Other links: Fat Basics | Healthy Food Fats

WHAT ARE FATS?

Fats and Fatty Acids: Fats, as we know them in foods and our body upholstery, are a storage form of fatty acids. It is the different types of fatty acids that make one kind of fat different from another. For instance, olive oil contains mainly oleic acid, while corn oil is predominantly linoleic acid. Individual fatty acids serve different purposes in the body - some are "burned" or oxidized for energy, some are structural features of cell membranes, some are converted to other types of fatty acids or other substances such as sterols, and others perform special duties in tissues such as nerve cells.

Fats are known technically as triglycerides because they contain 3 fatty acids attached to a 3-carbon backbone. When a doctor tests your blood for lipids, one component of the analysis is triglycerides. These may be elevated above normal in certain diseases like type 2 diabetes and pancreatic disease. They will also shoot up if you're too fond of alcohol!

Types of Fatty Acids: Fatty acids are of three types: saturated, monounsaturated, and polyunsaturated. These terms describe the structure of the fatty acid in terms of whether it is fully loaded with hydrogen. A saturated fatty acid has all the hydrogen it can hold and forms a straight chain. These stack tightly in cells providing rigidity and making food fats like butter solid at room temperature. Saturates have a similar effect in cell membranes. Many saturated fatty acids increase blood cholesterol levels and for that reason, are considered less healthful. However, they are an important source of energy.

Unsaturated Fatty Acids: Monounsaturated fatty acids, such as oleic acid, have lost a pair of hydrogens. Where hydrogens are missing, a "double bond" forms in the chain, causing the fatty acid to bend. The presence of double bonds in a fatty acid makes it unsaturated.

If two or more double bonds occur in the fatty acid, it is called "polyunsaturated." The more double bonds there are, the more unsaturated and kinkier the fatty acid becomes. This is illustrated in the fatty acids linoleic acid, alpha-linolenic acid, and arachidonic acid (shown right). As unsaturated fatty acids become more bent out of shape, they occupy more space and this makes a fat liquid (an oil) and our cell membranes more fluid. Arachidonic acid, which has four double bonds, is a good example of a polyunsaturated fatty acid.

Omega-3s and Omega-6s: Why Greek names? To distinguish one end of the fatty acid from the other, each of these fatty acid classes has a name. The end with the acid or carboxyl portion (shown in blue in the illustrations) is the acidic end, whereas the far end is the omega end. The location of double bonds in a fatty acid is determined by counting the number of carbons from the omega end of the chain. So, omega-3 fatty acids have their first double bond 3 carbons away from the omega end. Similarly, omega-6 fatty acids have their first double bond 6 carbons from the omega end. Oleic acid, a monounsaturated fatty acid, has its single double bond 9 carbons away from the omega end. Humans cannot convert omega-3 and omega-6 fatty acids to each other.

Omega-3s: These come in "short" and "long" chain varieties. The short-chain form is alpha-linolenic acid, the only omega-3 found in plants (except for some algae). It has 18 carbons and 3 double bonds. It is found in flaxseed oil (53%), canola oil (11%), English walnuts (9%), and soybean oil (7%). Alpha-linolenic acid is considered essential because we cannot make it and we need it. It is the precursor of EPA and DHA, the long-chain forms.

Long-chain or marine omega-3s occur primarily in fish and shellfish. The two main marine omega-3s are EPA or eicosapentaenoic acid, with 20 carbons and 5 double bonds, and DHA or docosahexaenoic acid, with 22 carbons and 6 double bonds.

Long-chain omega-3s can be made from alpha-linolenic acid, but humans perform this conversion very inefficiently (less than 5% is converted).

Essential Fatty Acids: These are polyunsaturated fatty acids the body needs and cannot make for itself or derive from other fatty acids. Linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3) are usually considered essential because they can be converted to long-chain forms, which are the fatty acids most critical to the body's needs. The term "conditionally" essential has been added to the vocabulary of essential fatty acids to denote those fatty acids needed in greater amounts under certain conditions. An example would be the need for DHA during the last trimester of pregnancy and early infancy when the brain is growing rapidly and incorporating substantial amounts of DHA into brain cells.

Lipids: A general term for substances that usually dissolve in organic solvents, but generally not in water. Lipids include fats, fatty acids, sterols, phospholipids, glycolipids, waxes, and other substances. They are essential components of every cell membrane.

*Words in blue are described in the glossary.