Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Carbohydrate and Lipid Metabolism
Lipid Transport and Storage
Metabolic Pathways in Adipose Tissue and Fat Mobilization: Comparative Aspects

Human adipose tissue does not appear to be a major site of Lipogenesis. This is indicated by experiments using glucose and Pyruvate labeled with radioactive tracers, which have shown that only a very small fraction of the label is incorporated into long-chain Fatty acids. Adipose tissue apparently lacks a key enzyme of lipogenesis, ATP-citrate lyase; it should also be noted that The activity of this enzyme in Liver Cells is very low. Other Enzymes, such as glucose-6-phosphate dehydrogenase and malic enzyme, whose activity adaptively increases in rat tissue during enhanced lipogenesis, show no change in activity in human adipose tissue. It is hypothesized that humans exhibit a so-called "excess carbohydrate syndrome," caused by the body's limited capacity to transform excess CARBOHYDRATES via lipogenesis. In birds, lipogenesis takes place in The Liver and assumes special significance due to the high lipid demand for egg formation. Human Adipose tissue is insensitive to most Hormones that stimulate lipolysis, with the exception of catecholamines. Of additional interest is the lack of a "lipolytic" response to epinephrine in rabbits, guinea pigs, pigs, and chickens; the pronounced lipolytic effect of Glucagon in birds coupled with the absence of an antilipolytic effect of Insulin; and, finally, the absence of acylglycerol synthesis from glucose in pigeons. Taken together, these findings clearly indicate that diverse mechanisms regulate metabolic processes in the adipose tissue of the animal species studied.

Considering, on the one hand, that patients with Diabetes Mellitus suffer profound metabolic disturbances primarily resulting from an excessive influx of free fatty acids from fat depots, and, on the other hand, that insulin largely corrects these abnormalities, it can be concluded that this hormone plays a vital role in regulating METABOLISM within adipose tissue.