The EPA is the English abbreviation of eicosapentaenoic acid, while DHA is for doco-sahexaenoic acid. The Eicosapentaenoic acid and docosahexaenoic acid both belong to w - 3 series of polyunsaturated fatty acids (PUFA), namely the long-chain PUFA of the third terminal methyl from a number of carbon atoms is a double bond. This exists mainly in deep-sea cold water fish in nature. With the difference of seasons and producing areas, fish oil content of EPA and DHA is between 4% ~ 40%. The transformed from the desaturation enzyme to the linolenic acidin human body. However, the linolenic acid cannot be compounded in human body, which needs one of the essential fatty acids provided by food. EPA and DHA are associated with many human body phenomenon and play an important role to maintain the body health.
The physiological function of EPA and DHA
It is an important composition of cephalin that the DHA maintains brain and retinal function and delays the aging of the brain, which abounds in brain tissue and relates to the brain cell function. The insufficiency of DHA renders the infant brain developmental disorders, adolescent mental retardation, middle-aged and old brain premature deterioration. It benefits for maintaining neural functioning that human can eat more fish which riches in DHA in brain, especially the synapses have more DHA distribution of phospholipids.
Michael, the professor of brain nutrition chemical research institute first proposed in the international academic seminar of DHA that:” DHA can not only promote human brain activity, but make it intelligent. “Later research has proved that DHA is one of the main materials of the human brain, accounting for about 10% of the human brain fat, which mainly exists in the central nervous system cells and retinal cells. Animal experiments confirmed that, DHA accumulation deficiency in the nervous system and the retina can cause vision electroretinogram waveform change and sensitivity reduction. Demon found that the EPA can quickly transfer into DHA and accumulate in the brain when he studied the EPA of chicken brain development, which shows DHA is the material that really plays an important role in brain development. During the womb, the need for DHA is from zygote division in mother's womb therefore the pregnant women should intake enough DHA, which promotes the development of the fetal brain and brain cells proliferation. Under normal circumstances, a pregnant woman should intake 0.5 ~ 0.5 g DHA every day (ordinary people need to 0.5 ~ 1.0 g), the intake of DNA is significant especially in the fetal period from the 4th month gravid to the baby after the birth of late l namely the period that did not finish this important of brain development. Therefore in early this process of the formation of the brain, timely and moderately replenishing the essential polyunsaturated fatty acids in a right amount is very necessary.
To prevent diseases of heart head blood-vessel
DHA and EPA theoretically have the effect of thrombosis, regulating blood lipid, anticoagulant, dilate blood vessels, lower blood pressure, resisting arrhythmia, and so on, thus they can prevent blood brain diseases.

　　In addition, the more fish people eating rich in DHA and EPA, the lower incidence of disease of heart head blood-vessel they get, proved by a great deal of researches. For instance, eskimos in Greenland eat more marine mammals and fish that leads to a lower morbidity and mortality of the serum cholesterol, triglycerides, and coronary heart disease (CHD) than that in immigrants moved to Europe, reported by Danish scientists in 1975. The coronary heart disease mortality rate of Japanese fishermen is lower than the ordinary farmers, reported by Hirai in 1982. The Eskimo in Canada and Quebec has a higher consumption of fish with a lower coronary heart disease risk, reported by Dewailly in 2001. What’s more, compared with the group of “little fish” and “no fish” meal, the “fish meal” group has a 52% of reduction in sudden cardiac sudden death rate, 32% in total heart disease mortality. The effective intake of EPA/DHA: 0.3~2.7g/d, namely eating fish more than once a week. Another study---11324 patients with first myocardial infarction, shows that taking EPA + DHA 850 mg per day leads a 15% reduction of the total mortality, and 30% coronary heart disease mortality. The 20-year follow-up survey for 852 people made by Netherlands and the 10-year follow-up survey for 12866 people made by the US indicate that fish consumption is negatively correlated with coronary heart disease mortality, namely the more fish people eat the lower rate of coronary heart disease mortality is. And provided the data analysis of 36 countries, we know adding fish intake can reduce coronary heart disease and stroke mortality, total mortality. The protection from fish for cardiovascular: mainly mediated by n - 3 PUFA (EPA and DHA) without ruling out the role of other components.
Inhibit the growth of tumor epidemiology survey found that the Eskimo woman mainly eating sea food has a low mortality rate result from breast cancer. Chengze said EPA and DHA in fish oil have the effect of inhibitory colorectal cancer while the inhibitory effect of DNA is stronger. Dustin found DHA can restrain the activation of macrophages and the activity of killing tumor cells. DHA can also reduce the antitumor drug resistance of gastric cancer, bladder cancer and ovarian cancer. The high purity of DHA can produce abnormal gland nest on inhibition of colon mucosa and restrain the growth of the abnormal gland nest.
Anti-inflammatory and inhibiting allergic reaction
The Eskimos have lower morbidity than the local white people in diseases caused by abnormal autoimmune chronic inflammatory such as asthmatic bronchitis, rheumatoid arthritis, lupus erythematosus (sle). The great amount intake of EPA and DHA from fish is one of the important reasons. The supplementation with fish oil can reduce collagen caused by arthritis and resist the dermatitis, decreasing the morbidity of parapsoriasis guttata. Animal experiments found that animals with EPA can reduce their experimental inflammatory edema degree.

　　The reasonable application of DHA and EPA
Although EPA and DHA have many beneficial effects, they are not the more the better. EPA and DHA contain five and six double bond respectively, which are highly unsaturated fatty acids, vulnerably to be attacked by active oxygen free radicals in the body and cause lipid peroxidation, leading to the lipid peroxide. Lipid peroxide can damage cell membranes, while the function of immune cells highly depends on the normal membrane structure and function. Therefore, the lipid peroxidation of immune cell membrane structure and function of damage will cause adverse effect on the immune function. In addition, lipid peroxide can destroy DHA in the body and cause cancer, while the malondialdehyde in lipid peroxide render protein cross-linking which leads muscle losing elasticity, increasing in melanin with the senile plaque appearance those results in atherosclerosis. Excessive intake of EPA and DHA can also cause other adverse reactions. Experimental results show that in 100 days, with marine animals only that can cause weight reduction, hemorrhage time interval prolonging, thrombocytopenia, reduction in prostaglandins in semen and the activity of sperm even the disappearance.
Given that the supplementation with fish oil may lead to the lipid peroxidation, a large dose is needed to achieve the desired effect （4g/d）, people who intake it often have intestinal discomfort. Therefore the WTO suggests that people should regard fish as the main source of EPA and DHA. As for seawater and freshwater fish, their contents accord to their sorts. Generally speaking, as long as eating freshwater fish in a high frequency people can still meet the needs of the human body of EPA and DHA..