Lipids include fats, oils, waxes, cholesterol, other sterols, and most steroids. In the body, fat serves as a source of Energy, a thermal insulator and cushion around organs, and an important cellular component. The fat-soluble vitamins are A, D, E, and K. You are probably most familiar with the nutritional aspects of dietary fats and oils. Since fats have 2.25 times the energy content of carbohydrates and proteins, most people try to limit their intake of dietary fat to avoid becoming overweight. The food industry has a big market for low-fat and non-fat foods. Just take a look around your local grocery stores! Lipids are classified as organic compounds that are soluble (dissolvable) in organic solvents, but only sparingly soluble in water. Lipids are biologically important for making barriers (membranes of animal cells), which control the flow of water and other materials into a cell.