Evidence-based human studies have shown that obtaining the desired health benefits of DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid) omega-3 fatty acids—including brain and visual support, cardio care, blood triglyceride lowering, and others—requires sufficient daily intake and duration (weeks to several months). (For more information, visit www.dhaomega3.org.) Of course, the relative bioavailability of omega-3 fatty acids from different sources (e.g., fish versus supplements versus functional foods) and when consumed from different structural forms (triglyceride, ethyl ester, free fatty acid, and phospholipid) is of considerable importance as well.
While the amounts of DHA/EPA in a product are often known to the consumer, no information on the relative bioavailability of the omega-3 fatty acids present is generally provided. However, both the DHA/EPA amounts in the product, and the bioavailability of the DHA/EPA, are of utmost importance to ensure appropriate delivery of the omega-3 fatty acids to the bloodstream following their digestion and passage across the intestinal wall and eventual assimilation to target tissues.
Most fish-derived DHA/EPA omega-3 is present in the “natural” triglyceride form, with a much lesser amount present in the phospholipid form. From fish, fish oil is derived, processed, and sold in encapsulated supplements (or bottled oils) as a source of DHA/EPA in the triglyceride form.
Further, the omega-3 fatty acids therein can be industrially converted to the ethyl ester forms of DHA/EPA, which allows for their concentration via procedures such as molecular distillation. Such oil concentrates of DHA/EPA can be sold as supplements or be converted industrially back to the triglyceride form prior to being incorporated in commercial supplements.
The term bioavailability generally refers to the capability of the orally ingested omega-3 fatty acids to be digested in the gastrointestinal tract and cross the intestinal wall, thereby entering the bloodstream and, subsequently, the various body tissues and organs. The bioavailability of DHA/EPA in humans has been measured via both acute studies (very short-term studies, such as a few hours after consumption) or via chronic studies (long-term daily consumption over weeks or months). The net rise in omega-3 fatty acid levels in blood samples at the end of the study relative to baseline levels is regularly used to assess the relative bioavailability of different sources/forms of omega-3 fatty acids when compared at essentially equal levels of intake.
Because of the wide variability in the results between individual subjects and other factors, acute studies appear not to be as dependable as chronic studies in making conclusions with respect to bioavailability.
Triglyceride versus Ethyl Ester
There have been a number of short-term acute studies in human subjects which have attempted to determine if the bioavailability/absorption of EPA and DHA, when taken as supplements, is any different when the “natural” triglyceride form versus the ethyl ester form is consumed. The results from such human studies appear to be somewhat influenced by the experimental designs used for such evaluations, including the doses, duration, and timing of the blood measurements for the resulting omega-3 accumulations following supplementation.
The early short-term studies by Lawson and Hughes (1,2) indicated a much better apparent human absorption of both EPA and DHA (at intake levels of 1.00 and 0.67 g, respectively) in the triglyceride form compared to the ethyl ester form. Furthermore, they reported that the marked differences in absorption between the two forms were less pronounced after a high-fat meal compared to a low-fat meal.
Subsequent studies by el Boustani et al. (3) reported a greater incorporation of EPA into circulating blood plasma fat (as triglyceride) when EPA (1 g) was consumed as the triglyceride form relative to the ethyl ester form.
Maximal plasma levels of DHA/EPA were also found to be significantly lower with ethyl ester forms as compared to triglyceride forms in a German study. (4)
In contrast to the previous studies, Luley and colleagues (5) found similar bioavailability for triglyceride versus ethyl ester preparations of EPA/DHA. In support of the latter study, Nordoy et al. (6) provided test meals to human subjects, with very high doses of omega-3 fatty acids (28 g) as triglyceride or ethyl ester forms. Blood sampling and measurements conducted at 24 hours after consumption of the omega-3 meals indicated their concentrations to be similar. The authors concluded that fish oil omega-3 fatty acids given as ethyl ester or triglyceride form were equally well absorbed, and that EPA and DHA were also equally absorbed.
Food versus Supplements
Long-term chronic studies have compared the relative bioavailability of DHA/EPA when ingested as fish or supplements.
