However there are a few more serious side effects that may be seen with high dose fish oil consumption which may warrant caution.
The first of these is that high doses of fish oil may decrease immune function (in opposition to lower intakes, which may enhance it) [40-42]. While this is beneficial in some autoimmune diseases it is not always desirable. For example, high amounts of fish oil may impair bacterial resistance . Modest doses, in the range of 1-2 g of EPA/DHA daily, do not appear to have a negative impact on immune function over 6 months . This effect may also be avoided with supplemental vitamin E . A second potential problem is increased lipid peroxidation, resulting in an increase in oxidative stress [44-47]. However, this effect can also be remedied with vitamin E [9, 48].
Another possible side effect is an increase in LDL cholesterol and a decrease in HDL cholesterol [49-50]. The first of these effects can be seen with doses as low as 3.6 g/day in humans . However, LDL increase is generally less than 5% , and fish oil on balance has a very positive impact on the cardiovascular system. Since fish oil thins the blood, it may also increase the likelihood of bleeding, but it does not appear to do this at lower doses. 2-5 grams a day, even when combined with other blood thinners such as aspirin, do not appear to increase bleeding time, but intake over 20 grams a day will increase bleeding times . Other possible side effects of high or very high dose fish oil consumption reported in animals are increased liver and spleen weight, adverse effects on iron metabolism, and red blood cell deformities [45, 46, 50], but it is doubtful that these are relevant in moderate doses.
What all of this amounts to is, fish oil consumption in a healthy individual should probably be kept within a reasonable range, and additional supplementation with vitamin E is also a good choice. The amount of fish oil one takes should be dependent on both goals and the amount of EPA/DHA present in the fish oil. The optimal range for both safety and effectiveness in most healthy individuals is 1-4 g of EPA/DHA daily, and this amount shouldn't be exceeded without medical supervision. Most fish oils are standardized to 30% EPA/DHA, so this would be about 3-12 one gram caps daily. If the fish oil is standardized to a different amount the dosage should be changed accordingly, for example 2-8 caps of a 50% EPA/DHA product. Most of the benefits (other than possibly the change in body composition, for which there is presently little functional data) can be seen with 1-2 grams of EPA/DHA daily. Most fish oil capsules also contain vitamin E, but if they don't, a vitamin E supplement should be taken also.
45 - IRP1 activity and expression are increased in the ...[J Nutr. 2003] - PubMed Result
46 - Effects of fish oil- and olive oil-rich diets on i...[Br J Nutr. 2003] - PubMed Result
47 - Subchronic toxicity of fish oil concentrates in male and female rats.