fat or Oil
Canola oil 7 53 22 Monounsaturated
Corn oil 13 24 59 Polyunsaturated
Olive oil 14 74 9 Monounsaturated
Palm oil 52 38 10 Saturated
Peanut oil 17 46 32 Monounsaturated
Safflower oil 9 12 74 Polyunsaturated
Soybean oil 15 23 51 Polyunsaturated
Soybean- cottonseed oil 18 29 48 Polyunsaturated
Butter 62 30 5 Saturated
Lard 39 45 11 Saturated

Because more than one-third of its fats are saturated, nutritionists label lard a saturated fat. Nutritive Value of Foods (Washington, D.C.: U.S. Department of Agriculture); Food and Life (New York: American Council on Science and Health).