Fatty acids, n-3 as well as n-6, are essential for normal physiological functioning and for the health of humans and all domestic species. In humans, not all fatty acids can be produced endogenously due to the absence of certain desaturases. Thus, specific fatty acids termed essential (linoleic, alpha-linolenic) need to be taken from the diet. Other fatty acids whose synthesis depends on essential fatty acid intake include eicosapentaenoic acid and docosahexaenoic acid, found in oily fish. Dietary sources of saturated fatty acids are animal products and tropical plant oils, whereas sources of unsaturated fatty acids are vegetable oils and marine products. In recent years there has been a greater demand for foods with increased levels of functional fatty acids, such as long-chain omega-3 fatty acids and conjugated linoleic acids, because of their biological roles in cells. Hence, there is a need to develop alternative food sources to increase consumption of n-3 polyunsaturated fatty acid. The main dietary sources of eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) are marine algae, fish, and fish oils. However, this review consists of two parts. The present part aims at describing fatty acid chemistry, nutrition, metabolism functions and increase essential fatty acid content of milk and meat in ruminants. The second part of further article, present knowledge and approaches to increase essential fatty acid content of egg and meat in poultry will reviewed.