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 converts sugars to fats, which are subsequently deposited and stored in fat tissue. Lipolysis is the process of fat breakdown, typically to generate energy. These two metabolic activities are controlled by hormones secreted by your pancreas, pituitary and adrenal glands, and ovaries or testes. The pancreatic hormone insulin is particularly important in fat metabolism and lipolysis.
Insulin is secreted from pancreatic 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. In contrast, fasting and a falling blood sugar reduce insulin production and release. Insulin triggers the absorption of glucose by muscle, liver and fat cells, thereby lowering the blood sugar level. In addition, insulin stimulates the uptake of fatty acids by fat cells, which convert these molecules into triglycerides -- the primary storage form of fat in your body. Thus, insulin promotes lipogenesis.
When your energy needs increase or your blood glucose level falls, the production of hormones that mobilize your energy stores begins to rise. These hormones -- such as glucagon and adrenalin -- stimulate lipolysis, which involves the breakdown of triglycerides stored in your fat tissue. The fatty acids and glycerol molecules liberated by lipolysis are then metabolized to generate energy to meet your needs. The hormones that oppose insulin and stimulate lipolysis are called glucose counter-regulatory hormones.
Insulin Prevents Lipolysis
The hormones that control lipolysis and lipogenesis do more than simply antagonize each other's actions in fat tissue. Hormones that promote lipogenesis may actually interfere with the production or secretion of other hormones that stimulate lipolysis. For example, when your blood sugar level rises, insulin does not simply drive glucose into your cells. It also inhibits the secretion of the counter-regulatory pancreatic hormone glucagon. In this way, insulin not only promotes lipogenesis, it effectively prevents lipolysis.
Insulin inhibits lipolysis and slows the breakdown of adipose tissue. This has important implications if you have type 2 diabetes or prediabetes. With these conditions, your body resists the action of insulin. Your pancreas tries to compensate by producing excessive amounts of insulin -- which promotes fat storage. Therefore, attempts to lose weight may be partially thwarted by hormonal imbalances. However, 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 and weight loss.