Despite the fact that eating a jelly doughnut seems to deposit fat directly on your hips, converting sugar to fat is actually a relatively complex chemical process. Sugar conversion to fat storage depends not only upon the type of foods you eat, but how much energy your body needs at the time you eat it.
Your body converts excess dietary glucose into fat through the process of fatty acid synthesis. Fatty acids are required in order for your body to function properly, playing particularly important roles in proper brain functioning. There are two kinds of fatty acids; essential fatty acids and nonessential fatty acids. Essential fatty acids refer to fatty acids you must eat from your diet, as your body cannot make them. Nonessential fatty acids are made through the process of fatty acid synthesis.
Fatty Acid Synthesis
Fatty acids are long organic compounds having an acid group at one end and a methyl group at the other end. The location of their first double bond dictates whether they are in the omega 3, 6, or 9 fatty acid family. Fatty acid synthesis takes place in the cytoplasm of cells and requires some energy input. In other words, your body actually has to expend some energy in order to store fat.
Glucose is a six-carbon sugar molecule. Your body first converts this molecule into two three-carbon pyruvate molecules through the process of glycolysis and then into acetyl CoA. When your body requires immediate energy, acetyl CoA enters the Citric Acid Cycle creating energy molecules in the form of ATP. When glucose intake exceeds your body's energy needs--for example, you eat an ice-cream sundae and then go relax on the sofa for five hours--your body has no need to create more energy molecules. Therefore, acetyl CoA begins the process of fatty acid synthesis becoming triglycerides that are stored in the fat tissues of your body. These triglycerides are stored energy molecules which can be broken down later to give you the energy you need to, for example, get up off the couch and go for a bike ride.
Regulation of Fatty Acid Synthesis
Fatty acid synthesis is influenced by foods you eat and hormones you release. When blood glucose levels are high, such as after eating a sugary meal, your body releases insulin. Insulin stimulates the formation of Fatty Acid Synthase, an enzyme that increases fat storage. On the other hand, polyunsaturated fatty acids decrease the formation of the Fatty Acid Synthase enzyme, implying that eating foods containing polyunsaturated fats may not lead to as much increased fat storage as eating sugary foods. In addition, when your fat cells increase their fat storage, a molecule called leptin is produced. Leptin leads to decreased food intake, increased energy expenditure, as well as inhibition of fatty acid synthesis.
Fat is broken down through the process of lipolysis and beta-oxidation. These reactions occur in the mitochondria or energy warehouses of the cells. The process is cyclical; two carbons are removed from the long fatty acid chain per turn of the cycle, forming acetyl CoA. At this point, acetyl CoA enters the Citric Acid Cycle to produce energy in the form of ATP. This process uses the identical pathway glucose molecules used after glycolysis when they were directed to make energy for the cell rather than being stored as fat.