Omega-3 fatty acids are important nutrients involved in many body activities, especially immune system responses. Your body doesn't produce omega-3 fatty acids and must get them from the food you eat.
Research has shown that omega-3 fatty acids can help reduce the risk of heart disease by decreasing the risk of arrhythmias (abnormal heart rhythms), which can lead to sudden cardiac death. Omega-3 fatty acids also slow the growth of plaque in arteries and reduce levels of the unhealthy type of cholesterol (low-density lipoproteins) and triglycerides in your blood.
Omega-3 fatty acids are a good source of lignans -- compounds that may have a weak estrogen effect. When a weak estrogen-like substance takes the place of your body’s natural strong estrogen in a breast cell’s estrogen receptor, then the weak substance can act as a relative anti-estrogen. By acting in this way, lignans might help work against breast cancer that depends on estrogen for its growth. But research so far on whether omega-3 fatty acids affect breast cancer risk has shown no conclusive association.
The highest concentrations of omega-3 fatty acids are found in coldwater fish, such as sardines, salmon, herring, tuna, cod, mackerel, halibut, and shark. These fatty acids are also found in lower concentrations in plant foods such as flaxseed, walnuts, Great Northern beans, kidney beans, navy beans, and soybeans. Some registered dietitians recommend eating a diet rich in fish with high levels of omega-3 fatty acids or eating 1 or 2 teaspoons of flaxseed every day.
But eating fish has become more of a health concern. Many fish with high levels of omega-3 fatty acids caught in the wild also have high levels of mercury and other environmental pollutants. Research on farm-raised salmon (the most popular farm-raised fish) found that it had higher levels of toxins (other than mercury) than fish caught in the wild. The levels of toxins in other types of wild fish compared to those in farm-raised fish aren't known. Some waters are probably safer than others for wild fish, and some farms are likely to be more health-conscious than others. For now, experts recommend varying the type of fish you eat to reduce the risk of eating too many contaminants. They also recommend eating wild-caught fish about twice a week and farm-raised salmon only about once a month.