The fat stores in your body are metabolically active and dynamic tissues. Two opposing forces determine the amount of fat you carry around from day to day. Lipogenesis is the process that leads to the deposition of lipids and the growth of fat tissue, while lipolysis is the process of fat breakdown and removal. These two physiologic activities are controlled by hormones secreted by your pancreas, gonads, pituitary and adrenal glands. Epinephrine, norepinephrine, glucagon, growth hormone, estrogen, testosterone, cortisol and insulin all play roles in fat metabolism.
Insulin
Insulin is secreted from pancreatic beta cells in response to rising levels of glucose in your bloodstream. The consumption of food -- particularly proteins and carbohydrates -- prompts the release of insulin from your pancreas, while fasting reduces insulin production. Insulin triggers the absorption of glucose by muscle, liver and fat cells, thereby lowering blood glucose concentrations. In addition, insulin stimulates the uptake of fatty acids by fat cells, which convert these lipid molecules to triglycerides. Triglycerides are then stored for future use. Thus, insulin promotes lipogenesis.
Lipolysis
Whenever your energy needs increase or your blood glucose level falls, the production of hormones that mobilize your energy stores begins to rise. These hormones -- glucagon and epinephrine, for example -- stimulate lipolysis, which involves the breakdown of triglycerides stored in your fat tissue. The fatty acids and glycerol molecules liberated by lipolysis are then oxidized to meet your energy needs. The hormones that oppose insulin and stimulate lipolysis are called counterregulatory hormones. A review published in September 1999 in "Endocrinology and Metabolism Clinics of North America" reports that counterregulatory hormones are vital for maintaining the constant supply of glucose required for normal brain function.
Insulin Prevents Lipolysis
The hormones that control lipolysis and lipogenesis do more than simply antagonize each other's actions in fat tissue. One hormone that promotes lipogenesis may actually interfere with the production or secretion of another hormone that stimulates lipolysis. For example, according to a 2005 "Advances in Pharmacology" review, when your blood glucose level rises, insulin does not simply drive glucose into your cells; it also inhibits the secretion of glucagon, which is another pancreatic hormone. In this way, insulin not only promotes lipogenesis; it effectively prohibits lipolysis.
Considerations
Insulin inhibits lipolysis and slows the breakdown of adipose tissue. This has important implications if you have type 2 diabetes or metabolic syndrome, because your pancreas could be producing excessive amounts of insulin in an attempt to compensate for your insulin resistance. Therefore, your need to lose weight may be partially thwarted by hormonal imbalances that perpetuate your underlying condition. However, as an article in the June 2006 issue of "Pharmacological Research" explains, the interplay among the various factors that contribute to lipolysis is complex, and insulin is only one of those factors. Dietary modifications, exercise and weight loss are among the most important therapeutic interventions for accelerating lipolysis.
References
- "Endocrinology and Metabolism Clinics of North America"; Physiology of Glucose Counterregulation to Hypoglycemia; G.B. Bolli, C.G. Fanelli; September 1999
- "Advances in Pharmacology" Inhibition of Glucagon Secretion; A. Young; 2005
- "Pharmacological Research"; Adipose Tissue Lipolysis as a Metabolic Pathway to Define Pharmacological Strategies against Obesity and the Metabolic Syndrome; D. Langin; June 2006
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