There are some fats you want to avoid. In The Big Fat Surprise, Nina Teicholz covers this topic in detail. Here’s my synopsis, with some of my own petroleum spin added.
We’re biologically adapted to process the fats derived from animals and animal products, which are high in “saturated” fats. Cooking with those fats, e.g. lard and butter, is healthy and produces tasty results. In the 20th Century as U.S. agriculture ramped up due to mechanization using petroleum fuels and chemicals, food producers and food companies started to have vast quantities of cheap vegetable oils, squeezed out of corn, soybeans, cotton, and other plants. Initially those oils had a different consistency and texture than animal fats. The chemists working for the food industry figured out how to “hydrogenate” the oils to make them act like lard and butter, only cheaper. Crisco was substituted for lard, margarine for butter. However, hydrogenation produced “trans” fat, a weirdly shaped fat molecule not found in nature.
It’s not usually a good idea to eat and drink stuff never seen in the history of the human race. In the case of trans fats, they end up in cell membranes in place of omega-3 fatty acids that our bodies normally use as part of the structure of cell. Trans fats interfere with the way a cell functions. The example Nina used that’s telling is that in some tissues, like the heart muscle, that can result in calcification–heart disease. This suggests that the increased incidence of heart disease in this country in the 20th Century could have had something to do with trans fat consumption, although sugar consumption was on the rise, too. Either way the nutrition scientists in the mid-20th Century latched onto cholesterol and saturated fat as the possible cause of heart disease and recommended the low-fat diets, using vegetable oils for what little fat was included. Those diets made the problems worse, not better.
In recent years, trans fats have finally been getting the bad rap they deserve, although maybe not for the best scientific reasons. Food manufacturers have hurriedly developed new vegetable oil formulations that don’t contain trans fats. Instead those oils produce a nasty compound called an “aldehyde” when heated, like when you fry a french fry or chicken wing. The aldehydes may be even worse than the trans fats, but haven’t been studied properly. Almost any fried food from a fast food places is likely using the vegetable oils that turn nasty when heated. This stuff has caused a varnish that coats surfaces and sticks poor test rats to the bottom of their cages
There are vegetable oils that are fine to eat and cook with. They are more heat stabile. Nina cites palm oil, coconut oil, and olive oil. Avocado oil as well, I think. People have lived on coconuts and coconut oil for thousands of years. Palm oil is closely related and similarly edible. Olive oil is reasonably edible, although not as wonderful as Mediterranean Diet promoters claim. For awhile, tropical oils were forced out our food supply by the temperate zone oil growers, like the American Soybean Association, who didn’t want the competition. Lately coconut oil has been making a comeback.