The Mechanisms by Which Excess Fat Harms You
Excess fat leads to chronic inflammation and damage over time. From ScienceDaily: "Fat tissue is no longer considered simply a storage place for excess calories, but in fact is an active tissue that secretes multiple compounds, thereby communicating with other tissues, including the liver, muscles, pancreas and the brain. Normal communication is necessary for optimal metabolism and weight regulation. However, in obesity, fat (adipose) tissue becomes dysfunctional, and mis-communicates with the other tissues. This places fat tissue at a central junction in mechanisms leading to common diseases attributed to obesity, like type 2 diabetes and cardiovascular diseases. ... Fat tissue dysfunction is believed to be caused by obesity-induced fat tissue stress: Cells over-grow as they store increasing amounts of fat. This excessive cell growth may cause decreased oxygen delivery into the tissue; individual cells may die (at least in mouse models), and fat tissue inflammation ensues. Also, excess nutrients (glucose, fatty acids) can also result in increased metabolic demands, and this in itself can cause cellular stress." You might also look at the role of macrophages in fat-induced inflammation.
Link: http://www.sciencedaily.com/releases/2009/07/090714125002.htm