The most desirable type of fat, the "cis" fats, are found in most unmodified dietary sources, such as olive oil. Fatty acids in cis fats have their carbons on the same side of the double bond, either above or below when drawn horizontally. Even if their double bonds are all in the cis configuration, polyunsaturated fats should be regarded with caution unless "stabilized" by a higher vitamin E content to offset any increased propensity to autoxidize. In other words, when one considers reactions of vitaletheine modulators with lipid epoxides, oils with lower proportions of saturated and polyunsaturated fats and higher vitamin E contents might be the safest sources of dietary fat. For example, palm oil may have distinct advantages over butter because of its higher vitamin E content.
It should be noted that safflower, palm, and cottonseed oil are the richest sources of vitamin E, considered to be one of the most beneficial of the antioxidant vitamins for general health. Unfortunately, cottonseed oil, corn oil, and peanut oil are more likely to be contaminated with carcinogenic aflatoxins. Olive oil also has many redeeming qualities that may account for its health benefits:
Other terms perhaps mistakenly associated with "better fat" include "partially-hydrogenated" and "poly-unsaturated". Unfortunately, these terms indicate chemical modifications that usually result in either the un-beneficial "trans" form of fatty acids (i.e. fat in which the carbons are attached to opposite corners of its double bonds) or in fat having characteristics of the less desirable, saturated variety. "Trans-free" is an interesting term applied to some new products on the market, but it should be noted that this term unless stated otherwise on the package doesn't necessarily guarantee the exclusive presence of desirable "cis" fatty acids, and it even could be applied to fat substitutes having no nutritional value.
If you need further incentive to select health foods with regards to fat content, remember the old margarine commercial that said "It's not nice to fool with mother nature"? Could this be an ironically fine example of a legal disclaimer for products presumed to contain chemically modified (and presumably mostly trans) fatty acids?
See also Tumor Promoters and Dietary Fat.
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