The following was originally published 02/05/2004.
Many people try to lose weight by eating diet foods that claim to be prepared in a healthy fashion. Whether they are "nutritionally balanced" frozen dinners, supplement shakes or meal replacement bars, they may not be truly healthy alternatives to whole food.
Many of these prepared foods contain poor ingredients - especially unhealthy fats. Manufacturers love partially hydrogenated oils because they extend shelf life, but in my opinion they are dangerous. For one thing, these oils are sources of trans fatty acids (TFAs), which increase risks of both cardiovascular disease and cancer. TFAs are just as bad, if not worse, for the heart and arteries than saturated fats. They increase total cholesterol, raise "bad" LDL (low density lipoprotein) and lower "good" HDL (high density lipoprotein). Beyond that, TFAs may also have adverse effects on cell membranes and the immune system, and may promote inflammation and aging.
As a matter of fact, the U.S. Food and Drug Administration recently ruled that starting in 2006, food companies must list a product's amount of trans fats on its label. While other fats now are listed in the "Nutrition Facts" box, TFAs can only be found in the long list of side-panel ingredients, where they usually are listed as "partially hydrogenated vegetable oils." The amount of trans fat typically is not given, however a general rule is that the higher these oils are on the ingredient list, the more trans fat there is.
Until 2006, I suggest that you avoid TFAs altogether by checking the labels of any processed foods you buy - even those claiming to be healthy - and stay away from those containing margarine, vegetable shortening and fractionated or partially hydrogenated oils of any kind. I have no objection to frozen and other convenience foods made from healthy ingredients. Read labels!
Andrew Weil, M.D.