Fat is our body's best energy reserve. Each gram of sugar and protein can only provide 4 calories, but each gram of fat can produce 9 calories, which is more than twice the previous two. 2. Make up body cells and tissues. There are 60 trillion cells in our body, and the outer layer of each cell is composed of a double lipid membrane. Oil is an important component of the cell membrane. The oil on the cell membrane determines the hardness and fluidity of the cell membrane, which in turn affects the entry and exit of the cell.