Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated. Carboxylic acids as short as butyric acid (4 carbon atoms) are considered to be fatty acids, whereas fatty acids derived from natural fats and oils may be assumed to have at least 8 carbon atoms, e.g., caprylic acid (octanoic acid).  Most of the natural fatty acids have an even number of carbon atoms, because their biosynthesis involves acetyl-CoA, a coenzyme carrying a two-carbon-atom group (see fatty acid synthesis).
In industry, fatty acids are produced by the hydrolysis of the ester linkages in a fat or biological oil (both of which are triglycerides), with the removal of glycerol. See oleochemicals.

# Types of fatty acids

## Saturated fatty acids
Saturated fatty acids do not contain any double bonds or other functional groups along the chain. The term "saturated" refers to hydrogen, in that all carbons (apart from the carboxylic acid  group) contain as many hydrogens as possible. In other words, the omega () end contains 3 hydrogens (CH3-), and each carbon within the chain contains 2 hydrogen
Saturated fatty acids form straight chains and, as a result, can be packed together very tightly, allowing living organisms to store chemical energy very densely.  The fatty tissues of animals contain large amounts of long-chain saturated fatty acids. In IUPAC nomenclature, fatty acids have an  suffix. In common nomenclature, the suffix is usually -ic.
The shortest descriptions of fatty acids include only the number of carbon atoms and double bonds in them (e.g., C18:0 or 18:0). C18:0 means that the carbon chain of the fatty acid consists of 18 carbon atoms, and there are no (zero) double bonds in it, whereas C18:1 describes an 18-carbon chain with one double bond in it. Each double bond can be in either a cis- or trans- conformation and in a different position with respect to the ends of the fatty acid; therefore, not all C18:1s, for example, are identical. If there is one or more double bonds in the fatty acid, it is no longer considered saturated, rather mono- or polyunsaturated.
Most commonly-occurring saturated fatty acids are:

## Unsaturated fatty acids
File:Isomers of oleic acid.png
Unsaturated fatty acids are of similar form, except that one or more alkenyl functional groups exist along the chain, with each alkene substituting a single-bonded " -CH2-CH2-" part of the chain with a double-bonded "-CH=CH-" portion (that is, a carbon double-bonded to another carbon).
The two next carbon atoms in the chain that are bound to either side of the double bond can occur in a cis or trans configuration.
In most naturally-occurring unsaturated fatty acids, each double bond has 3n carbon atoms after it, for some n, and all are cis bonds.  Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation).
The differences in geometry between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).

### Nomenclature
There are several different ways to make clear where the double bonds are located in molecules. For example:
- cis/trans-Delta-x or cis/trans-x: The double bond is located on the xth carbon-carbon bond, counting from the carboxylic acid end. The cis or trans notation indicates whether the molecule is arranged in a cis or trans conformation. In the case of a molecule's having more than one double bond, the notation is, for example,  cis,cis-9,12.
- Omega-x or -x : A double bond is located on the xth carbon-carbon bond, counting from the , (methyl carbon) end of the chain. Sometimes, the symbol  is replaced with a lowercase letter n, making it n-6 or n-3.
- In IUPAC nomenclature, a systematic naming system for all chemical compounds, counting is begins from the carboxylic acid end and cis double bonds are labelled Z and trans double bonds are labelled E.  (See IUPAC nomenclature of organic chemistry for details.)

### Examples of unsaturated fatty acids
Myristoleic is omega-5 fatty acid, palmitoleic is omega-7 fatty acid, and oleic and erucic acid are omega-9 fatty acids. Stearic and oleic acid are both C18 fatty acids. They differ only in that stearic acid is saturated with hydrogen, whereas oleic acid is an unsaturated fatty acid with two fewer hydrogens.
Docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) are examples of long chain omega-3 fatty acids (LCn3) in fish oil. Alpha-linolenic is a long chain omega-3 fatty acid from plants. LCn3 may benefit health.
Linoleic acid and arachidonic acid are omega-6 fatty acids. These fatty acids may be harmful.

