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For over a decade, our team has combed through thousands of research articles published in reputable journals. To help you make educated decisions, and to better understand controversial or confusing supplements, our medical experts have digested the science into these three easy-to-follow ratings. We hope this provides you with a helpful resource to make informed decisions towards your health and well-being.
3 Stars Reliable and relatively consistent scientific data showing a substantial health benefit.
2 Stars Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1 Star For an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support.
This supplement has been used in connection with the following health conditions:
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Docosahexaenoic acid (DHA), an omega-3 fatty acid present in cod liver oil and other fish oils, is important for normal development of the brain and eyes. Studies have shown that higher concentrations of DHA in mothers' milk are associated with better visual acuity in the infants.2 Other studies have suggested that DHA improves the development of infants, although not all research agrees.3 Because DHA in the mother's diet passes into the breast milk,4 some doctors advise nursing mothers to supplement their diet with cod liver oil or another fish-oil supplement. Women wishing to use this or any supplement while breast-feeding should consult their doctors and use only under the supervision of a qualified healthcare practitioner.
1.5 to 1.8 grams daily
Animal and human studies suggest that deficiencies of omega-3 fatty acids may contribute to behaviors associated with unhealthy responses to stress.5 , 6 A double-blind study of students with a low dietary intake of DHA (docosahexaenoic acid, an omega-3 fatty acid) reported that taking 1.5 to 1.8 grams of DHA per day for three months prevented an increase in aggressiveness in these students during a stressful final exam period.7 This group of researchers reported in another double-blind study that 1.5 grams per day of DHA given to medical students during a stressful exam period resulted in changes in some, though not all, blood measurements indicating improved responses to stress.8
How It Works
How to Use It
Most healthy people do not supplement with fish oil containing DHA or vegetarian sources of DHA. The level of DHA given to premature infants who are not breast-fed should be determined by a pediatrician. Much of the research in adults has been based on 1-3 grams per day of DHA from fish oil, although higher levels have been taken when isolated DHA from microalgae sources is used.
Because cod liver oil contains large amounts of vitamin A and vitamin D , women who are or who could become pregnant should consult a doctor before taking cod liver oil. Adults should make sure the total amount of vitamin A and vitamin D from cod liver oil and other supplements does not exceed 25,000 IU (7,500 mcg) per day for vitamin A (15,000 IU per day for those over age 65) and 800 IU per day for vitamin D, unless they are being supervised by a doctor.
Where to Find It
Cold-water fish, such as mackerel, salmon, herring, sardines, black cod, anchovies, and albacore tuna, are rich sources of DHA and EPA. Similarly, cod liver oil contains large amounts of DHA and EPA. Certain microalgae contain DHA and are used as a vegetarian source of this nutrient in some supplements. Most fish oil supplements contain 12% DHA.
Premature infants who are not breast-fed are often DHA-deficient.9 A link has appeared between DHA deficiency and Alzheimer's disease ; however, no evidence at this time indicates that supplementation with DHA will help Alzheimer's patients.10 Similarly, preliminary evidence shows that children with attention deficit disorder (ADD) have low DHA levels. However, no evidence demonstrates that DHA supplementation improves ADD.11 Preliminary evidence suggests that people with a variety of rare but related congenital diseases (Zellweger's syndrome, neonatal adrenoleukodystrophy, and infantile Refsum's disease) may be DHA-deficient, and may even benefit from DHA supplementation.12 Many doctors believe the diets of most people eating a Western diet do not provide optimal amounts of omega-3 fatty acids.
At least four studies have reported a reduced blood level of omega-3 fatty acids in people with depression .13 , 14 , 15 , 16
Interactions with Supplements, Foods, & Other Compounds
Fish oil is easily damaged by oxygen, so small amounts of vitamin E are often included in fish oil supplements to prevent such oxidative damage.17 Doctors often recommend that people who supplement with fish oil or DHA take vitamin E supplements to protect EPA and DHA within the body from oxidative damage. Some evidence indicates that vitamin E may be protective against oxidative damage caused by fish oil.18 However, animal researchers have reported that the oxidative damage caused by DHA alone was not prevented with vitamin E supplementation.19 The level of oxidative damage caused by DHA has not been shown to result in significant health problems.
