Not all fats are harmful; many are essential for normal bodily and brain functions. Some fats, known as good fats, can even decrease the risk of cardiovascular diseases.

A fat molecule consists of a glycerol head and three fatty acid tails with hydrocarbon chains. Saturated fats have hydrocarbon chains where all carbons are fully bound to hydrogen atoms, resulting in single bonds and a straight shape. This makes them compact and solid at room temperature.

Unsaturated fats have fewer hydrogen atoms, with some carbon atoms forming double bonds. These double bonds can bend the hydrocarbon chain, creating gaps that make the fats less compact and usually liquid at room temperature. Monounsaturated fats have one double bond, while polyunsaturated fats have multiple.

Dietary fats provide fatty acids for cell membrane synthesis. Unsaturated fatty acids contribute to membrane fluidity, aiding transport and cellular signaling. Too much saturated fat can make the membrane rigid, hindering cellular responsiveness. Membrane fluidity is crucial for the nervous system, and a specific ratio of unsaturated to saturated fatty acids is needed for myelin formation, which speeds up electrical signals in neurons.

The body can synthesize most fatty acids, but omega-3 and omega-6 polyunsaturated fatty acids are essential and must be obtained from the diet. Unsaturated fats are generally healthier than saturated fats, as they decrease bad cholesterol (LDL) and increase good cholesterol (HDL), while saturated fats increase both.

Trans fats are the unhealthiest type of unsaturated fat. A double bond can be cis (hydrogens on the same side, bending the molecule) or trans (hydrogens on opposite sides, making it more symmetric and straight). Trans fats are structurally similar to saturated fats and rarely occur naturally, meaning the body lacks enzymes to break them down effectively. They increase bad cholesterol (LDL) and reduce good cholesterol (HDL), having detrimental effects on blood vessels.

Trans fats are primarily found in partially hydrogenated oil products like margarine. Food manufacturers partial hydrogenate unsaturated fats to prolong shelf-life and create solid or semi-solid textures preferred by bakers. This process converts some cis double bonds into trans configurations. The FDA banned the production of partially hydrogenated oils in June 2018, though older products might have been in use until January 2020.