Adipose tissue triglyceride, fuel

Adipose tissue triglyceride, fuel reserve, 31t Aerobic exercise... 

In the post-absorptive state stored fat is presented to the tissues in four forms free fatty acids derived directly from adipose tissue triglycerides formed in the liver from fatty acids and acetoacetate and hydroxybutyrate formed by the partial oxidation of free fatty acids. Free fatty acids and ketone bodies are readily oxidizable. It may be that more than one form is necessary because free fatty acids alone, owing to their low solubility and their relative toxicity, cannot be transported in sufficient quantities to meet the fuel requirements of some tissues, such as heart muscle. The other possibility for the existence of multiple forms of this substrate is that it allows direction of particular fuels to specific organs. 

The liver also plays a central role in lipid metabolism. When excess fuel is available, the liver synthesizes fatty acids. These are used to produce triglycerides that are transported from the liver to adipose tissues by very low density lipoprotein (VLDL) complexes. In fact, VLDL complexes provide adipose tissue with its major source of fatty acids. This transport is particularly active when more calories are eaten than are burned During fasting or starvation conditions, however, the liver converts fatty acids to acetoacetate and other ketone bodies. The liver cannot use these ketone bodies because it lacks an enzyme for the conversion of acetoacetate to acetyl CoA. Therefore the ketone bodies produced by the liver are exported to other organs where they are oxidized to make ATP. 

Triglycerides are the major form of fuel in birds, and the principal stores are in the adipose tissue and liver in adult birds, and in the yolk in developing embryos. Skeletal muscle and cardiac muscle are the tissues which use most lipid, but most tissues can use... 

Current interest in the study of adipose tissue metabolism has been stimulated by the work of many researchers in various countries. It is not my purpose to give a full account of the historical development of the subject, but a few highlights should be mentioned. Among these (a) the demonstration of the rapid turnover of stored fat " (b) the publication in 1948 of the classic review on the physiology of adipose tissue by Wertheimer and Shapiro (c) the identification of the plasma free fatty acids (FFA) as the form in which the triglycerides stored in the adipose tissue are mobilized and made available as fuel to the body tissues, - and the introduction of the concept of caloric homeostasis . ... 

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