Mercola finally put it all together in simple layman's terms for us at: http://v.mercola.com/blogs/public_blog/Finally-Science-Confirms-the-Secret-Key-to-Weight-Loss-19046.aspx.

The gist of it: The way Americans eat (constant junk consumption) causes us to release a lot of insulin from the pancreas into the blood. Physical inactivity causes our insulin receptors to shut down, because they are not being used. These receptors are how sugar gets from the blood stream into the cells. So the insulin and sugar are stuck in the blood, and we have high blood sugar and low energy. The high insulin levels that result inhibit the pruduction of lipase, a fat-burning enzyme. Without lipase our bodies have to break down carbs and proteins for energy. If we try to diet we lose muscle mass instead of fat because we don't have lipase, because we've been eating junk so long. Burning up our muscles makes us weak and low energy, and we can't exercise, and the whole situation gets worse until we die of heart disease, high cholesterol, diabetes and other systemic issues that were caused at the very beginning by our crappy diets and inactivity.