Polyunsaturated fat

The position of the carbon-carbon double bonds in carboxylic acid chains in fats is designated by Greek letters. The carbon atom closest to the carboxyl group is the alpha carbon, the next carbon is the beta carbon and so on. In fatty acids the carbon atom of the methyl group at the end of the hydrocarbon chain is called the omega carbon because omega is the last letter of the Greek alphabet. Omega-3 fatty acids have a double bond three carbons away from the methyl carbon, whereas omega-6 fatty acids have a double bond six carbons away from the methyl carbon. The illustration below shows the omega-6 fatty acid, linoleic acid.

Polyunsaturated fatty acids can be classified in various groups by their chemical structure:

• methylene-interrupted polyenes
• conjugated fatty acids
• other PUFAs

Based on the length of their carbon backbone, they are sometimes classified in two groups:{{cite journal |vauthors=Buckley MT et al. |year=2017 |title=Selection in Europeans on Fatty Acid Desaturases Associated with Dietary Changes |journal=Mol Biol Evol |volume=34 |issue=6 |pages=1307–1318 |doi=10.1093/molbev/msx103 |pmc=5435082 |pmid=28333262}} All feature pentadiene groups.

• short chain polyunsaturated fatty acids (SC-PUFA), with 18 carbon atoms. These are more common. Key members include linoleic acid, α-linolenic acid, and arachidonic acid.
• long-chain polyunsaturated fatty acids (LC-PUFA) with 20 or more carbon atoms

PUFAs with 18 carbon atoms, which are the most common variety, are not produced by mammals. Since they have important dietary functions, their biosynthesis has received much attention. Plants produce PUFAs from oleic acid. Key enzymes are called fatty acid desaturases, which introduce additional double bonds. Desaturases convert oleic acid into linoleic acid the precursor to alpha-linolenic acid, gamma-linolenic acid and dihomo-gamma-linolenic acid.{{cite journal |doi=10.1021/cr300007p |title=Transgenic Biosynthesis of Polyunsaturated Fatty Acids: A Sustainable Biochemical Engineering Approach for Making Essential Fatty Acids in Plants and Animals |date=2013 |last1=Jiao |first1=Jingjing |last2=Zhang |first2=Yu |journal=Chemical Reviews |volume=113 |issue=5 |pages=3799–3814 |pmid=23421688 }}

Industrial PUFAs are generally obtained by hydrolysis of fats that contain PUFAs. The process is complicated by the sensitive nature of PUFAs, leading to side reactions and colorization. Thus, steam hydrolysis often fails for this reason. Alkaline hydrolysis of fats followed by acidification is expensive. Lipases, a family of enzymes, show potential as mild and green catalysts for the production of PUFAs from triglycerides.

In general, outside of dietary contexts, PUFAs are undesirable components of vegetable oils, so there is great interest in their removal from, say, olive oil. One technology for lowering the PUFA contact is by selective formation of derivatives with ureas.

From the perspective of chemical analysis, PUFA's have high iodine numbers. These high values are simply a reflection of the fact that PUFAs are polyunsaturated. Hydrogenation of PUFAs gives less unsaturated derivatives. For unsaturated products from partial hydrogenation often contain some trans isomers. The trans monounsaturated C20 species elaidic acid can be prepared in this way.

{{main|Lipid peroxidation}}

Polyunsaturated fatty acids are susceptible to lipid peroxidation, far more so than monounsaturated or saturated analogues. The basis for this reactivity is the weakness of doubly allylic C-H bonds. They are drying oils, i.e. film-forming liquids suitable as painting. One practical consequence is that polyunsaturated fatty acids have poor shelf life, owing to their tendency toward autoxidation, leading, in the case of edibles, to rancidification. Metals accelerate the degradation. A range of reactions with oxygen occur. Products include fatty acid hydroperoxides, epoxy-hydroxy polyunsaturated fatty acids, jasmonates, divinylether fatty acids, and leaf aldehydes. Some of these derivatives are signalling molecules, some are used in plant defense (antifeedants), some are precursors to other metabolites that are used by the plant.{{cite journal |doi=10.1146/annurev.arplant.53.100301.135248 |title=The Lipoxygenase Pathway |date=2002 |last1=Feussner |first1=Ivo |last2=Wasternack |first2=Claus |journal=Annual Review of Plant Biology |volume=53 |pages=275–297 |pmid=12221977 }}

These fatty acids have 2 or more cis double bonds that are separated from each other by a single methylene bridge ({{chem2|\sCH2\s}}). This form is also sometimes called a divinylmethane pattern.

