10 Triacylglycerol metabolism
10.1 Digestion and utilization of dietary triacylglycerol
10.2 Utilization of fatty acids: β-Oxidation
10.3 Triacylglycerol utilization
10.4 Fatty acid synthesis
While polar lipids and cholesterol are found in the cell membranes of every cell, triacylglycerol is essentially confined to fat tissue, which stores and releases it, and to the cells in the intestine and the liver that synthesize and degrade it.1 Yet, triacylglycerol is the most abundant lipid species, and the only one with an important role in energy metabolism. We will therefore here focus on triacylglycerol. Cholesterol, which is not important in energy metabolism, will be covered in a separate chapter as well because of its medical importance.
Triacylglycerol is important in human metabolism in two functions:
- As a foodstuff. A significant fraction of our caloric intake is triacylglycerol.
- As an endogenous storage of metabolic energy. This storage can be replenished from dietary fat, or by endogenous synthesis of fat from carbohydrates or proteins.
- One gram of triacylglycerol itself contains roughly twice as many calories as one gram of carbohydrates or protein. This is because triacylglycerol contains much less oxygen than carbohydrates, in which oxygen contributes half the mass but essentially no metabolic energy. Similarly, protein contains oxygen, nitrogen and sulfur and is very close in energy content to carbohydrates.
- Triacylglycerol in fat cells coalesces to droplets that are entirely free of water. In contrast, protein and carbohydrates, including glycogen, always remain hydrated.
Because of this efficiency of storage, it makes sense that most of the excess glucose or protein is converted to fat, while only a limited fraction is stored as glycogen. There is, however, one limitation to the use of triacylglycerol: Once carbohydrate or protein carbon has been converted to fat, it can go back and forth between fat, fatty acids, acetyl-CoA, and ketone bodies, but it is no longer available for the regeneration of glucose (Figure 10-1b). Therefore, when starving, we will always have to degrade some protein along with fat to keep up a minimum supply of glucose, and therefore starving people from day zero will not only deplete their fat stores but also their muscle tissue.
1: There is also some intracellular storage of triacylglycerol e.g. in the muscle cells.