Polyunsaturated fatty acids contain two or more double bonds in the fatty acyl chain. They are essential because they cannot be synthesized by the body and have to be obtained from dietary sources.  There are two families of PUFAS that are essential: Omega-3 and Omega-6. The number following "Omega-" represents the position of the first double bond, counting from the terminal methyl group on the molecule.

Omega-3 fatty acids are derived from linolenic acid, they have a double bond three carbons from the methyl end of the acyl chain. Alpha Linolenic Acid (ALA) is the principal Omega-3 fatty acid, which a healthy human converts into eicosapentaenoic acid (EPA), and later into docosahexaenoic acid (DHA) . EPA and the GLA synthesized from linoleic (Omega-6) acid are later converted into hormone-like compounds known as eicosanoids, which aid in many bodily functions including vital organ function and intracellular activity. Good dietary sources include: Fish such as mackerel and salmon, linseed (flaxseeds, rapeseed (canola), soy beans, sesame seeds, avocados, and some dark leafy green vegetables.

Omega-3s are used in the formation of cell walls, making them supple and flexible and improving circulation and oxygen uptake with proper red blood cell flexibility and function. EPA and DHA have a distinctive serum triglycerides lowering effect by inhibiting hepatic very low density lipoprotein (VLDL)-triglyceride synthesis. Omega-3 fatty acids inhibit inflammatory process by influencing the eicosanoid metabolism , they also reduce blood coagulation  by reducing platelet adhesion and aggregation  and provides protective benefits against Coronary heart diseases.

However, Omega-3 deficiencies are linked to decreased memory and mental abilities, tingling sensation of the nerves, poor vision, increased tendency to form blood clots, diminished immune function, increased triglycerides and low density lipoprotein  levels, impaired membrane function, hypertension, irregular heart beat, learning disorders, menopausal discomfort, itchiness on the front of the lower leg(s), and growth retardation in infants, children, and pregnant women.

Omega-6 is the most important linoleic acid, they lower total and LDL cholesterol concentrations when supplied instead ofsaturatted fatty acids. They also support the cardiovascular, reproductive, immune, and nervous systems.

However, Omega-6 PUFAs increase the susceptibility of LDL to oxidative modifications resulting in increased risk of acute myocardial infarction and coronary thrombosis. LA consumption may reduce the level HDL cholesterol increasing the risk for CHD mortality. Omega 6/3 imbalance is linked with serious health conditions, such as heart attacks, cancer, insulin resistance, asthma, lupus, schizophrenia, depression, postpartum depression, accelerated aging, stroke, obesity, diabetes, arthritis, ADHD, and Alzheimer's Disease,

Omega-6 fatty acids in the form of gamma linolenic acid (GLA) and LA are found in the plant seed oils of evening primrose, black currant, borage, and fungal oils, while arachidonic acid (AA) of the omega-6 series is found in egg yolk, meats in general, particularly organ meats, and other animal-based foods.

Trans-fatty acids are polyunsaturated fatty acids that have at least one double bond in the tran position. There is no known role of trans-fatty acids in human health. Their major sources are partially hydrogenated vegetable fat and animal fat. Intake of trans- fatty acids is associated with LDL and increased risk of developing CVD.

Although intake of polyunsaturated fatty acids is encouraged there is need for additional data on the misconceptions so rounding them. Fat intake should basically be based on overall body weight existing health conditions, cardiovascular diseases, medications, and the total  daily intake of carbohydrates and proteins.

 

The following links also have dietary information related to lowering cholesterol:
Cholesterol: good or bad for health?
Cholesterol: benefits and how to raise HDL cholesterol level
More food stuffs high/low in cholesterol
Foods of animal origin high in cholesterol- USDA
Foods low in cholesterol content - USDA
Plant food that help lower cholesterol
List of food totally free of cholesterol
Cholesterol content in seafoods (tuna, salmon, shrimp)
Cookbooks for low cholesterol diet


Related links

Fruits and vegetables moderate lipid cardiovascular risk factor in hypertensive patients
References