Depending on your caloric balance -- that is, how many calories you consume each day compared to how many you use -- your body can store fat through fatty acid synthesis or burn fat through fatty acid oxidation. You may also synthesize or oxidize fatty acids to meet other physiological or structural needs of your body. These processes occur through different pathways and in different parts of the cell, allowing your body to regulate the occurrence of each.
The fats you consume provide energy and fat-soluble vitamins to your diet. They also are incorporated into cell membranes and hormones. Your dietary fats are predominantly triglycerides, molecules containing three fatty acids attached to a glycerol. Fatty acids comprise chains of carbon atoms of varying lengths, with hydrogen atoms bound to the carbons. The more hydrogens the carbon atoms hold, the more saturated the fatty acid. Two fatty acids are essential to your diet, meaning your body cannot synthesize them: linoleic acid and alpha-linolenic acid.
Fatty Acid Synthesis
Fatty acid synthesis takes place in the cytoplasm of your cells, the thick liquid matrix within your cells that holds your organelles in place. The initiation of fatty acid synthesis occurs when your pancreas senses high levels of blood glucose, indicating your body has sufficient energy intake. Your pancreas then secretes insulin, which not only promotes the uptake of glucose from blood into your cells but also stimulates the synthesis of two enzymes, fatty acid synthase and acetyl-CoA carboxylase. These enzymes work together to convert acetyl-CoA, a product of glucose metabolism, to malonyl-CoA and then to the fatty acid palmitate. Your cells can then modify palmitate to create the specific fatty acids you need, either for storage or for a fatty-acid dependent process or structure.
Fatty Acid Oxidation
In contrast to fatty acid synthesis, fatty acid oxidation occurs in the mitochondria, a cell organelle that functions to release energy from the food components you eat. The signal for fatty acid oxidation begins with the secretion of glucagon -- a hormone that works in opposition to insulin -- or, in some cases, epinephrine. These hormones stimulate enzymes that clip off fatty acids from triglyceride molecules in your blood or fat stores. Your cells then absorb the circulating fatty acids into their cytoplasm, and once in the cytoplasm they are transported into the mitochondria for oxidation. During fatty acid oxidation, two-carbon units are sequentially cleaved from the fatty acid chain, each producing one molecule of acetyl-CoA. Acetyl-CoA then enters the pathway of glucose metabolism and energy production.
Although fatty acid synthesis and fatty acid oxidation both require the same nucleotide co-factors, they each use different forms of the co-factors. For example, during synthesis, a nucleotide co-factor called NADPH is oxidized, while during oxidation, this co-factor is reduced. From a chemical perspective, this difference allows one process to conserve energy and the other to release energy.