{{cite book

|author= Baggott, James|year= 1997|title= The divinylmethane pattern in fatty acids

|location= Salt Lake City, UT|publisher= Knowledge Weavers}}

The essential fatty acids are all omega-3 and -6 methylene-interrupted fatty acids. See more at Essential fatty acids—Nomenclature{{cite web|publisher=United States Department of Agriculture, Agricultural Research Service|year=2011|title=National nutrient database for standard reference, release 23|url=http://www.nal.usda.gov/fnic/foodcomp/search/|access-date=2009-02-22|archive-url=https://web.archive.org/web/20150303184216/http://www.nal.usda.gov/fnic/foodcomp/search/|archive-date=2015-03-03|url-status=dead}}

The biological effects of the ω-3 and ω-6 fatty acids are largely mediated by their mutual interactions, see Essential fatty acid interactions for detail.

Because of their effects in the diet, unsaturated fats (monounsaturated and polyunsaturated) are often referred to as good fats; while saturated fats are sometimes referred to as bad fats. Some fat is needed in the diet, but it is usually considered that fats should not be consumed excessively, unsaturated fats should be preferred, and saturated fats in particular should be limited.{{cite web |title=Fats explained |url=https://www.heartuk.org.uk/downloads/factsheets/fats-explained.pdf |website=HEART UK – The Cholesterol Charity |accessdate=20 February 2019}}{{cite web |title=Key Recommendations: Components of Healthy Eating Patterns |url=https://health.gov/dietaryguidelines/2015/guidelines/chapter-1/key-recommendations/ |website=Dietary Guidelines 2015-2020 |accessdate=20 February 2019}}{{cite web |title=Live Well, Eat well, Fat: the facts |url=https://www.nhs.uk/live-well/eat-well/different-fats-nutrition/ |website=NHS |accessdate=20 February 2019}}{{cite web |title=Dietary Guidelines for Indians - A Manual |url=http://ninindia.org/DietaryGuidelinesforNINwebsite.pdf |website=Indian Council of Medical Research, National Institute of Nutrition |access-date=2019-02-20 |archive-url=https://web.archive.org/web/20181222101538/http://www.ninindia.org/DietaryGuidelinesforNINwebsite.pdf |archive-date=2018-12-22 |url-status=dead }}

In preliminary research, omega-3 fatty acids in algal oil, fish oil, fish and seafood have been shown to lower the risk of heart attacks.{{cite web|url=http://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/|publisher=US National Institutes of Health, Office of Dietary Supplements|title=Omega-3 Fatty Acids and Health: Fact Sheet for Health Professionals|date=2 November 2016|access-date=5 April 2017}} Other preliminary research indicates that omega-6 fatty acids in sunflower oil and safflower oil may also reduce the risk of cardiovascular disease.{{cite journal | vauthors = Willett WC | title = The role of dietary n-6 fatty acids in the prevention of cardiovascular disease | journal = Journal of Cardiovascular Medicine | volume = 8 | issue = Suppl 1| pages = S42-5 | date = September 2007 | pmid = 17876199 | doi = 10.2459/01.JCM.0000289275.72556.13 | s2cid = 1420490 }}

Among omega-3 fatty acids, neither long-chain nor short-chain forms were consistently associated with breast cancer risk. High levels of docosahexaenoic acid (DHA), however, the most abundant omega-3 polyunsaturated fatty acid in erythrocyte (red blood cell) membranes, were associated with a reduced risk of breast cancer.{{cite journal | vauthors = Pala V, Krogh V, Muti P, Chajès V, Riboli E, Micheli A, Saadatian M, Sieri S, Berrino F | title = Erythrocyte membrane fatty acids and subsequent breast cancer: a prospective Italian study | journal = Journal of the National Cancer Institute | volume = 93 | issue = 14 | pages = 1088–95 | date = July 2001 | pmid = 11459870 | doi = 10.1093/jnci/93.14.1088 | doi-access = }} DHA is vital for the grey matter structure of the human brain, as well as retinal stimulation and neurotransmission.

Contrary to conventional advice, an evaluation of evidence from 1966–1973 pertaining to the health impacts of replacing dietary saturated fat with linoleic acid found that participants in the group doing so had increased rates of death from all causes, coronary heart disease, and cardiovascular disease.{{cite journal | vauthors = Ramsden CE, Zamora D, Leelarthaepin B, Majchrzak-Hong SF, Faurot KR, Suchindran CM, Ringel A, Davis JM, Hibbeln JR | title = Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis | journal = BMJ | volume = 346 | pages = e8707 | date = February 2013 | pmid = 23386268 | pmc = 4688426 | doi = 10.1136/bmj.e8707 }} Although this evaluation was disputed by many scientists,{{cite web|url=https://www.hsph.harvard.edu/nutritionsource/2016/04/13/diet-heart-ramsden-mce-bmj-comments/|title=Research Review: Old data on dietary fats in context with current recommendations: Comments on Ramsden et al. in the British Medical Journal|author=Interview: Walter Willett|publisher=TH Chan School of Public Health, Harvard University, Boston|date=2017|access-date=24 May 2017}} it fueled debate over worldwide dietary advice to substitute polyunsaturated fats for saturated fats.{{cite journal | vauthors = Weylandt KH, Serini S, Chen YQ, Su HM, Lim K, Cittadini A, Calviello G | title = Omega-3 Polyunsaturated Fatty Acids: The Way Forward in Times of Mixed Evidence | journal = BioMed Research International | volume = 2015 | pages = 143109 | year = 2015 | pmid = 26301240 | pmc = 4537707 | doi = 10.1155/2015/143109 | doi-access = free }}

