Food substances which are the chief energy storage mechanism in organisms such as plants and animals. Fat molecules are composed of fatty acid chains attached to a glycerol backbone, usually in threes. Fats vary in the details of just which fatty acid variants are attached. Possibilities include saturated (the usual case in animal source fats), mono or poly unsaturated (from many plant oils, have one (mono) or more (poly) double carbon bonds in the fatty acid chain), and in where the double bond is (at the omega carbon in the 3 position, or the 6 position or ...). Some fats are required in the human diet, they are the 'essential oils'. All other fats humans need can be made, at the cost of some energy, from those fats, or from other chemicals. Transfats are a class of fats which are very rare in nature, but very common in industrially processed oils as a consequence of "hydrogenation". It is becoming increasingly clear that ingestion of more than small quantities of transfats distorts some aspects of human biochemistry in ways which increase artery disease and heart disease, and so increase death rates. Saturated fats have a much smaller deleterious health effect.