Omega-3 and omega-6 fatty acids are known as essential fatty acids because these nutrients are necessary for health and can only be obtained via the diet. British researchers studying cultured prostate cancer cells found that omega-3 and omega-6 fatty acids have opposite effects. The researchers found that omega-6 fatty acids tended to promote growth of the cells whereas omega-3 fatty acids slowed growth. Omega-6 fats are over-supplied by typical diets in the form of animal fats, red meats, cheese and most vegetable oils. Omega-3 fatty acids are scarce in most diets. The best sources are fatty fish, flax seed and walnuts.