The early study by Visioli et al. (7) indicated a greater net rise in human blood plasma levels of DHA after six weeks when DHA from salmon was consumed daily, compared to DHA intakes via supplementation (as the ethyl ester form)—even when the latter DHA intakes were higher than those from fish.
A subsequent study by Harris et al. (8) compared blood levels of omega-3 fatty acids in subjects having similar intakes of DHA and EPA from salmon (mainly triglyceride form) or supplemental ethyl ester form. The EPA level in the red blood cells was found to rise faster in the fish group by four weeks, with no apparent differences in bioavailability of DHA/EPA exhibited by the end of the study at 16 weeks.
Based on a two-week chronic study that measured the rise in blood levels of DHA in subjects consuming equivalent daily intakes of DHA from cooked salmon (mainly triglyceride form) versus supplemental DHA from an algal source (as triglyceride), a bio-equivalence of the two DHA sources was found. (9)
1. L Lawson and B Hughes, “Human absorption of fish oil fatty acids as triglycerides, free acids, or ethyl esters,” Biochemical and Biophysical Research Communications, vol. 152, no. 1 (Apr 15, 1988): 328-335.
2. L Lawson and B Hughes, “Absorption of eicosapentaenoic acid and docosahexaenoic acid from fish oil triglycerols or fish oil ethyl esters co-ingested with a high-fat meal,” Biochemical and Biophysical Research Communications, vol. 156, no. 2 (Oct 31, 1988): 960-963.
3. S el Boustani et al., “Eternal absorption in man of eicosapentaenoic acid in different chemical forms,” Lipids, vol. 22, no. 10 (Oct 1987): 711-714.
4. B Beckermann et al., “Comparative bioavailability of eicosapentaenoic acid and docosahexaenoic acid from triglycerides, free fatty acids and ethyl esters in volunteers,” Arzneimittelforschung, vol. 40, no. 6 (June 1990): 700-704.
5. C Luley et al., “Bioavailability of omega-3 fatty acids: ethyl ester preparations are as suitable as triglyceride preparations,” Akt Ernahrungsmed, 15 (1990): 122-125.
6. A Nordoy et al., “Absorption of the n-3 eicosapentaenoic and docosahexaenoic acids as ethyl esters and triglycerides by humans,” American Journal of Clinical Nutrition, vol. 53, no. 5 (May 1991): 1185-1190.
7. F Visioli et al., “Dietary intake of fish vs. formulations leads to higher plasma concentrations of n-3 fatty acids,” Lipids, vol. 38, no. 4 (April 2003): 415-418.
8. WS Harris et al., “Comparison of the effects of fish and fish-oil capsules on the n-3 fatty acid content of blood cells and plasma phospholipids,” American Journal of Clinical Nutrition, vol. 86, no. 6 (Dec 2007): 1621-1625.
9. LM Arterburn et al., “Algal-oil capsules and cooked salmon : nutritionally equivalent sources of docosahexaenoic acid,” Journal of the American Dietetic Association, vol. 108, no. 7 (July 2008): 1204-1209.
10. SA Mathews et al., “Comparison of triglycerides and phospholipids as supplemental sources of dietary long-chain polyunsaturated fatty acids in piglets,” Journal of Nutrition, vol. 132, no. 10 (Oct 2002): 3081-3089.
11. VP Carnielli et al., “Intestinal absorption of long-chain polyunsaturated fatty acids in preterm infants fed breast milk or formula,” American Journal of Clinical Nutrition, vol. 67, no. 1 (Jan 1998): 97-103.
12. CJ Barrow et al., “Bioequivalence of encapsulated and microencapsulated fish-oil supplementation,” Journal of Functional Foods, 1 (2009): 38-43.
13. J Dyerberg et al., “Bioavailability of marine n-3 fatty acid formulations,” Prostaglandins, Leukotrienes, and Essential Fatty Acids, vol. 83, no. 3 (Sept 2010): 137-141.
14. J Neubronner et al., “Enhanced increase of omega-3 index in response to long-term n-3 fatty acid supplementation from triacylglycerides versus ethyl esters,” European Journal of Clinical Nutrition, vol. 65, no. 2 (February 2011): 247-254.