Fat is one of the three main macronutrients: fat, carbohydrate, and protein. Fats are a wide group of compounds whose basis is in long-chain organic acids, called fatty acids. More particularly fats are esters of such organic acids formed with the alcohol glycerol. Glycerol is a triol, meaning that it has three chemically active -OH (hydroxyl) groups. Fats are made when each of these three -OH groups reacts with a fatty acid. The resulting fats are called triglycerides. Because of their preponderant aliphatic structure, fats are hydrophobic, generally soluble in organic solvents but generally insoluble in water. Fats made up of shorter chain fatty acids are usually liquid at room temperature, whereas the longer chain fats will be solid. Some ambiguity in terminology arises because the words "oil", "fat", and "lipid" are often used interchangeably. Of these lipid is the general term, because a lipid is not necessarily a triglyceride. Oil is the term usually used to refer to fats that are liquids at normal room temperature, while fat is usually used to refer to fats that are solids at normal room temperature.Fat is important foodstuff for many forms of life, and fats serve both structural and metabolic functions. They are necessary part of the diet of most heterotrophs (including humans). Some fatty acids that are set free by the digestion of fats are called essential because they cannot be synthesized in the body from simpler constituents. There are two essential fatty acids (EFAs) in human nutrition: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). Other lipids needed by the body can be synthesized from these and other fats. Fats and other lipids are broken down in the body by enzymes called lipases produced in the pancreas.Fats and oils are categorized according to the number and bonding of the carbon atoms in the aliphatic chain. Fats that are saturated fats have no double bonds between the carbons in the chain. Unsaturated fats have one or more double bonded carbons in the chain. The nomenclature is based on the non-acid (non-carbonyl) end of the chain. This end is called the omega end or the n-end. Thus alpha-linolenic acid is called an omega-3 fatty acid because the 3rd carbon from that end is the first double bonded carbon in the chain counting from that end. Some oils and fats have multiple double bonds and are therefore called polyunsaturated fats. Unsaturated fats can be further divided into cis fats, which are the most common in nature, and trans fats, which are rare in nature. Unsaturated fats can be altered by reaction with hydrogen effected by a catalyst. This action, called hydrogenation, tends to break all the double bonds and makes a fully saturated fat. To make vegetable shortening, then, liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce saturated fats, which have more desirable physical properties e.g., they melt at a desirable temperature (30–40 °C), and store well, whereas polyunsaturated oils go rancid when they react with oxygen in the air. However, trans fats are generated during hydrogenation as contaminants created by an unwanted side reaction on the catalyst during partial hydrogenation. Consumption of such trans fats has shown to increase the risk of coronary heart diseaseSaturated fats can stack themselves in a closely packed arrangement, so they can solidify easily and are typically solid at room temperature. For example, animal fats tallow and lard are high in saturated fatty acid content and are solids. Olive and linseed oils on the other hand are unsaturated and liquid.Fats serve both as energy sources for the body, and as stores for energy in excess of what the body needs immediately. Each gram of fat when burned or metabolized releases about 9 food calories (37 kJ = 8.8 kcal). Fats are broken down in the healthy body to release their constituents, glycerol and fatty acids. Glycerol itself can be converted to glucose by the liver and so become a source of energy.