Omega-3 Fatty Acid

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Omega-3 Fatty Acids (Omega-3s) are polyunsaturated fats with a double bond at the third carbon atom from the end of the carbon chain. As omega is the last letter in the Greek alphabet, the term "omega-3" denotes the double bond occurring three carbons from the end. They are also sometimes referred to as "n-3 fatty acids" in scientific literature.

Relationship of Omega-3s to Omega-6s

Within the human body, omega-3s and omega-6 fatty acids essentially "compete" with one another for a limited amount of enzymes available to desaturate and elongate them into the long chain highly unsaturated fatty acids the body needs. Therefore, the amount of omega-3s needed for optimal health depends on the amount of omega-6s consumed as well. One study sets the ideal ratio is 2.3:1, or 2.3g omega-6s for every 1g of omega-3s consumed.[1] However, a more commonly cited optimal ratio is 4:1.[2]

A study published in 2000 estimated that polyunsaturated fatty acids make up about 19-22% of fat intake in the diets of adult Americans.[1] The vast majority (89%) of the polyunsaturated fat consumed is linoleic acid, an omega-6 fatty acid, compared to only 9%-11% from alpha-linoleic acid (ALA), an omega-3. The most common sources of ALA in the American diet are canola and soybean oils. During the study period, American adults consumed an estimated 1.1–1.6g of ALA per day. They consumed less than 0.2g/day of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) combined.

The study states:

"The ratio of 2.3:1 translates to 6.7g n-6 fatty acids and 2.9g n-3 fatty acids in a 8360 kJ (2000 kcal) diet. The difficulty in meeting the recommended ratio is that many foods typically consumed in the American diet simply have a ratio of n-6 to n-3 fatty acids far above 2.3:1. Even if fish consumption is increased to achieve the goal of 0.65 g/d of EPA and DHA, the ratio will not be markedly lowered unless n-6 fatty acid consumption is decreased markedly."

The study estimated per capita linoleic acid (omega-6) consumption at 11-16g/day.[1] Therefore, reducing omega-6 consumption to 6.7g/day would mean cutting it roughly in half.

According to another source, author Susan Allport, reducing our omega-6:omega-3 ratio in our diets to 4:1 "produces a 1:1 ratio of HUFAs [highly unsaturated fatty acids] in cell membranes."[3]

Types of Omega-3 Fatty Acids

Fats are categorized by the number of carbons in them, the number of double bonds, and the placement of the double bonds in their chemical structures. Omega-3s are notated as "n-3."

The following are all omega-3s:

The human body converts ALA to EPA and DHA using a series of elongation and desaturation processes.[4] It elongates the chain by adding additional carbons, and it desaturates it (of hydrogen) by adding more double-bonds between carbons (in place of what was previously bonds with hydrogen molecules).

First, the body uses an enzyme called delta-6-desaturase to turn ALA (18:3n-3) into SDA (18:4n-3). Then it uses the enzyme elongase-5 to add two carbons, making 20:4n-3. Then it uses an enzyme called delta-5-desaturase to add another double-bond, creating EPA (20:5n-3).[4]

The body can then elongate EPA using elongase-2 to form 22:5n-3 and then 24:5n-3. Using delta-6-desaturase, it can then add one more double-bond to create 24:6n-3. Then it can undergo one round of beta-oxidation to form DHA (22:6n-3).

These same enzymes are used in a parallel process converting the omega-6 fatty acid linoleic acid (LA) (18:2n-6) to longer chain, highly unsaturated fatty acids. Delta-6-desaturase first converts LA to gamma-linolenic acid (GLA) (18:3n-6). This is elongated by elongase to 20:3n-6 and then desaturated once again using delta-5-desaturase to arachidonic acid 20:4n-6. Further elongation creates 22:4n-6 and 24:4n-6 and then delta-6-desaturase desaturates it to 24:5n-6. Last, one round of beta-oxidation creates 22:5n-6.[4]

Because the two families of polyunsaturated fats (omega 3 and omega 6) are competing for the use of the same enzymes, the consumption of one influences the metabolism of the other. If one consumes them at a ratio of 14:1 (omega-6/omega-3), the presence in the tissues will be equal. Thus, omega-3s are stronger at competing for the enzymes than omega-6s.[5]

Sources of Omega-3 Fatty Acids

A 2013 study by Washington State University research Charles Benbrook (who is also on the Science Advisory Board of the Organic Center) and others found that "organic milk contained 25% less ω-6 fatty acids and 62% more ω-3 fatty acids than conventional milk, yielding a 2.5-fold higher ω-6/ω-3 ratio in conventional compared to organic milk (5.77 vs. 2.28)" and that "dairy products supply far more α-linolenic acid than seafoods, about one-third as much eicosapentaenoic acid, and slightly more docosapentaenoic acid, but negligible docosahexaenoic acid."[6]

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References

  1. 1.0 1.1 1.2 PM Kris-Etherton, Denise Shaffer Taylor, Shaomei Yu-Poth, Peter Huth, Kristin Moriarty, Valerie Fishell, Rebecca L Hargrove, Guixiang Zhao, and Terry D Etherton, "Polyunsaturated fatty acids in the food chain in the United States," American Journal of Clinical Nutrition, 2000;71(suppl):179S–88S.
  2. Yehuda S. and Carasso R. L., (1993) Modulation of learning, pain thresholds, and thermoregulation in the rat by preparations of free purified alpha-linolenic and linoleic acids. Determination of the optimal ω3/ω6 ratio. Proc. Natl. Acad. Sci. U.S.A. 90:10345–10349.
  3. Susan Allport, The Queen of Fats: Why Omega-3s Were Removed From the Western Diet and What We Can Do To Replace Them, University of California Press, Los Angeles, 2006.
  4. 4.0 4.1 4.2 Gwendolyn Barceló-Coblijn, Eric J. Murphy, "Alpha-linolenic acid and its conversion to longer chain n−3 fatty acids: Benefits for human health and a role in maintaining tissue n−3 fatty acid levels," Progress in Lipid Research, Volume 48, Issue 6, November 2009, Pages 355–374.
  5. Holman RT. "The slow discovery of the importance of omega 3 essential fatty acids in human health," J Nutr 1998;128:427S–33S.
  6. Charles Benbrook, Gillian Butler, Maged A. Latif, Carlo Leifert, and Donald R. Davis, Organic Production Enhances Milk Nutritional Quality by Shifting Fatty Acid Composition: A United States–Wide, 18-Month Study, PLoS ONE 8(12): e82429, December 9, 2013.