Triglycerides, or triacylglycerols, are the main storage form of fats within the body. They are predominantly found as solids in peripheral adipocytes and circulate in all the subtypes of lipoprotein carrier.
They consist of a glycerol molecule conjugated to 3 fatty acid molecules.
Triglycerides are primarily an energy source; they liberate the largest amount of energy per unit mass of any of the fuel sources.
A lean adult has 15kg of triglyceride (which yields about 38 kilojoules for each gram respired - it thus represents an energy store of 570,000 kilojoules - this is approximately enough energy to survive for 3 months).
The adipose tissue in which triglyceride is stored performs various roles:
- triglyceride is a liquid at body temperature - the layers of fat around organs such as the kidney (perinephric fascia) and in the omentum provide protection via acting as fluid cushions
- thermal insulation
Triglycerides are digested in the gut to fatty acids and monoglycerides. These are in turn absorbed into the enterocytes and chylomicrons are synthesized for transport to the tissues. The liver is also able to synthesize triglycerides using fatty acids (either from the circulation or synthesized from glucose). The liver assembles triglycerides within the very low density lipoproteins (VLDL) which are released into the circulation.
Deriving fatty acids from adipose tissue stores:
- an intracellular enzyme, hormone-sensitive lipase, becomes active
- fatty acids are then released from glycerol (which they are bound in the stored triglyceride)
- fatty acids move out of the adipose cell and are transported (bound to albumin)
to other tissues - generally referred to as non-esterified fatty acids (NEFA)
- tissues such as skeletal and cardiac muscle can use NEFA directly for respiration
- many tissues lack the enzymes required to break down NEFA to 2-carbon acetyl CoA which is necessary to enter the Krebs cycle. The liver undertakes this task and produces ketone bodies (acetone, acetoacetate,3-hydroxy-3-butyrate) - these are water soluble and transported to tissues where they can be converted to acetyl CoA
- hormone-sensitive lipase is inhibited by insulin (therefore becomes active when insulin levels are low); also activated by certain stress hormones