<P> Most of the fat found in food is in the form of triglycerides, cholesterol, and phospholipids . Some dietary fat is necessary to facilitate absorption of fat - soluble vitamins (A, D, E, and K) and carotenoids . Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acid (an omega - 6 fatty acid) and alpha - linolenic acid (an omega - 3 fatty acid) because they cannot be synthesized from simple precursors in the diet . Both of these fatty acids are 18 - carbon polyunsaturated fatty acids differing in the number and position of the double bonds . Most vegetable oils are rich in linoleic acid (safflower, sunflower, and corn oils). Alpha - linolenic acid is found in the green leaves of plants, and in selected seeds, nuts, and legumes (in particular flax, rapeseed, walnut, and soy). Fish oils are particularly rich in the longer - chain omega - 3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A large number of studies have shown positive health benefits associated with consumption of omega - 3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses, such as depression, attention - deficit hyperactivity disorder, and dementia . In contrast, it is now well - established that consumption of trans fats, such as those present in partially hydrogenated vegetable oils, are a risk factor for cardiovascular disease . </P> <P> A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity and diabetes . However, a number of very large studies, including the Women's Health Initiative Dietary Modification Trial, an eight - year study of 49,000 women, the Nurses' Health Study and the Health Professionals Follow - up Study, revealed no such links . None of these studies suggested any connection between percentage of calories from fat and risk of cancer, heart disease, or weight gain . The Nutrition Source, a website maintained by the Department of Nutrition at the Harvard School of Public Health, summarizes the current evidence on the impact of dietary fat: "Detailed research--much of it done at Harvard--shows that the total amount of fat in the diet isn't really linked with weight or disease ." </P>

What are the four main types of lipids