What's the difference between fat, trans fat, and saturated fat?

More simply saturated fat is solid at room temperature while unsaturated fat is liquid. While animal fats tend to be saturated and vegetable unsaturated coconut and palm seed oils are saturated vegetable oils. What makes fats saturated or unsaturated is the number of hydrogen atoms bonded to the carbon atoms in the fatty acid chains. Years ago they discovered that if they broke the double carbon bond in fatty acids in vegetable oil and bonded the carbon atoms with hydrogen that liquid vegetable oil became solid. This is called hydrogenated oil. Most vegetables shortenings used to be partially hydrogenated vegetable oil. Since the fatty acids are not fully saturated they are called trans fat. When I was in high school chemistry in ‘78 my teacher warned us about partially hydrogenated vegetable oil. At the time vegetable shortening was sold as a healthier alternative to butter or lard. My teacher warned us that hydrogenating vegetable oils made them just as bad for us as saturated animal fats. While his advice was far ahead of its time it turned out that he didn’t go far enough. Now we know that trans fats are much worse for us than lard or butter. I recently noticed that “hydrogenated vegetable oil” is listed in some products that brag to be trans fat free. Now food makers are using fully hydrogenated vegetable oil in products. Fully hydrogenated vegetable oil is not a trans fat and food producers claim that fully hydrogenated vegetable oil is not unhealthy but considering that they were wrong about partially hydrogenated vegetable oil 50 years ago I’m inclined to be skeptical about their claims for fully hydrogenated vegetable oil now. I prefer natural fats untouched by chemists.

The value of each type of fat in the human diet is still widely debated. People in the Weston Price cult believe that saturated animal fats and cholesterol are almost a panacea while others point out that our bodies can make all the cholesterol and saturated fats it needs and that there is no dietary requirement of either. Most studies have leaned in the favor of reducing saturated animal fats including ones you list as healthy. High consumption of animal fats is a risk factor for dyslipidemia as well as breast, ovarian, colorectal, and prostate cancers.