Fat
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| See Also |
- For other uses, see Fat (disambiguation).
Fats (also known as neutral fats, triglycerides, or triacylglycerides) are uncharged esters of glycerol, a three-carbon alcohol. They belong to a larger class of biomolecules called lipids, which are water-insoluble but highly soluble in organic solvents.
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).
Chemical structure and properties
The basic components of triacylglycerols are fatty acids, a class of compounds that consist of a long hydrocarbon chain and a terminal caroxylate group. In a fat molecule, three fatty acid chains are bonded to a glycerol backbone. Chemically speaking, fat is considered a triester of glycerol, as it is a molecule formed from the reaction of three acids and an alcohol.
Fatty acids vary in two important characteristics: chain length and degree of unsaturation; the chemical properties of triacylgycerols are thus determined by their particular fatty acid components.
Fatty acids in naturally occurring triglycerides typically range from 14 to 24 carbon atoms, with 16- and 18-carbon lengths being the most common. Fatty acids found in plants and animals are typically composed of an even number of carbon atoms, due to the process of biosynthesis in these organisms. Bacteria, however, possess the ability to synthesise odd- and branched-chain fatty acids. Consequently, ruminant animal fat contains significant proportions of branched-chain fatty acids, due to the action of bacteria in the rumen.
Most natural fats contain a complex mixture of individual triglycerides; because of this, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, one of which contains palmitic, oleic and stearic acids in that order. This gives rise to a fairly sharp melting point, causing chocolate to melt in the mouth without feeling greasy.
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 fats
A fat's constituent fatty acids may also differ in the number of hydrogen atoms that branch off of the chain of carbon atoms. Each carbon atom is typically bonded to two hydrogen atoms. When a fatty acid has this typical arrangement, it is called "saturated", because the carbon atoms are saturated with hydrogen; meaning they are bonded to as many hydrogens as they possibly could be.
Unsaturated fats
In other fats, a carbon atom may instead bond to only one other hydrogen atom, and have a double bond to a neighboring carbon atom. This results in an "unsaturated" fatty acid. A fat containing only saturated fatty acids is itself called saturated. A fat containing at least one unsaturated fatty acid is called unsaturated, and a fat containing more than one unsaturated fatty acid is called polyunsaturated.
Double bonds may be in a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogens are on the same side of the double bond, whereas in the trans conformation they are on the opposite side. Trans fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and more solid at room temperature than cis fat. But trans fats reduce the fluidity (and functionality) of cell membranes.
Both mono- and polyunsaturated fats can replace saturated fat in the diet; trans unsaturated fats should be avoided. Substituting saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. This effect is attributed to the low melting point of unsaturated fats found in food. Trans unsaturated fats are particularly bad because the double bond stereochemistry allows the fat molecules to assume a linear conformation which leads to efficient packing (i.e., plaque formation). The geometry of the cis double bond introduces a bend in the molecule precluding stable formations (see specific fatty acid links above for drawings that illustrate this). Natural sources of fatty acids (see above) are rich in the cis isomer.
Examples of common unsaturated fats are palmitoleic acid, oleic acid, linoleic acid, and arachidonic acid. Foods containing unsaturated fats include avocado, nuts, and soybean, canola, and olive oils.
Saturated and unsaturated fats differ in their energy content and melting point. Since an unsaturated fat contains fewer carbon-hydrogen bonds than a saturated fat 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. But the rigid double bond in an unsaturated fat fundamentally changes the chemistry of the fat. There are two ways the double bond may be arranged: the isomer with the both parts of the chain on the same side of the double bond (the cis-isomer; also the only naturally occurring type of fat), or the isomer with the parts of the chain on opposite sides of the double bond (the trans-isomer, generally a product of partial hydrogenation of natural unsaturated fats). The cis-isomer introduces a kink into the molecule that prevents the fats from stacking efficiently like 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, however, may still stack like saturated fats, but are not as susceptible to metabolization. Trans-fats have been associated with many health problems, but their biochemistry is poorly understood.
Fats function as long-term energy stores
They also serve as energy stores for the body. Fats 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. The fatty acids are a good source of energy for many tissues, especially heart and skeletal muscle. Triglycerides play an important role in metabolism as energy sources. They contain more than twice as much energy (9 kcal/g) as carbohydrates and proteins. In the intestine, triglycerides are split into glycerol and fatty acids (this process is called lipolysis) (with the help of lipases and bile secretions), which can then move into blood vessels. The triglycerides are rebuilt in the blood from their fragments and become constituents of lipoproteins, which deliver the fatty acids to and from fat cells among other functions. Various tissues can release the free fatty acids and take them up as a source of energy. Fat cells can synthesize and store triglycerides. When the body requires fatty acids as an energy source, the hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids.
what (chemically speaking) makes fats such an efficient energy store?
Adipose or fatty tissue is the human 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 degrades stored fat to supply fatty acids and 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). It was briefly thought that visceral fat produced a hormone involved in insulin resistance, but this has been disproven by clinical tests (see, resistin, a hormone, ultimately misnamed, which is produced by adipose tissue and does cause insulin resistence in mice but not in humans).
Other biological roles of fats include insulation, transport, and hormone synthesis
Vitamins A, D, E, and K are fat-soluble, meaning they can only be digested, absorbed, and transported in conjunction with fats. Fats are 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. They are also known as lipids.
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.
Fats in human health and nutrition
In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, and, by extension, the risk of heart disease and stroke. However, the negative impact of raised levels of triglycerides is lower than that of LDL:HDL ratios. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level.
Other diseases caused by high triglycerides include pancreatitis and depression.
ReferencesISBN links support NWE through referral fees
- Donatelle, Rebecca J. 2005. Health: The Basics, 6th edition. San Francisco, CA: Pearson.
- Stryer, Lubert. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.
External links
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