Absolutely Small - Michael D. Fayer [112]
Trans fats have been strongly linked to heart disease because of their effect on cholesterol levels in the blood. Trans fats may also have a harmful effect on the nervous system. Myelin is a covering that protects neurons. It is composed of about 30% protein and about 70% fat. The two main fats are oleic acid (see Figures 16.2 and 16.4) and docosahexaenoic acid (DHA, see below). Trans fats replace DHA in brain cell membranes and in myelin. Trans fat alters the electrical signals that comprise the messages in the nervous system, affecting communication between neurons. It is remarkable that a change in shape of a molecule, without changing its chemical composition, can make a beneficial food a harmful food.
WHEN ZERO IS ZERO
Because of the mounting evidence that trans fats are harmful to our health, they should be avoided. Consumer advocacy groups have encouraged the required removal of trans fats from cooking oils used in fast food restaurants and in various commercial products. Because of the bad publicity surrounding trans fats, manufacturers try to avoid alerting consumers to their presence. Now that most people know that partial hydrogenation will produce trans fats, some food labels use the term modified instead of the phrase “partially hydrogenated.” Even more amazing is the U.S. government’s definition of 0% trans fat. Government regulations permit a product label to say that an oil contains 0% trans fat if one serving contains less than 0.5 grams of trans fat, but the manufacturer is permitted to define the size of a serving. Let’s say that there are 0.6 grams of trans fat in a tablespoon of oil. One tablespoon is three teaspoons. So, the manufacturer defines a serving as two teaspoons, which contain 0.4 grams of trans fat. Thus, by changing the definition of a serving, the oil has 0% trans fat. This type of labeling is not permitted in Europe and other countries. Minimizing the amount of partially hydrogenated oil that you consume will reduce your exposure to trans fats.
OMEGA 3 FATTY ACIDS
Figure 16.5 is a ball-and-stick model of docosahexaenoic acid (DHA). As mentioned, DHA is an important component of the lining of nerves. It has 22 carbons and six carbon-carbon double bonds. It is highly unsaturated. All of the double bonds are in a cis configuration. DHA is one of a class of unsaturated fatty acids (fats) popularly called omega-3 (ω-3) fatty acids. These fats are thought to be beneficial to human health.
FIGURE 16.5. Ball-and-stick model of docosahexaenoic acid (DHA). DHA is a polyunsaturated fat with 22 carbon atoms and six carbon-carbon double bonds all in the cis conformation.
The better name for this class of fats is n-3 fatty acids, with n referring to the number of carbons. The carbons are numbered beginning with the carbon that forms the carboxylic acid. This carbon is labeled 1. So, for DHA counting around the chain, the last carbon at the opposite end from the carboxylic acid group is carbon 22 (see Figure 16.5). This is n, the number of carbons in the chain. Carbon n-3 is the carbon number that results when 3 is subtracted from the total chain length, n. For DHA, that number is 19 as indicated in Figure 16.5. The fat is an ω-3 fatty acid if the n-3 carbon is double bonded as shown in the figure. α-linolenic acid displayed in Figure 16.3 is also an ω-3 fatty acid. It has 18 carbons, so n-3 is 15. As seen in Figure 16.3, there is a double bond between carbons 15 and 16.
TRIGLYCERIDES
The fats we have discussed so far involve single chains. However, a large class of fats commonly found in the body contains three fatty acid molecules tied together into one molecule. These are called triglycerides. Capric acid is a saturated fat with 10 carbons. Capric acid triglyceride is shown in Figure