Proper nutrition must include essential fatty acids of adequate quantity and quality (of which fats consist). Physiological functions of fats are complex since fats are the most significant source of energy in cells (particularly the nervous system), certain hormones, important for normal digestion. Besides, the human body can use vitamins A, D, E and K only with the presence of fats. There are several types of fats: saturated fats, found mostly in animal products; polyunsaturated fats, to which omega-3 and omega-6 fatty acids pertain (vegetable oils and seafood); monounsaturated fats are mainly in olives and olive oil, nuts, hazelnuts and peanuts.