Some evidence suggests that adding vitamin E to EPA/DHA may prevent the fish oil -induced increase in serum glucose.20 Similarly, the impairment of glucose tolerance sometimes caused by the omega-3 fatty acid has been prevented by the addition of half an hour of moderate exercise three times a week.21 The effect of DHA by itself on glucose levels has not been adequately studied.
People who take fish oil containing EPA and DHA and who also take 15 grams of pectin per day have been reported to have reductions in LDL cholesterol .22 This suggests that pectin may overcome the occasional problem of increased LDL cholesterol from fish oil supplementation. The LDL cholesterol-raising effect of EPA and DHA may also be successfully prevented by taking garlic supplements (or presumably adding garlic to the diet) along with EPA and DHA.23 Adding pectin or garlic when people supplement with DHA by itself has yet to be studied.
Interactions with Medicines
While those with heart disease and diabetes often benefit from fish oil (the primary source of DHA in the diet),24 , 25 such people should check with their doctor before taking more than 3 or 4 grams of fish oil per day for several months. Elevations in blood sugar have sometimes been reported,26 though this may simply be due to small increases in weight resulting from high dietary fish oil.27 While DHA combined with EPA from fish oil consistently lowers triglycerides , it occasionally increases LDL cholesterol .28
According to a report in a Japanese medical journal, three people at high risk for colon cancer developed a variety of cancers after one to two years of supplementation with DHA.29 To date, this report has not been confirmed by other researchers. To the contrary, test tube studies report that DHA is toxic to cancer cells30 and may someday be considered as an adjunct to conventional treatment for cancer.31 Similarly, animal studies suggest that DHA may inhibit cancer.32
1. Helland IB, Smith L, Saarem K, et al. Maternal supplementation with very-long-chain n-3 fatty acids during pregnancy and lactation augments children's IQ at 4 years of age. Pediatrics 2003;111:e39-e44.
2. Jorgensen MH, Hernell O, Hughes E, Michaelsen KF. Is there a relation between docosahexaenoic acid concentration in mothers' milk and visual development in term infants? J Pediatr Gastroenterol Nutr 2001;32:293-6.
3. Gibson RA, Makrides M. Long-chain polyunsaturated fatty acids in breast milk: are they essential? Adv Exp Med Biol 2001;501:375-83.
4. Harris WS, Connor WE, Lindsey S. Will dietary w-3 fatty acids change the composition of human milk? Am J Clin Nutr 1984;40:780-5.
5. Reisbick S, Neuringer M, Hasnain R, Connor WE. Home cage behavior of rhesus monkeys with long-term deficiency of omega-3 fatty acids. Physiol Behav 1994;55:231-9.
6. Stevens LJ, Zentall SS, Abate ML, et al. Omega-3 fatty acids in boys with behavior, learning, and health problems. Physiol Behav 1996;59:915-20.
7. Hamazaki T, Sawazaki S, Itomura M, et al. The effect of docosahexaenoic acid on aggression in young adults. A placebo-controlled double-blind study. J Clin Invest 1996;97:1129-33.
8. Sawazaki S, Hamazaki T, Yazawa K, Kobayashi M. The effect of docosahexaenoic acid on plasma catecholamine concentrations and glucose tolerance during long-lasting psychological stress: a double-blind, placebo-controlled study. J Nutr Sci Vitaminol 1999;45:655-65.
9. Crawford MA, Costeloe K, Ghebremeskel K, et al. Are deficits of arachidonic and docosahexaenoic acids responsible for the neural and vascular complications of preterm babies? Am J Clin Nutr 1997;66(4Suppl):1032S-41S [review].
10. Soderberg M, Edlund C, Kristensson K, et al. Fatty acid composition of brain phospholipids in aging and in Alzheimer's disease. Lipids 1991;26:421-5.
11. Stevens LJ, Zentall SS, Deck JL, et al. Essential fatty acid metabolism in boys with attention-deficit hyperactivity disorder. Am J Clin Nutr 1995;62:761-8.
12. Martinez M, Vazquez E. MRI evidence that docosahexaenoic acid ethyl ester improves myelination in generalized peroxisomal disorders. Neurology 1998;51:26-32.
13. Maes M, Smith R, Christophe A, et al. Fatty acid composition in major depression: decreased omega 3 fractions in cholesteryl esters and increased C20: 4 omega 6/C20:5 omega 3 ratio in cholesteryl esters and phospholipids. J Affect Disord 1996;38:35-46.