Taking isotope-reinforced polyunsaturated fatty acids, for example deuterated linoleic acid where two atoms of hydrogen substituted with its heavy isotope deuterium, with food (heavy isotope diet) can suppress lipid peroxidation and prevent or treat the associated diseases.{{cite journal | last1 = Hill | first1 = S. | display-authors = etal | year = 2012 | title = Small amounts of isotope-reinforced PUFAs suppress lipid autoxidation | journal = Free Radical Biology & Medicine| volume = 53 | issue = 4| pages = 893–906 | doi = 10.1016/j.freeradbiomed.2012.06.004 | pmid = 22705367 | pmc = 3437768 }}{{Cite journal|url=https://pubmed.ncbi.nlm.nih.gov/32113652/|pmid = 32113652|year = 2020|last1 = Shchepinov|first1 = M. S.|title = Polyunsaturated Fatty Acid Deuteration against Neurodegeneration|journal = Trends in Pharmacological Sciences|volume = 41|issue = 4|pages = 236–248|doi = 10.1016/j.tips.2020.01.010|s2cid = 211724987}}

Polyunsaturated fat supplementation does not decrease the incidence of pregnancy-related disorders, such as hypertension or preeclampsia, but may increase the length of gestation slightly and decreased the incidence of early premature births.

Expert panels in the United States and Europe recommend that pregnant and lactating women consume higher amounts of polyunsaturated fats than the general population to enhance the DHA status of the fetus and newborn.

Results from observational clinical trials on polyunsaturated fat intake and cancer have been inconsistent and vary by numerous factors of cancer incidence, including gender and genetic risk. Some studies have shown associations between higher intakes and/or blood levels of polyunsaturated fat omega-3s and a decreased risk of certain cancers, including breast and colorectal cancer, while other studies found no associations with cancer risk.{{cite journal | vauthors = Patterson RE, Flatt SW, Newman VA, Natarajan L, Rock CL, Thomson CA, Caan BJ, Parker BA, Pierce JP | title = Marine fatty acid intake is associated with breast cancer prognosis | journal = The Journal of Nutrition | volume = 141 | issue = 2 | pages = 201–6 | date = February 2011 | pmid = 21178081 | pmc = 3021439 | doi = 10.3945/jn.110.128777 }}

{{Vegetable oils, composition}}

Polyunsaturated fat can be found mostly in nuts, seeds, fish, seed oils, and oysters. "Unsaturated" refers to the fact that the molecules contain less than the maximum amount of hydrogen (if there were no double bonds). These materials exist as cis or trans isomers depending on the geometry of the double bond.

{{Fat composition in different foods (table)}}

PUFA's are significant components of alkyd resins, which are used in coatings.

{{reflist}}

• {{cite web| url=http://www.cyberlipid.org/fa/acid0003.htm| title=Polyenoic Fatty Acids| author=Cyberlipid| access-date=2007-01-17| archive-url=https://web.archive.org/web/20180930034042/http://www.cyberlipid.org/fa/acid0003.htm| archive-date=2018-09-30| url-status=dead}}
• {{cite web

|title=Lipid Glossary 2

|url=http://www.plantbiology.msu.edu/ohlrogge/labweb/lipid_glossary.pdf

|author=Gunstone, Frank D | access-date=2007-01-17|archive-url = https://web.archive.org/web/20060813111917/http://www.plantbiology.msu.edu/ohlrogge/labweb/lipid_glossary.pdf |archive-date = 2006-08-13}}

• {{cite web

| url=http://aocs.org/member/division/analytic/fanames.asp

|author1=Adlof, R. O. |author2=Gunstone, F. D. |name-list-style=amp |title=Common (non-systematic) Names for Fatty Acids|date= 2003-09-17 | access-date=2007-01-24 |archive-url = https://web.archive.org/web/20061206090944/http://www.aocs.org/member/division/analytic/fanames.asp |archive-date = 2006-12-06}}

• {{cite journal |author= Heinz|title=Similarities and Differences in Lipid Metabolism of Chloroplasts Isolated from 18:3 and 16:3 Plants |journal=Plant Physiol |volume=72 |issue=2 |pages=273–279 |year=1983 |pmid=16662992 |doi= 10.1104/pp.72.2.273 |last2= Roughan |first2= PG |pmc= 1066223}}

{{Fatsandoils}}

Category:Fatty acids

Category:Nutrition

Category:Nutrients