An unsaturated fat is a fat or fatty acid in which there are one or more double bonds 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. In cellular metabolism hydrogen-carbon bonds are broken down – or oxidized – to produce energy, thus an unsaturated fat molecule contains somewhat less energy (i.e fewer calories) than a comparable sized saturated fat. 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.

[edit] Chemistry and nutrition

Amounts of fat types in selected foods (click to enlarge).

Double bonds may be in either a cis or a trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogen atoms are on the same side of the double bond, whereas in the trans conformation they are on opposite sides (see also Trans fat). Saturated fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and are generally more solid at room temperature than unsaturated fats. Unsaturated chains have a lower melting point, hence increasing fluidity of the cell membranes.

Although both monounsaturated and polyunsaturated fats can replace saturated fat in the diet, trans unsaturated fats should be avoided. Replacing saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. Trans unsaturated fats are particularly risky 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.^[1] 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.^[2]

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

Although unsaturated fats are healthier than saturated fats,^[3] the Food and Drug Administration (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. Most foods contain both unsaturated and saturated fats. Marketers only advertise one or the other, depending on which one makes up the majority. Thus, various unsaturated fat vegetable oils, such as olive oils, also contain saturated fat.

[edit] Role of dietary fats in the prevention of prostate cancer

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.^[4]

[edit] 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.^[5]

[edit] Component in different foods

Fat composition in different foods (click at right to hide or show)

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Food	Saturated	Monounsaturated	Polyunsaturated
	As weight percent (%) of total fat
Cooking oils
Canola oil	7	59	29
Corn oil	13	24	59
Olive oil	13	74	8
Soybean oil	15	24	58
Dairy Products
Cheese,regular	64	29	3
Cheese,light	60	30	0
Milk, whole	62	28	4
Milk, 2%	62	30	0
Ice cream, gourmet	62	29	4
Ice cream, light	62	29	4
Meats
Beef	33	38	5
Ground sirloin	38	44	4
Pork chop	35	44	8
Ham	35	49	16
Chicken breast	29	34	21
Chicken	34	23	30
Turkey breast	30	20	30
Turkey drumstick	32	22	30
Fish,orange roughy	23	15	46
Salmon	28	33	28
Hot dog, beef	42	48	5
Hot dog, turkey	28	40	22
Burger, fast food	36	44	6
Cheeseburger, fast food	43	40	7
Breaded chicken sandwich	20	39	32
Grilled chicken sandwich	26	42	20
Sausage, Polish	37	46	11
Sausage, turkey	28	40	22
Pizza, sausage	41	32	20
Pizza, cheese	60	28	5
Nuts
Almonds dry roasted	9	65	1
Cashews dry roasted	20	59	17
Macadamia dry roasted	15	79	2
Peanuts dry roasted	14	50	31
Pecans dry roasted	8	62	25
Flaxseeds, ground	8	23	65
Sesame seeds	14	38	44
Soybeans	14	22	57
Sunflower seeds	11	19	66
Walnuts dry roasted	9	23	63
Sweets and baked goods
Candy, chocolate bar	59	33	3
Candy, fruit chews	14	44	38
Cookie, oatmeal raisin	22	47	27
Cookie, chocolate chip	35	42	18
Cake, yellow	60	25	10
Pastry, Danish	50	31	14
Fats added during cooking or at the table
Butter, stick	63	29	3
Butter, whipped	62	29	4
Margarine, stick	18	39	39
Margarine, tub	16	33	49
Margarine, light tub	19	46	33
Lard	39	45	11
Shortening	25	45	26
Chicken fat	30	45	21
Beef fat	41	43	3
Dressing, blue cheese	16	54	25
Dressing, light Italian	14	24	58
Other
Egg yolk fat	36[6]	44[6]	16[6]
Unless else specified in boxes, then reference is: [7]

[edit] References

[edit] See also

• Saturated fat
• Monounsaturated fat
• Polyunsaturated fat
• Trans fat
• Iodine number - a chemical analysis method to determine the proportion of unsaturated fat.