14. Edwards R, Peet M, Shay J, Horrobin D. Omega-3 polyunsaturated fatty acid levels in the diet and in red blood cell membranes of depressed patients. J Affect Disord 1998;48:149-55.
15. Peet M, Murphy B, Shay J, Horrobin D. Depletion of omega-3 fatty acid levels in red blood cell membranes of depressive patients. Biol Psychiatry 1998;43:315-9.
16. Maes M, Christophe A, Delanghe J, et al. Lowered omega3 polyunsaturated fatty acids in serum phospholipids and cholesteryl esters of depressed patients. Psychiatry Res 1999;85:275-91.
17. Piche LA, Draper HH, Cole PD. Malondialdehyde excretion by subjects consuming cod liver oil vs. a concentrate of n-3 fatty acids. Lipids 1988;23:370-1.
18. Wander RC, Du S-H, Ketchum SO, Rowe KE. Effects of interaction of RRR-alpha-tocopheryl acetate and fish oil on low-density-lipoprotein oxidation in postmenopausal women with and without hormone-replacement therapy. Am J Clin Nutr 1996;63:184-93.
19. Kubo K, Saito M, Tadokoro T, Maekawa A. Changes in susceptibility of tissues to lipid peroxidation after ingestion of various levels of docosahexaenoic acid and vitamin E. Br J Nutr 1997;78:655-69.
20. Luostarinen R, Wallin R, Wibell L, et al. Vitamin E supplementation counteracts the fish oil-induced increase of blood glucose in humans. Nutr Res 1995;15:953-68.
21. Dunstan DW, Burke V, Mori TA, et al. The independent and combined effects of aerobic exercise and dietary fish intake on serum lipids and glycemic control in NIDDM. Diabetes Care 1997;20:913-21.
22. Sheehan JP, Wei IW, Ulchaker M, Tserng KY. Effect of high fiber intake in fish oil-treated patients with non-insulin-dependent diabetes mellitus Am J Clin Nutr 1997;66:1183-7.
23. Adler AJ, Holub BJ. Effect of garlic and fish-oil supplementation on serum lipid and lipoprotein concentrations in hypercholesterolemic men. Am J Clin Nutr 1997;65:445-50.
24. Leaf A, Weber PC. Cardiovascular effects of n-3 fatty acids. N Engl J Med 1988;318:549-57.
25. Malasanos TH, Stacpoole PW. Biological effects of omega-3 fatty acids in diabetes mellitus. Diabetes Care 1991;14:1160-79.
26. Schectman G, Kaul S, Kassebah AH. Effect of fish oil concentrate on lipoprotein composition in NIDDM. Diabetes 1988;37:1567-73.
27. Toft I, Bonaa KH, Ingebretsen OC, et al. Effects of n-3 polyunsaturated fatty acids on glucose homeostasis and blood pressure in essential hypertension. Ann Intern Med 1995;123:911-8.
28. Harris WS, Zucker ML, Dujovne CA. Omega-3 fatty acids in type IV hyperlipidemia: fish oils vs. methyl esters. Am J Clin Nutr 1987;45:858 [abstract].
29. Akedo I, Ishikawa H, Nakamura T, et al. Three cases with familial adenomatous polyposis diagnosed as having malignant lesions in the course of a long-term trial using docosahexanoic acid (DHA)-concentrated fish oil capsules. Jpn J Clin Oncol 1998;28:762-5.
30. Kafrawy O, Zerouga M, Stillwell W, Jenski LJ. Docosahexaenoic acid in phosphatidylcholine mediates cytotoxicity more effectively than other omega-3 and omega-6 fatty acids. Cancer Lett 1998;132:23-9.
31. Bougnoux P, Germain E, Hubert B, et al. Cytotoxic drugs efficacy correlates with adiposte tissue docosahexanenoic level in locally advanced breast carcinoma. Br J Cancer 1999;79:1765-9.
32. de Bravo MG, de Antueno RJ, Toledo J, et al. Effects of an eicosapentaenoic and docosahexaenoic acid concentrate on a human lung carcinoma grown in nude mice. Lipids 1991;26:866-70.
Last Review: 10-16-2013
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