The first line of defense in maintaining energy is to break down carbohydrates, or glycogen, into simple glucose molecules -- this process is called glycogenolysis. Next, your body breaks down fats into glycerol and fatty acids in the process of lipolysis. The fatty acids can then be broken down directly to get energy, or can be used to make glucose through a multi-step process called gluconeogenesis. In gluconeogenesis, amino acids can also be used to make glucose.

In the fat cell, other types of lipases work to break down fats into fatty acids and glycerol. These lipases are activated by various hormones, such as glucagon, epinephrine and growth hormone. The resulting glycerol and fatty acids are released into the blood, and travel to the liver through the bloodstream. Once in the liver, the glycerol and fatty acids can be either further broken down or used to make glucose.

Quoted:
Where does it go? Does it get pushed into digestion at some point and you shit it out? It has to be moved somewhere?

Quoted:
Where does it go? Does it get pushed into digestion at some point and you shit it out? It has to be moved somewhere?

When fatty acids are required by other tissues for energy or other purposes, they are released from the triacylglycerols (triglycerides) mainly by the actions of three enzymes, hormone-sensitive lipase, adipose triacylglycerol lipase and monoacylglycerol lipase (see our web page on acylglycerol lipases). Hormone-sensitive lipase is stimulated by the action of the hormones insulin and noradrenalin by a mechanism that ultimately involves phosphorylation of the enzyme by cAMP-protein kinase, thereby increasing its activity and causing it to translocate from the cytosol to the lipid droplet. Perilipin has been described as "the gatekeeper of the adipocyte lipid storehouse". Thus, the lipolytic process is regulated by perilipin, which acts as a barrier to lipolysis in non-stimulated cells, but on stimulation as during fasting is phosphorylated by the cAMP-protein kinase also. This changes its shape and reduces its hydrophobicity, and in the process activates lipolysis. In addition to its activity towards triacylglycerols, hormone-sensitive lipase will rapidly hydrolyse diacylglycerols, monoacylglycerols, retinyl esters and cholesterol esters. In fact, diacylglycerols are hydrolysed ten times as rapidly as triacylglycerols. Within the triacylglycerol molecule, hormone-sensitive lipase preferentially hydrolyses ester bonds in the sn-1 and sn-3 positions, leaving free acids and 2-monoacylglycerols as the main end products.

Less is known of the properties of the adipose triacylglycerol lipase, which was discovered relatively recently, but it is structurally related to the plant acyl-hydrolases in that it has a patatin-like domain in the NH2-terminal region (patatin is a non-specific acyl-hydrolase in potato). It is specific for triacylglycerols yielding diacylglycerols and free fatty acids as the main products. It has low activity only towards diacylglycerols, and none to monoacylglycerols, retinyl esters and cholesterol esters, although it also has transacylase and phospholipase activities. It is located on the surface of the lipid droplet both in the basal and activated states. Adipose triacylglycerol lipase can be activated at the same time as hormone-sensitive lipase and is now believed to be rate limiting for the first step in triacylglycerol hydrolysis. Regulation of the enzymatic activity involves a number of factors, which are now being revealed. For example, a lipid droplet protein, designated 'CGI-58' or 'ABHD5', is known to be an important activating factor. In the resting state this protein binds to perilipin, but on phosphorylation of the latter, it dissociates and interacts with adipose triacylglycerol lipase to activate triacylglycerol hydrolysis. Mutations in adipose triacylglycerol lipase or CGI-58 are believed to be responsible for a syndrome in humans known as ‘neutral lipid storage disease’.

The monoacylglycerol lipase is believed to be the rate-limiting enzyme in monoacylglycerol hydrolysis, i.e. the final step in triacylglycerol catabolism releasing free glycerol and fatty acids, and is found in the cytoplasm, the plasma membrane, and in lipid droplets. It is specific for monoacylglycerols and has no activity against di- or triacylglycerols. As it is the enzyme mainly responsible for deactivation of the endocannabinoid 2-arachidonoylglycerol, and is highly active in malignant cancers, it is attracting pharmaceutical interest.

Free fatty acids released by the combined action of these enzymes are exported into the plasma for transport to other tissues in the form of albumin complexes. The glycerol released is transported to the liver for metabolism by either glycolysis or gluconeogenesis. Eventually, the whole organelle can disappear, including the proteins, the fate of which is uncertain.

Not only does the adipocyte provide a store of energy but it manages the flow of energy through the formation of the hormone leptin, which secretes various factors that communicate with other tissues including cytokines, adiponectin and resistin. The synthesis of leptin is tightly controlled by adipocytes and its main function is believed to be the provision of information on the state of fat stores to other tissues. Lipid droplets may play a role in this process, since perilipin is required for the sensing function. It is evident that caveolae, which contain the proteins caveolins (and presumably sphingolipids) and are particularly abundant in adipocytes, have a major role in lipid metabolism. They modulate the flux of fatty acids across the plasma membrane, and they are involved in signal transduction and membrane trafficking pathways. In addition, insulin is the main hormone that affects metabolism, and the receptor at the plasma membrane is located in caveolae. Thus, adipose tissue metabolism has profound effects on whole-body metabolism, and defects in these processes can have severe implications for the pathogenesis of diabetes and obesity in humans.