Adipose tissue triacylglycerol stores

What would happen to a 70-kg man if the 135,000 kcal stored as triacylglycerols in his 18 kg of adipose tissue were stored instead as skeletal muscle glycogen It would take approximately 34 kg glycogen to store as many calories. Glycogen, because it is a polar molecule with -OH groups, binds approximately 4 times its weight in water, or 136 kg. Thus, his fuel stores would weigh 170 kg. 

The triacylglycerols of chylomicrons and VLDL are digested by lipoprotein lipase (LPL), an enzyme found attached to capillary endothelial cells (see Fig. VI.4). The fatty acids that are released are taken up by muscle and many other tissues and oxidized to CO2 and water to produce energy (see Chapter 23). After a meal, these fatty acids are taken up by adipose tissue and stored as triacylglycerols. 

ADIPOSE TISSUE IS THE MAIN STORE OF TRIACYLGLYCEROL IN THE BODY... 

Otfier fiormones accelerate tfie release of free fatty acids from adipose tissue and raise tfie plasma free fatty acid concentration by increasing the rate of lipolysis of the triacylglycerol stores (Figure 25—8). These include epinephrine, norepinephrine, glucagon, adrenocorticotropic hormone (ACTH), a- and P-melanocyte-stimulat-ing hormones (MSH), thyroid-stimulating hormone (TSH), growth hormone (GH), and vasopressin. Many of these activate the hormone-sensitive hpase. For an optimal effect, most of these lipolytic processes require the presence of glucocorticoids and thyroid hormones. These hormones act in a facilitatory or permissive capacity with respect to other lipolytic endocrine factors. 

Adipose tissue This is a specialised connective tissue that stores much of the energy reserve of the body in the form of fat (triacylglycerol) within the cells, known as adipocytes, which are close packed within the connective tissue. Adipose tissue is located in a number of anatomically distinct deposits in the body, e.g. below the skin, around major organs and between muscles. 

Most of the triacylglycerol is stored in adipose tissue depots. Smaller amounts are found in muscle and liver. Adipose tissue is connective tissue that contains adipocytes, blood vessels, collagen and lymphocytes. The last latter are present in lymph nodes, most of which are present within adipose tissue depots throughout the body. Indeed, there may be more lymphocytes in adipose... 

There is no gold standard for measuring the content of the triacylglycerol store, since adipose tissue is distributed in many different depots (see Appendix 2.4 for some of the methods that are available). 

Glutamine is found in all cells in a combined form in peptides or proteins, but also in a free form. The highest free concentration of glutamine is found in muscle, where it acts as a store for use by other tissues. In fact, the total amount in all the skeletal muscle in the body is about 80 g, which is synthesised in the muscle from glucose and branched-chain amino acids (see Chapter 8). As with glycogen in the liver and triacylglycerol in adipose tissue. 

To reduce body mass by burning off stored fuel. This is put forward as one mechanism by which the amount of triacylglycerol stored in adipose tissue can be reduced (Chapters 7, 12 and 15). 

The largest store of fuel in the body occurs in adipose tissue. Approximately 80% of adipose tissue is triacylglycerol (the remainder is connective tissue, water, proteins and DNA). Approximately 90% of an individual adipocyte is triacylglycerol (Figure 7.5). Despite this, triacylglycerol is not released from the adipose tissue. Instead hydrolysis (lipolysis) of the triacylglycerol within adipose tissue... 

After prolonged physical activity, the uptake of fatty acids is directed to the muscles involved in the activity and away from adipose tissue and the inactive muscles, due to a change in the activity of one enzyme. This allows restoration of the triacylglycerol store within the muscle. 

The resulting triacylglycerol is stored in adipose tissue. In the liver, some is combined with protein and phospholipids to form a complex, known as very low density lipoprotein (VLDL), which is secreted from the liver into the blood. Details of the formation of VLDL are presented in Appendix 11.2. Failure to form VLDL or secrete can cause accu-... 

Fuels are stored within the muscle, liver and adipose tissue (Table 13.4) and amounts vary according to the nutritional status of the subject and the previous physical activity (see also Chapter 2). Triacylglycerol that is stored within the muscle can be used but the most significant fat fuel is long-chain fatty acids, derived from triacylglycerol in adipose tissue. 

In the ebb phase, there is increased activity of the sympathetic nervous system and increased plasma levels of adrenaline and glucocorticoids but a decreased level of insulin. This results in mobilisation of glycogen in the liver and triacylglycerol in adipose tissue, so that the levels of two major fuels in the blood, glucose and long-chain fatty acids, are increased. This is, effectively, the stress response to trauma. These changes continue and are extended into the flow phase as the immune cells are activated and secrete the proinflammatory cytokines that further stimulate the mobilisation of fuel stores (Table 18.2). Thus the sequence is trauma increased endocrine hormone levels increased immune response increased levels of cytokines metabolic responses. 

The increased mobilisation of fatty acids from adipose tissue raises the plasma concentration, which increases the rate of fat oxidation by muscle. It also releases some essential fatty acids from the store in the triacylglycerol in adipose tissue. These are required for formation of new membranes in proliferating cells and those involved in repairing the wound (e.g. fibroblasts) (Chapters 11 and 21 Figure 21.22). 

Fat metabolism in adipose tissue (top). Fats (triacylglycerols) are the most important energy reserve in the animal organism. They are mostly stored in insoluble form in the cells of adipose tissue—the adipocytes—where they are constantly being synthesized and broken down again. 

Adipose tissue removes free fatty acids from the lipoproteins, synthesizes triacylglycerols from them again, and stores these in the form of insoluble droplets. 

The liver is the most important site for the formation of fatty acids, fats (triacylglycewls), ketone bodies, and cholesterol. Most of these products are released into the blood, in contrast, the triacylglycerols synthesized in adipose tissue are also stored there. 

Chylomicrons deliver tiiacylglycerols to tissues, where lipoprotein lipase releases free fatty acids for entry into cells. Triacylglycerols stored in adipose tissue are mobilized by a hormone-sensitive triacylglycerol lipase. The released fatty acids bind to serum albumin and are carried in the blood to the heart, skeletal muscle, and other tissues that use fatty acids for fuel. 

When the diet contains more fatty acids than are needed immediately as fuel, they are converted to triacylglycerols in the liver and packaged with specific apolipoproteins into very-low-density lipoprotein (VLDL). Excess carbohydrate in the diet can also be converted to triacylglycerols in the liver and exported as VLDLs (Fig. 21-40a). In addition to triacylglycerols, VLDLs contain some cholesterol and cholesteryl esters, as well as apoB-100, apoC-I, apoC-II, apoC-III, and apo-E (Table 21-3). These lipoproteins are transported in the blood from the liver to muscle and adipose tissue, where activation of lipoprotein lipase by apoC-II causes the release of free fatty acids from the VLDL triacylglycerols. Adipocytes take up these fatty acids, reconvert them to triacylglycerols, and store the products in intracellular lipid droplets myocytes, in contrast, primarily oxidize the fatty acids to supply energy. Most VLDL remnants are removed from the circulation by hepatocytes. The uptake, like that for chylomicrons, is... 

The fuel reserves of a healthy adult human are of three types glycogen stored in the liver and, in relatively small quantities, in muscles large quantities of triacylglycerols in adipose tissues and tissue proteins, which can be degraded when necessary to provide fuel (Table 23-5). 

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