This in an excerpt from the article "The Skinny on Fats" from the following website:
http://www.westonaprice.org/know-your-fats/526-skinny-on-fats.html
The Dangers of Polyunsaturates
The public has been fed a great deal of misinformation about the relative virtues of saturated fats versus polyunsaturated oils. Politically correct dietary gurus tell us that the polyunsaturated oils are good for us and that the saturated fats cause cancer and heart disease. The result is that fundamental changes have occurred in the Western diet. At the turn of the century, most of the fatty acids in the diet were either saturated or monounsaturated, primarily from butter, lard, tallows, coconut oil and small amounts of olive oil. Today most of the fats in the diet are polyunsaturated from vegetable oils derived mostly from soy, as well as from corn, safflower and canola.
Modern diets can contain as much as 30% of calories as polyunsaturated oils, but scientific research indicates that this amount is far too high. The best evidence indicates that our intake of polyunsaturates should not be much greater than 4% of the caloric total, in approximate proportions of 1 1/2 % omega-3 linolenic acid and 2 1/2 % omega-6 linoleic acid.30 EFA consumption in this range is found in native populations in temperate and tropical regions whose intake of polyunsaturated oils comes from the small amounts found in legumes, grains, nuts, green vegetables, fish, olive oil and animal fats but not from commercial vegetable oils.
Excess consumption of polyunsaturated oils has been shown to contribute to a large number of disease conditions including increased cancer and heart disease; immune system dysfunction; damage to the liver, reproductive organs and lungs; digestive disorders; depressed learning ability; impaired growth; and weight gain.31
One reason the polyunsaturates cause so many health problems is that they tend to become oxidized or rancid when subjected to heat, oxygen and moisture as in cooking and processing. Rancid oils are characterized by free radicals—that is, single atoms or clusters with an unpaired electron in an outer orbit. These compounds are extremely reactive chemically. They have been characterized as "marauders" in the body for they attack cell membranes and red blood cells and cause damage in DNA/RNA strands, thus triggering mutations in tissue, blood vessels and skin. Free radical damage to the skin causes wrinkles and premature aging; free radical damage to the tissues and organs sets the stage for tumors; free radical damage in the blood vessels initiates the buildup of plaque. Is it any wonder that tests and studies have repeatedly shown a high correlation between cancer and heart disease with the consumption of polyunsaturates?32 New evidence links exposure to free radicals with premature aging, with autoimmune diseases such as arthritis and with Parkinson's disease, Lou Gehrig's disease, Alzheimer's and cataracts.33
Too Much Omega-6
Problems associated with an excess of polyunsaturates are exacerbated by the fact that most polyunsaturates in commercial vegetable oils are in the form of double unsaturated omega-6 linoleic acid, with very little of vital triple unsaturated omega-3 linolenic acid. Recent research has revealed that too much omega-6 in the diet creates an imbalance that can interfere with production of important prostaglandins.34 This disruption can result in increased tendency to form blood clots, inflammation, high blood pressure, irritation of the digestive tract, depressed immune function, sterility, cell proliferation, cancer and weight gain.35
Too Little Omega-3
A number of researchers have argued that along with a surfeit of omega-6 fatty acids the American diet is deficient in the more unsaturated omega-3 linolenic acid. This fatty acid is necessary for cell oxidation, for metabolizing important sulphur-containing amino acids and for maintaining proper balance in prostaglandin production. Deficiencies have been associated with asthma, heart disease and learning deficiencies.36 Most commercial vegetable oils contain very little omega-3 linolenic acid and large amounts of the omega-6 linoleic acid. In addition, modern agricultural and industrial practices have reduced the amount of omega-3 fatty acids in commercially available vegetables, eggs, fish and meat. For example, organic eggs from hens allowed to feed on insects and green plants can contain omega-6 and omega-3 fatty acids in the beneficial ratio of approximately one-to-one; but commercial supermarket eggs can contain as much as nineteen times more omega-6 than omega-3!37
