Fat

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Fats are a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides: triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure and composition. Although the words "oils", "fats", and "lipids" are all used to refer to fats, in reality, fat is a subset of lipid.^[1] "Oils" is usually used to refer to fats that are liquids at normal room temperature, while "fats" is usually used to refer to fats that are solids at normal room temperature. "Lipids" is used to refer to both liquid and solid fats, along with other related substances, usually in a medical or biochemical context, which are not soluble in water. The word "oil" is also used for any substance that does not mix with water and has a greasy feel, such as petroleum (crude oil), heating oil, and essential oils, regardless of its chemical structure.^[2]

Fats form a category of lipid, distinguished from other lipids by their chemical structure and physical properties. This category of molecules is important for many forms of life, serving both structural and metabolic functions. They are an important part of the diet of most heterotrophs (including humans). Fats or lipids are broken down in the body by enzymes called lipases produced in the pancreas.
Examples of edible animal fats are lard, fish oil, butter/ghee and whale blubber. They are obtained from fats in the milk and meat, as well as from under the skin, of an animal. Examples of edible plant fats include peanut, soya bean, sunflower, sesame, coconut and olive oils, and cocoa butter. Vegetable shortening, used mainly for baking, and margarine, used in baking and as a spread, can be derived from the above oils by hydrogenation.

These examples of fats can be categorized into saturated fats and unsaturated 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 but present in partially hydrogenated vegetable oils.

Chemical structure

A triglyceride molecule

Example of a natural triglyceride with three different fatty acids. One fatty acids is saturated (blue highlighted), another contains one double bond within the carbon chain (green highlighted). The third fatty acid (a polyunsaturated fatty acid, highlighted in red) contains three double bonds within the carbon chain. All carbon-carbon double bonds shown are cis isomers.

There are many different kinds of fats, but each is a variation on the same chemical structure. All fats are derivatives of fatty acids and glycerol. The molecules are called triglycerides, which are triesters of glycerol and with two carbon chains (one bonded to the single-bonded oxygen and the other to the main carbon), often formed from the reaction of the carboxylic acid and an organic alcohol). As a simple visual illustration, if the kinks and angles of these chains were straightened out, the molecule would have the shape of a capital letter E. The fatty acids would each be a horizontal line; the glycerol "backbone" would be the vertical line that joins the horizontal lines. Fats therefore have "ester" bonds.

The properties of any specific fat molecule depend on the particular fatty acids that constitute it. Different fatty acids are composed of different numbers of carbon and hydrogen atoms. The carbon atoms, each bonded to two neighboring carbon atoms, form a zigzagging chain; the more carbon atoms there are in any fatty acid, the longer its chain will be. Fatty acids with long chains are more susceptible to intermolecular forces of attraction (in this case, van der Waals forces), raising their melting point. Long chains also yield more energy per molecule when metabolized.

Saturated and unsaturated fats

A fat's constituent fatty acids may also differ in the C/H ratio. When all three fatty acids have the formula C_nH_(2n+1)CO₂H, the resulting fat is called "saturated". Values of n usually range from 13 to 17. Each carbon atom in the chain is saturated with hydrogen, meaning they are bonded to as many hydrogens as possible. Unsaturated fats are derived from fatty acids with the formula C_nH_(2n-1)CO₂H. These fatty acids contain double bonds within carbon chain. This results in an "unsaturated" fatty acid. More specifically, it would be a monounsaturated fatty acid. Polyunsaturated fatty acids would be fatty acids with more than one double bond; they have the formula, C_nH_(2n-3)CO₂H and C_nH_(2n-5)CO₂H. Unsaturated fats can be converted to saturated ones by the process of hydrogenation. This technology underpinned the development of margarine.
Saturated and unsaturated fats differ in their energy content and melting point. Since unsaturated fats contain fewer carbon-hydrogen bonds than saturated fats with the same number of carbon atoms, unsaturated fats will yield slightly less energy during metabolism than saturated fats with the same number of carbon atoms. Saturated fats can stack themselves in a closely packed arrangement, so they can freeze 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 highly unsaturated and are oily.

Trans fats

There are two ways the double bond may be arranged: the isomer with both parts of the chain on the same side of the double bond (the cis-isomer), or the isomer with the parts of the chain on opposite sides of the double bond (the trans-isomer). Most trans-isomer fats (commonly called trans fats) are commercially produced; trans fatty acids are rare in nature. The cis-isomer introduces a kink into the molecule that prevents the fats from stacking efficiently as in the case of fats with saturated chains. This decreases intermolecular forces between the fat molecules, making it more difficult for unsaturated cis-fats to freeze; they are typically liquid at room temperature. Trans fats may still stack like saturated fats, and are not as susceptible to metabolization as other fats. Trans fats may significantly increase the risk of coronary heart disease.^[3]

Importance for living organisms

Vitamins A, D, E, and K are fat-soluble, meaning they can only be digested, absorbed, and transported in conjunction with fats. Fats are also sources of essential fatty acids, an important dietary requirement.

Fats play a vital role in maintaining healthy skin and hair, insulating body organs against shock, maintaining body temperature, and promoting healthy cell function.

Fats also serve as energy stores for the body, containing about 37 kilojoules per gram (8.8 kcal/g).^[4] They are broken down in the body to release glycerol and free fatty acids. The glycerol can be converted to glucose by the liver and thus used as a source of energy.

Fat also serves as a useful buffer towards a host of diseases. When a particular substance, whether chemical or biotic, reaches unsafe levels in the bloodstream, the body can effectively dilute—or at least maintain equilibrium of—the offending substances by storing it in new fat tissue. This helps to protect vital organs, until such time as the offending substances can be metabolized and/or removed from the body by such means as excretion, urination, accidental or intentional bloodletting, sebum excretion, and hair growth.

While it is nearly impossible to remove fat completely from the diet, it would also be unhealthy to do so. Some fatty acids are essential nutrients, meaning that they can't be produced in the body from other compounds and need to be consumed in small amounts. All other fats required by the body are non-essential and can be produced in the body from other compounds.

Adipose tissue

The obese mouse on the left has large stores of adipose tissue. For comparison, a mouse with a normal amount of adipose tissue is shown on the right.

In animals, adipose, or fatty tissue is the body's means of storing metabolic energy over extended periods of time. Depending on current physiological conditions, adipocytes store fat derived from the diet and liver metabolism or degrade stored fat to supply fatty acids and also glycerol to the circulation. These metabolic activities are regulated by several hormones (i.e., insulin, glucagon and epinephrine).

The location of the tissue determines its metabolic profile: "visceral fat" is located within the abdominal wall (i.e., beneath the wall of abdominal muscle) whereas "subcutaneous fat" is located beneath the skin (and includes fat that is located in the abdominal area beneath the skin but above the abdominal muscle wall). Visceral fat was recently discovered to be a significant producer of signaling chemicals (i.e., hormones), among which several are involved in inflammatory tissue responses. One of these is resistin which has been linked to obesity, insulin resistance, and Type 2 diabetes. This latter result is currently controversial, and there have been reputable studies supporting all sides on the issue.