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What you should know about fish oils and omega-3 fatty acids
11.11.2009 | Reto Muggli, PhD* und Prof. Stefan Mühlebach, PhD**
* AdviServ Consulting, CH-4114 Hofstetten, Switzerland
** Chief Scientific Officer, Consumer Health Care Interantional, Vifor Pharma, CH-1752 Villars-sur-Glâne, Switzerland
Essential fatty acids
Omega-3 long chain polyunsaturated fatty acids (omega-3 LCPUFA) - primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) - are essential for health. In addition to serving the general function of fatty acids (FA), EPA and DHA are precursors of short-lived, potent signalling molecules (1). The unequal distribution into different tissues shows the complex kinetics of the PUFA, not only of EPA and DHA and may also depend on their form.
Omega-3 LCPUFA oils of natural concentration
There are several biological raw material sources for omega-3 LCPUFA and each has its own advantages and limitations. EPA and DHA are found naturally in fish, making fish oils a relatively inexpensive and easily available source of EPA/DHA.
Fish oils contain, beside saturated and mono-unsaturated FA, about 30% EPA+DHA. The amount, relative concentration – and therefore also the ratio EPA/DHA – depends on the fish species, the catch season and the geographical catch area. Fish oil is a by-product of fish meal production. Of the about 1 million metric tons of fish oil produced annually only about 2-5% is used for human consumption.
Typical examples are:
• Menhaden oil (herring family): 18% EPA, 12% DHA
• Tuna oil: 8% EPA, 25% DHA.
• Krill oil (extracted from tiny Antarctic crustaceans) 15% to 20% EPA, 9% to 14% DHA with seasonal variations. Krill oil is unique in that it is made up of 80% phospholipids (PL) whereas fish oils are 100% triglycerides (TG).
Omega-3 LCPUFA are also produced by fermentation of microalgae and/or lower fungi and subsequent extraction of the cell lipids. A single cell oil with 40% DHA in the form of TG is currently extensively used to fortify infant formulas.
All raw oils, whether from marine or microbial sources, are refined to remove possible undesirable by-products (free fatty acids, pigments, polymers, primary and secondary oxidation products) and environmental contaminants such as heavy metals and pesticides. The degree of refining and the expertise applied in refining the raw oils are decisive for the ultimate quality and stability of the products. Quality standards, such as those developed by the Omega-3 Working Group of the Council for Responsible Nutrition (2), ensure product quality and safety.
The chemical stability of omega-3 LCPUFA oils depends critically on the absence of (reactive) peroxides and transition metals (catalysts) as well as on the lipid class (TG, PL) and FA composition. PL are generally more stable than TG, and the stability decreases with increasing degree of unsaturation of the FA.
EPA and DHA can attach to the glycerol backbone of triglycerides in three different positions, two in phospholipids. The positional distribution is not at random. In fish oil 60% of the EPA and DHA are in the R2 position (Fig.1).
Fig. 1: Structure of triglycerides and phospholipids
Chemical and physical modification of natural omega-3 LCPUFA oils
To obtain an oil which contains 50% omega-3 LCPUFA or higher, or an oil containing EPA or DHA only, natural triglycerides are hydrolysed and the fatty acids fractionated by one of several methods
• urea crystallisation,
• distillation,
• supercritical fluid extraction
The resulting products are either non-physiological fatty acid ethyl esters or reconstituted glycerides consisting of about 60% tri-, 30% di- and 10% monoglycerides. Alternatively, lipids of defined structure can be obtained with enzymes that exhibit substrate or position specificity (fermentation).
Biodisposition of omega-3 LC-PUFA oils
Different forms of omega-3 LCPUFA oils may not be necessarily bioequivalent. In the pancreas enzymes called lipases act on the omega-3 LCPUFA oils. The hydrolysis (reaction with water) splits the omega-3 as follows (Fig.2):
• Triglyceride → two free fatty acids (FFA) and a monoglyceride
• Phospholipid → one FFA and a lysophospholipid
• Ethyl Ester → one FFA and ethanol.
Fig. 2: Intestinal hydrolysis of triglycerides and phospholipids
FFA and monoglycerides - together with bile salts, cholesterol and fat-soluble vitamins - are re-assembled to units known as micelles which diffuse into the cells lining the enteral tract (mucosal phase).
Once inside, the monoglycerides and FFA are converted back into a TG format. The TGs then combine with apoproteins, assemble into particles called chylomicrons and enter the circulation via the lymphatic route. In the plasma the chylomicron-TG are converted back to FFA and remnant particles that can be taken up by (extrahepatic/hepatic) tissues. Lysophospholipids are treated similarly.
Studies in animals and humans comparing the absorption of omega-3 fatty acids from TG versus absorption from non-physiological ethyl esters have been somewhat conflicting. In general, no significant differences in absorption were observed in studies in which the feeding spanned several weeks and the omega-3 LCPUFA were given together with a fatty meal. Nevertheless it is important to realize that omega-3 LCPUFA differ in their accretion rates and distribution into individual lipid pools (3).
Conclusion
Omega-3 LCPUFA supplements on the market today are available in different formats namely:
• Triglycerides(TG) – as un-processed natural fish oils
• Reconstituted glycerides – FA re-esterified with glycerol (mainly concentrates)
• Ethyl ester – FA re-esterified with ethanol (concentrates)
Caution should be exercised when comparing different omega-3 LCPUFA health care products. They may differ markedly in stability, accretion rates and distribution into individual lipid pools depending on raw material source, lipid form, composition and method of production.
Literatur
1. www.lipidlibrary.co.uk
2. www.crnusa.org
3. Katan MB et al. J Lipid Res 1997;38:2012-22.
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