Chemical structure of saturated fatty acids
Saturated fatty acids (SFA) contain no double bonds and have general formula R-COOH in which the R- group is a straight-chain hydrocarbon of the form CH3(CH2)n with varying length ranging from short chain length (volatile liquids) to chain lengths of 30 or more carbon atoms (waxy solids), though the most common and important fatty acids, found in many different plant and animal fats, contain between 12 and 22 carbon atoms.
They are stable to oxidation as a result of the saturation of their carbon chain.
In the human diet the most consumed saturated fatty acids are myristic, palmitic and stearic acids.
Saturated fatty acids and chain lenght
Short chain: up to 6 carbon atoms
- Formic acid (IUPAC name: methanoic acid): one carbon atom
- Acetic acid (IUPAC name: ethanoic acid): 2 carbon atoms
- Propionic acid (IUPAC name: propanoic acid): 3 carbon atoms
- Butyric acid: 4 carbon atoms
- Caproic acid: 6 carbon atoms
They are volatile molecules with high water solubility, a short chain length and small molecular size.
They are present ubiquitously in the gastrointestinal tract of mammals where they are the end products of bacterial digestion of carbohydrates, that is undigested starch and dietary fiber; in normal conditions the major short chain saturated fatty acids produced are acetic, propionic and butyric acid; caproic acid is present in small proportions except if diet contains high carbohydrate levels.
To some extent, these fatty acids can be absorbed and used by colonic enterocytes as metabolic substrates, also as an energy source (butyric acid is a significant metabolic fuel for enterocytes), and appear to be essential for normal colonic function (again, butyric acid formed in the colon may have some anticarcinogenic action).
They can also be produced by microbial proteolysis followed by deamination.
Short chain SFA are lower in caloric value than medium chain and long chain fatty acids with the following caloric values: acetic acid, 3.5 kcal/g, propionic acid 5.0 kcal/g, butyric acid 6.0 kcal/g and caproic acid, 7.5 kcal/g.
Short chain fatty acids in certain foods
Short chain saturated fatty acids formic, acetic and propionic acids are uncommon in naturally occurring fats but are present in many food products.
In triacylglycerols of fat milk from cows, sheep and goats they are mainly esterified in the sn-3 position (where sn– indicates stereospecific numbering).
Through the action of pancreatic and gastric lipase on triacylglycerols, that preferentially hydrolyze sn-3 esters, they are likely to be completely hydrolyzed in the lumen of the stomach and small intestine.
Butyric acid and caproic acid, together with smaller amounts of caprylic and capric acids plus lauric acid (medium chain fatty acids), are present in bovine milk (butyric acid represents approximately 5%-10% and caproic acid 3%-5% of fatty acids in bovine milk triacylglycerols). Milk from goats and sheep also contains butyric, caproic and caprylic acids but capric acid is present in larger amounts.
Butyric and caproic acids, with medium-chain saturated fatty acids caprylic and capric acids occur in a few seeds.
Medium chain: from 8 to 12 carbon atoms
They have good water solubility.
As aforementioned some of them are present, together with certain short chain fatty acids, in triacylglycerols of milk fat from bovine, goats and sheep.
Caprylic acid may be a precursor for de novo synthesis of long chain fatty acids such as myristic and palmitic acids.
Lauric acid predominates in seed fats of the Lauraceae family, from which its name derives.
Long and very long chain: from 14 atoms onwards
- Myristic acid (14 carbon atoms)
- Palmitic acid (16 carbon atoms)
- Stearic acid (18 carbon atoms)
- Arachidic acid (20 carbon atoms)
- Behenic acid (22 carbon atoms)
- Lignoceric acid (24 carbon atoms)
They have low or absent water solubility.
Myristic acid predominates in seed fats of the Myristicaceae family, from which its name derives.
Palmitic acid is the commonest saturated fatty acid in plant and animal (in fish oils, in the milk and storage fat of many mammals) lipids, but generally not present in very large proportions. With oleic acid and linoleic acid it is one of the most abundant fatty acids in triacylglycerols of adipose tissue and plasma lipoproteins.
Stearic acid is one of the most common long-chain fatty acids; it is abundant in beef fat, in cotton, coconut, palm kernel, castor beans, rapeseed, soybeans and sunflowers oils, in cocoa butter and it has been reported not to raise plasma cholesterol levels. Because stearic acid has a melting point higher than body temperature, triglycerides containing it in high amounts are poorly absorbed in humans.
Behenic acid is also poorly absorbed.
Long chain saturated fatty acids (20 carbon atoms and greater) are major components in only a few uncommon seed oils and up to 26 carbons in length are found in small amounts in most animal tissues.
They are used by the food industry to make low-calorie lipids.
In general, SFA in membranes are confined to the sn-1 position of phospholipids and have 16 to 18 carbon atoms (in sn-2 there are 18 to 20 carbon atom unsaturated fatty acids); in triacylglycerols saturated fatty acids, often long chain ones, (or monounsaturated fatty acids) tend to be in sn-1 and sn-3 positions (in sn-2 position there are polyunsaturated fatty acids).
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