### Essential fatty acids
The human body can produce all but two of the fatty acids it needs. These two, linoleic acid (LA) and alpha-linolenic acid (LNA), are widely distributed in plant oils. In addition, fish oils contain the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Other marine oils, such as from seal, also contain significant amounts of docosapentaenoic acid (DPA), which is also an omega-3 fatty acid. Although the body to some extent can convert LA and LNA into these longer-chain omega-3 fatty acids, the omega-3 fatty acids found in marine oils help fulfil the requirement of essential fatty acids (and have been shown to have wholesome properties of their own).
Since they cannot be made in the body from other substrates and must be supplied in food, they are called essential fatty acids.  Mammals lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10.  Hence linoleic acid and linoleinic acid are essential fatty acids for humans.
In the body, essential fatty acids are primarily used to produce hormone-like substances that regulate a wide range of functions, including blood pressure, blood clotting, blood lipid levels, the immune response, and the inflammation response to injury infection.
Essential fatty acids are polyunsaturated fatty acids and are the parent compounds of the omega-6 and omega-3 fatty acid series, respectively. They are essential in the human diet because there is no synthetic mechanism for them. Humans can easily make saturated fatty acids or monounsaturated fatty acids with a double bond at the omega-9 position, but do not have the enzymes necessary to introduce a double bond at the omega-3 position or omega-6 position.
The essential fatty acids are important in several human body systems, including the immune system and in blood pressure regulation, since they are used to make compounds such as prostaglandins. The brain has increased amounts of linolenic and alpha-linoleic acid derivatives.  Changes in the levels and balance of these fatty acids due to a typical Western diet rich in omega-6 and poor in omega-3 fatty acids is alleged to be associated with depression and behavioral change, including violence. The actual connection, if any, is still under investigation. Further, changing to a diet richer in omega-3 fatty acids, or consumption of supplements to compensate for a dietary imbalance, has been associated with reduced violent behavior and increased attention span, but the mechanisms for the effect are still unclear.  So far, at least three human studies have shown results that support this: two school studies as well as a double blind study in a prison.
Fatty acids play an important role in the life and death of cardiac cells because they are essential fuels for mechanical and electrical activities of the heart.    

### Trans fatty acids
A trans fatty acid (commonly shortened to trans fat) is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule less 'kinked' in comparison to fatty acids with cis double bonds. These bonds are characteristically produced during industrial hydrogenation of plant oils. Research suggests that amounts of trans fats correlate with circulatory diseases such as atherosclerosis and coronary heart disease more than the same amount of non-trans fats, for reasons that are not well understood.

# Free fatty acids
Fatty acids can be bound or attached to other molecules, such as in triglycerides or phospholipids.  When they are not attached to other molecules, they are known as "free" fatty acids.
The uncombined fatty acids or free fatty acids may come from the breakdown of a triglyceride into its components (fatty acids and glycerol).
Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of ATP.  Many cell types can use either glucose or fatty acids for this purpose.  In particular, heart and skeletal muscle prefer fatty acids.  The brain cannot use fatty acids as a source of fuel; it relies on glucose, or on ketone bodies. Ketone bodies are produced in the liver by fatty acid metabolism during starvation, or during periods of low carbohydrate intake.

# Fatty acids in dietary fats
The following table gives the fatty acid and cholesterol composition of some common dietary fats.

# Acidity
Short chain carboxylic acids such as formic acid and acetic acid are miscible with water and dissociate to form reasonably strong acids (pKa 3.77 and 4.76, respectively). Longer-chain fatty acids do not show a great change in pKa.  Nonanoic acid, for example, has a pKa of 4.96.  However, as the chain length increases the solubility of the fatty acids in water decreases very rapidly, so that the longer-chain fatty acids have very little effect on the pH of a solution. The significance of their pKa values therefore has relevance only to the types of reactions in which they can take part.
Even those fatty acids that are insoluble in water will dissolve in warm ethanol, and can be titrated with sodium hydroxide solution using phenolphthalein as an indicator to a pale-pink endpoint.  This analysis is used to determine the free fatty acid content of fats, i.e., the proportion of the triglycerides that have been hydrolyzed.

# Reaction of fatty acids
Fatty acids react just like any other carboxylic acid, which means they can undergo esterification and acid-base reactions. Reduction of fatty acids yields fatty alcohols. Unsaturated fatty acids can also undergo addition reactions, most commonly hydrogenation, which is used to convert vegetable oils into margarine. With partial hydrogenation, unsaturated fatty acids can be isomerized from cis to trans configuration. In the Varrentrapp reaction certain unsaturated fatty acids are cleaved in molten alkali, a reaction at one time of relevance to structure elucidation.

## Auto-oxidation and rancidity
Fatty acids at room temperature undergo a chemical change known as auto-oxidation. The fatty acid breaks down into hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols. Heavy metals present at low levels in fats and oils promote auto-oxidation. Fats and oils often are treated with chelating agents such as citric acid.