Free Mediterranean Diet

An unsaturated fat is a fat or fatty acid in which there is one or more double bond in the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond. Where double bonds are formed, hydrogen atoms are eliminated. Thus, a saturated fat is “saturated” with hydrogen atoms. The greater the degree of unsaturation in a fatty acid (ie, the more double bonds in the fatty acid), the more vulnerable it is to lipid peroxidation (rancidity). Antioxidants can protect unsaturated fat from lipid peroxidation.

Chemistry and Nutrition

Double bonds may be in a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogens are on the same side of the double bond, whereas in the trans conformation they are on the opposite side. Trans fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and more solid at room temperature than cis fat. But trans fats reduce the fluidity (and functionality) of cell membranes.

Both mono- and polyunsaturated fats can replace saturated fat in the diet; trans unsaturated fats should be avoided. Substituting saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. This effect is attributed to the low melting point of unsaturated fats found in food. Trans unsaturated fats are particularly bad because the double bond stereochemistry allows the fat molecules to assume a linear conformation which leads to efficient packing (i.e., plaque formation). The geometry of the cis double bond introduces a bend in the molecule precluding stable formations (see specific fatty acid links above for drawings that illustrate this). Natural sources of fatty acids (see above) are rich in the cis isomer.

Although polyunsaturated fats are protective against cardiac arrhythmias a study of post-menopauseal women with a relatively low fat intake showed that polyunsaturated fat was positively associated with progression of coronary atherosclerosis, whereas monounsaturated fat was not . This probably is an indication of the greater vulnerability of polyunsaturated fats to lipid peroxidation, against which Vitamin E has been shown to be protective.

Examples of common unsaturated fats are palmitoleic acid, oleic acid, linoleic acid, and arachidonic acid. Foods containing unsaturated fats include avocado, nuts, and soybean, canola, and olive oils. Meat products contain both saturated and unsaturated fats.

Although unsaturated fats are not as unhealthy as saturated fats, the old FDA recommendation stated that the amount of unsaturated fat consumed should not exceed 30% of one’s daily caloric intake (or 67 grams given a 2000 calorie diet). The new dietary guidelines have eliminated this recommendation.

Insulin resistance correlates positively with monounsaturated fat (especially oleic acid) and negatively with polyunsaturated fat (especially arachidonic acid) in the phospholipids of human skeletal muscle .

Membrane composition as a metabolic pacemaker

Cell membranes of mammals have a higher composition of polyunsaturated fat (DHA, omega-3 fatty acid) and a lower composition of monounsaturated fat than do reptiles. Higher polyunsaturated membrane content gives greater membrane fluidity (and functionality), commensurate with the higher metabolic rate of the warm-blooded species. In fish, however, increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, presumably to maintain greater membrane fluidity (and functionality) at the lower temperatures^[4]

Saturated fat is fat that consists of triglycerides containing only saturated fatty acids. Saturated fatty acids have no double bonds between the carbon atoms of the fatty acid chain (hence, they are fully saturated with hydrogen atoms). There are several kinds of naturally occurring saturated fatty acids, with their only difference being the number of carbon atoms - from 1 to 24. Some common examples of saturated fatty acids are butyric acid with 4 carbon atoms (contained in butter), lauric acid with 12 carbon atoms (contained in Breast milk, coconut oil, palm oil, and cocoa butter), myristic acid with 14 carbon atoms (contained in cow milk and dairy products), palmitic acid with 16 carbon atoms (contained in meat) and stearic acid with 18 carbon atoms (also contained in meat).

Fat that occurs naturally in living matter such as animals and plants is used as food for human consumption and contains a varying proportion of saturated and unsaturated fat. Foods that contain a high proportion of saturated fat are butter, ghee, suet, tallow, lard, coconut oil, cottonseed oil and palm kernel oil, dairy products (especially cream and cheese), meat as well as some prepared foods.

Dehydrogenation converts saturated fats to unsaturated fats, while hydrogenation accomplishes the rever

Health issues

Diets high in saturated fat correlate in some studies with an increased incidence of atherosclerosis and coronary heart disease. Some studies suggest replacing saturated fats in the diet with unsaturated fats will increase one’s ratio of HDL to LDL serum cholesterol.

Controversy

It has been alleged that the many studies of saturated fat in the diet do not distinguish between saturated fat and trans fat. Some claim that saturated fat (in the absence of trans fat) is healthful; for example, foods such as peanuts and pure peanut butter (peanut butter having no added partially hydrogenated vegetable oil) contain saturated fat but no trans fat. Such foods may be beneficial or may be a health hazard; no research specific to this question has as yet been done.

Another confounding issue may be the formation of exogenous (outside the body) Advanced Glycation End Products (AGEs) and oxidation products generated during cooking, which it appears some of the studies have not controlled for. It has been suggested that, “given the prominence of this type of food in the human diet, the deleterious effects of high-fat foods may be in part due to the high content in glycotoxins, above and beyond those due to oxidized fatty acid derivatives.” (Koschinsky, 1997) The glycotoxins, as he called them, are more commonly called AGEs.

Table

Source: Composition of Foods. Fats and Oils. AH No. 8-4. U.S.D.A.