September 23, 2011
By Marie Spano, M.S., R.D., Contributing Editor
Its not the fats, its the mix of fatty acids that determines the health benefit or detriment of a given fat or oilfrom monounsaturated and polyunsaturated to saturated, trans and cis, and essential or non-essential fatty acids.
Fatty acid structure
Fatty acids are basically linked carbon chains with attached hydrogen ions, and the structures give rise to the nomenclature. There are two types of unsaturated fatty acids: monounsaturated and polyunsaturated. Monounsaturated fatty acids have one double bond between carbon atoms , whereas polyunsaturated fatty acids have more than one double bond between the carbons. When the term omega" is used, it refers to the position of the double bond relative to the last carbon in the chain.
Hydrogen atoms on the same side of the double bond result in cis fatty acids, creating a distinct kink in the chemical structure. Due to these kinks, cis fatty acids do not stack neatly on top of one another, typically resulting in a fat that is liquid at room temperature. Trans fatty acids have hydrogen atoms on opposite sides of the double bond, creating a more linear structure that can stack tightly on top of other trans fatty acids, making a fat that tend to be more solid at room temperature. Though trans fatty acids are elongated and linear in nature at room temperature, heating can create structural kinks, making typically solid fats that contain trans fatty acids soften or melt.
Essential fatty acids
The human body cannot synthesize the polyunsaturated short-chain fatty acids linoleic acid (LA), an omega-6 fatty acid, and alpha-linolenic acid (ALA), an omega-3 fatty acid, yet they are required for structural and functional purposes in the body. A long-chain omega-6, arachidonic acid (AA), is essential only if LA is missing from the diet. Both LA and ALA are required in small amounts for membrane fluidity, synthesis of eicosanoids and as substrates for biological pathways that produce metabolic products necessary for structural and functional roles within the human body (Biochemistry," 2nd edition, 1994; 2010 Dietary Guidelines for Americans Policy Document).
Vegetable oils are a primary source of LA in the diet and, in developed countries, ALA deficiency is not only very rare, but most adults consume several times more ALA than they need. Less than 0.01% of ALA is converted to AA. Though AA may be an important fatty acid for infant health, there is no clear reason to be concerned about adequate intake or tissue levels of AA in adults (Applied Physiology, Nutrition, and Metabolism, 2007; 32:619-634).
Soybean oil, canola oil, flaxseeds and walnuts are the top food sources of ALA. In vivo studies indicate that approximately 5% of ALA is converted to eicosapentaenoic acid (EPA) and less than 0.5% of ALA is converted to docosahexaenoic acid (DHA) in adults. In infants, approximately 1.0% of ALA is converted to DHA (Applied Physiology, Nutrition, and Metabolism, 2007; 32:619-634; Prostaglandins, Leukotrienes & Essential Fatty Acids, 2009; 80:85-91).
Consumption of ALA-rich foods may improve some risk factors of cardiovascular disease; however, it isnt clear whether ALA is responsible or the other compounds in these nutrient-rich foods, or a combination of the two (American Journal of Clinical Nutrition, 2001; 74:612-619). And though some studies have found that supplementation with ALA may improve endothelial function (Current Opinions in Clinical Nutrition and Metabolic Care, 2011; 14:121-131), others question the biological importance of ALA (Lipids in Health and Disease, 2009; 10:33).
EPA plus DHA are tied to a dose-dependent decrease in triglycerides, a slight decrease in blood pressure (especially in the elderly) and potential antiarrhythmic effects (Clinical and Experimental Pharmacology and Physiology, 2006; 33(9):842-846; Nutrition and Health, 2009; 20:41-49).
Omega-9s are non-essential fatty acids. The most common omega-9 fatty acid is oleic acid, a monounsaturated fatty acid found in soybean (particularly the newer high-oleic varieties), canola, sunflower and olive oils," notes Robert Reeves, head of public affairs working group, Qualisoy, St. Louis.
Omega-9 fatty acids are generally stable and, therefore, are good substitutes for oils containing trans fat," says Reeves. Studies have shown that omega-9 fatty acids help increase HDL (high-density lipoprotein) and decrease LDL (low-density lipoprotein) cholesterol, thus helping reduce plaque buildup in artery walls."
There are two types of trans fatty acids, naturally occurring and man-made. Naturally occurring trans fats are produced by bacteria in the digestive system of ruminants, like cows, and, therefore, are found in dairy and beef. Man-made trans fatty acids are produced when some of the double bonds from liquid oils are broken during hydrogenation (creating partially hydrogenated oils because some double bonds remain).
Naturally occurring trans fats may not be harmful and, in fact, may be beneficial (Lipids, 2011; 46:105-119). However, the 2010 Dietary Guidelines for Americans recommends keeping intake of synthetic trans fat as low as possible, because intake of synthetic trans fats may increase risk of some common chronic diseases, namely, cardiovascular disease.
Speaking of saturation
Monounsaturated fatty acids have one double bond. Substituting saturated fatty acids with monounsaturated fatty acids may lower cholesterol;, however, this substitution does not decrease existing atherosclerosis (Current Atherosclerosis Reports, 2010; 12:391-396).
Saturated fatty acids have no double bonds between carbon atoms and, therefore, all carbon atoms are attached to as many hydrogen atoms as they can hold. The majority of saturated fatty acids raise total cholesterol, LDL and HDL, and the 2010 Dietary Guidelines for Americans recommends consuming less than 10% of calorie intake from saturated fat, and replacing saturated fat in the diet with monounsaturated and polyunsaturated fats to lower blood cholesterol and reduce risk of cardiovascular disease.
However, stearic acid has been found in many clinical studies to not increase LDL cholesterol," says Reeves. It has, therefore, been suggested by the 2010 U.S. Dietary Guidelines Advisory Committee that stearic acid not be considered a cholesterol-raising fatty acid as currently implied by food labels."
The blend of fatty acids in a fat determines its overall effect on health. And the current consensus is to decrease intake of saturated fatty acids (with the exception of stearic acid) and replace these with monounsaturated and polyunsatured fatty acids, with a particular focus on EPA and DHA.
Marie Spano, M.S., R.D., CSCS, is a nutrition communications expert whose work has appeared in popular press magazines, e-zines and nutrition-industry trade publications. She has been an expert guest on NBC, ABC and CBS affiliates on the East Coast. For more information, visit mariespano.com.
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