Composition of Adipose Tissue
Normal body fat levels are considered to be a protective and insulating layer of fatty acids for body organs. Triglyceride storage in adipose tissue is the difference between energy consumed and energy needed for daily activities. The endocrine system controls the body fat storage signals through thyroid hormones, and the sympathetic and parasympathetic nervous systems . Extra body fat in obese individuals causes the adipose tissue to create dysfunction in the immune system. Adipocytes can then produce lipid toxicity in other tissues [9, p. 21].
Lipids in cells serve two purposes—structure of cellular membrane and as fat storage depots. Fat depots are depleted during starvation to ensure membrane preservation. In the nonobese individual, body fat is approximately 15% body weight in males and 21% in females [9, p. 21]. Adipocytes are active in metabolism with both endocrine and paracrine functions [9, p. 22].
Those with higher BMIs are associated with greater percentages of body fat and increased leptin. A BMI >35 causes increased leptin, which should act as a negative regulator of body weight but instead has no effect in obesity [9, p. 23]. Elevated leptin levels in obese individuals are indicative of increased anorexigenic and decreased orexi- genic peptide synthesis in the hypothalamus . Leptin influences the hypothalamus through the signaling mechanism of neuropeptide Y and agouti-related protein, which stimulates eating similar to a cocaine-amphetamine transcription factor. Research into leptin functions in the body are ongoing, but it appears that leptin signals to the brain, the status of body fat stores, and failure of proper signaling are related to weight gain .
Another hormone, adiponectin, is derived from adipocytes and can lead to obesity-induced insulin resistance, cardiovascular events, and atherosclerosis . Other secretory effects of adipokines include Interleukin 6 (IL-6), tumor necrosis factor (TNF), renin-angiotensin system, plasminogen activator inhibitor (PAI-1), adipsin (Complement D), and resistin .