chapter 6 脂肪 英文版 营养学.docx

1、chapter 6 脂肪 英文版 营养学Lipid BasicsI dont have any clever song parodies this week. :( I know, its disappointing. Maybe you can make one up and post it in the forums?Our fabulous and exciting topic this week is lipids! So tasty, and so abused. Lets start with the basics, as usual.Technically speaking, t

2、here are three types of molecules that fall under the category of lipids. These aretriglycerides,phospholipids, andsterols. Its pretty easy to see that both triglycerides and phospholipids have fatty acid and glycerol components. but what about sterols? Their chemical structure looks completely diff

3、erent. Even so, the one thing that all three have in common is their hydrophobic nature, and this is the basis of their categorization as lipids.As I just mentioned, both phospholipids and triglycerides have fatty acid components, so lets take a closer look at these for a moment.Eachfatty acidhas am

4、ethyl groupon one end and acarboxyl groupon the other. These are connected by a carbon backbone. This backbone can be just a few or many carbons in length. The fatty acids in foods most commonly contain an even number of carbons in their chains, and this is important because its the carbon backbone

5、that ultimately determines the consistency and the taste of the fat.There are two features of the carbon backbone that most influence its consistency. The first is thelengthof the carbon backbone. If the fatty acid has 2-4 carbons, its considered to be a short-chain fatty acid. If it has 6-10, its a

6、 medium-chain and if it has 12 or more carbons its a long-chain fatty acid. The shorter the fatty acid, the more likely it is to be liquid at room temperature; conversely, the longer the fatty acid, the more likely it is to be solid at room temperature.The second major factor affecting consistency i

7、s the saturation of the carbon chain. Remember from chemistry class that carbon is capable of making four bonds. When all the carbons in a fatty acid chain are all making four bonds each, the fatty acid issaturated. If any of the carbons in the chain are making a double bonds, though, then that fatt

8、y acid is unsaturated. If theres just one double bond, its amonounsaturatedfat, while two or more double bonds make a fatty acidpolyunsaturated. As far as consistency goes, the more saturated a fatty acid is, the more likely it is to be solid at room temperature. Fatty acids that are polyunsaturated

9、 are more likely to be liquid. At the end of the day, both the length and saturation of the chain influence how liquid or solid the fat is. Scientists go back and forth repeatedly between mono- versus polyunsaturated as far as which type of fat is the healthiest to include in our diets. Current thin

10、king is that once we fulfil our minimum need for polyunsaturated fat (discussed in the next section) its best to get the rest of our fat from monounsaturated sources. We should minimize the amount of saturated fats in our diet as much as possible.Its only unsaturated fats that can beessential. Satur

11、ated fats, while extemely important, are not essential by our nutrition definition - your body has the ability to make all it needs so theres no need for you to get them in your diet. If a fat is unsaturated, we can look atthe position of the first double bondin order to figure out whether its an es

12、sential fatty acid or not. When making the determination, we number the methyl carbon as carbon #1. I know, I know, this does not follow the rules you were taught in organic chemistry. I dont know why its done this way for fatty acids, but it is. If it makes you feel better at all, the methyl carbon

13、 is also called the omega carbon. From carbon #1 we count down the chain to the first carbon involved in a double bond. If this ends up being before the ninth carbon in the chain, its an essential fatty acid (a.k.a. essential fat).Check out the polyunsaturated fatty acid in the picture, above. Notic

14、e that the first double bond occurs at carbon number six. This is anomega-6fatty acid! The most common example of an omega-6 fatty acid is linoleic acid. On the other hand, if the first double bond were at carbon number three, it would be anomega-3fatty acid. The omega-3s that youll hear the most ab

15、out are alpha-linolenic acid, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Now, although humans do need to obtain these essential fatty acids in our diet, it doesnt take much - the average adult needs only 2-4 tablespoons of plant oils each day to fulfill the requirement. Fatty fish l

16、ike salmon, halibut, whitefish, etc. are also a great source - as long as theyre baked or broiled and not deep-fried.The omega-3 and omega-6 fatty acids have some vital biological roles. Most importantly, they are the precursors to molecules calledeicosanoids. I know your textbook goes into some pretty gory detail about the various eicosanoids and their functions. However, for our purposes, I would be thrilled