They can be found in fish, such as salmon, tuna, and halibut, other marine life such as algae and krill, certain plants, and nut oils. Also known as polyunsaturated fatty acids (PUFAs), they play an important role in brain function as well as normal growth and development. 

The American Heart Association recommends eating fish at least twice a week. It is advised that pregnant women and mothers, nursing mothers, young children, and women who might become pregnant not eat several types of fish, including swordfish, shark, and king mackerel. These individuals should also limit consumption of other fish, including albacore tuna, salmon, and herring. 

There are three major types of  fatty acids that are ingested in foods and used by the body: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). 

Once eaten, the body converts ALA to EPA and DHA, the two types readily used by the body. Extensive research indicates that they reduce inflammation and help prevent risk factors associated with chronic diseases such as heart disease, cancer, and arthritis. These essential fatty acids are highly concentrated in the brain and appear to be particularly important for brain memory and performance. In fact, infants who do not get enough from their mothers during pregnancy are at risk for developing vision and nerve problems. 

Symptoms of deficiency include extreme tiredness and fatigue, poor memory, dry skin, heart problems, mood swings or depression, and poor circulation.

It is important to maintain an appropriate balance in the diet, as they work  to promote health. Omega-3 help reduce inflammation, and omega-6 tend to help inflammation. An inappropriate balance contributes to the development of disease while a proper balance helps maintain and even improve health. A healthy diet should consist of roughly 2 - 4 times more omega-6 than omega-3. 

The typical American diet tends to contain 14 - 25 times more 6 than 3, and many researchers believe this imbalance is a significant factor in the rising rate of inflammatory disorders.

In contrast, however, the Mediterranean diet consists of a healthier balance, and many studies have shown that people who follow this diet are less likely to develop heart disease. It also contains another fatty acid, omega-9 , which have been reported to help lower risks associated with cancer and heart disease. The diet does not include much meat and emphasizes foods like whole grains, fresh fruits and vegetables, fish, olive oil, garlic, as well as moderate wine consumption.

Clinical studies suggest they may be helpful in treating a variety of health conditions. The evidence is strongest for heart disease and problems that contribute to heart disease, but the range of possible uses acids include:

High Cholesterol

Those who follow a Mediterranean-style diet tend to have higher high density lipoprotein (HDL or "good" ) cholesterol levels. Similar to those who follow a Mediterranean diet, Inuit Eskimos, who consume high amounts of  fish, also tend to have increased HDL cholesterol and decreased triglycerides. 

In addition, fish oil supplements containing eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have been reported in several large clinical studies to reduce low density lipoprotein (LDL or "bad") cholesterol and triglyceride levels. Finally, walnuts have been shown to lower total cholesterol and triglycerides in individuals with high cholesterol levels.

High Blood Pressure

Several clinical studies suggest that they can lower blood pressure significantly in individuals with hypertension. An analysis of studies using fish oil supplements found that supplementation with three grams of fish oil daily can lead to significant reductions in blood pressure in individuals with untreated hypertension.

Diabetes

Individuals with diabetes tend to have high triglyceride and low HDL levels. The acids from the oil can help lower triglycerides and markers of diabetes, and raise HDL, so people with diabetes may benefit from eating foods or taking supplements that contain DHA and EPA. ALA (from flaxseed, for example) may not have the same benefit as DHA and EPA because some people with diabetes lack the ability to efficiently convert ALA to a form the body can use. 

Weight loss

Many individuals who are overweight suffer from poor blood sugar control, diabetes, and high cholesterol. Clinical studies suggest that overweight people who follow a diet and weight loss program that includes exercise tend to achieve better control over their blood sugar and cholesterol levels when fish such as salmon, mackerel, and herring is a staple in their low-fat diet.

Arthritis

Most clinical studies investigating the use of supplements for inflammatory joint conditions have focused almost entirely on rheumatoid arthritis. Several articles reviewing the research in this area conclude that supplements reduce tenderness in joints, decrease morning stiffness, and allow for a reduction in the amount of medication needed for people with rheumatoid arthritis.

Osteoporosis

Clinical studies suggest that EPA helps increase levels of calcium in the body, deposit calcium in the bones, and improve bone strength. Studies also suggest that people who are deficient in EPA and gamma-linolenic acid are more likely to suffer from bone loss than those with normal levels. 

Depression

People who do not maintain a healthy balance acids in their diet may be at an increased risk for depression. They help nerve cells communicate with each other, which is important in maintaining good mental health. In particular, DHA is involved in a variety of nerve cell processes.

Levels were found to be low and the ratio of 6 to 3 were particularly high in a clinical study of patients hospitalized for depression. In a clinical study, those who ate a healthy diet consisting of fatty fish two to three times per week for five years experienced a significant reduction in feelings of depression and hostility.

Bipolar disorder

In a clinical study with bipolar disorder, those who were treated with EPA and DHA (in combination with their usual mood stabilizing medications) for four months experienced fewer mood swings and recurrence of either depression or mania than those who received placebo. 

Menstrual Pain

In a clinical study of  women, those with the highest dietary intake of omega-3 fatty acids had the mildest symptoms, such as hot flashes and increased sweating, during menstruation.

Dietary Sources