Obesity - especially abdominal obesity - is part of a broad cluster of conditions collectively known as metabolic syndrome. This syndrome includes, among other things:

• Diabetes
• Cardiovascular disease
• High blood pressure
• High cholesterol (or more properly, high LDL and low HDL, the "bad" and "good" cholesterols respectively)
• High blood triglycerides (fats)

These things are all interrelated in complex ways.

For example, fat cells are related to insulin, in the sense that insulin stops fat cells from releasing their stored fat. Insulin, in turn, is related to high blood sugar, in that high blood sugar stimulates the release of insulin. But high insulin levels for extended periods can result in insulin resistance and hence diabetes, which causes high blood sugar to stick around and fat cells to improperly release fats into the bloodstream. Those floating fats can build up in blood vessels, causing high blood pressure and ultimately heart disease. And the poor circulation from that disease can combine with the poor circulation caused by diabetes to...

...yeah, like I said, complicated.

In part, becoming very obese is as much a symptom of underlying breakdowns in the way the body regulates energy intake as it is a cause of those breakdowns. It's all part of a bunch of feedback loops where which is cause and which is effect becomes kind of unimportant, because they're all causing and reinforcing one another.