We eat too much food (too many calories) and we get fat. Being fat brings a host of other health issues, such as heart and other cardiovascular diseases, high cholesterol, diabetes, and on the list could go. Pretty simple, right? Wrong.
Storing excess calories (energy) as fat is how the body tries to protect us from poor nutritional choices and excess calories, in order to prevent high cholesterol, heart disease, diabetes, and so on.
Let's break it down
We (human beings in Western nations) eat too much, most of which is not healthy, i.e., filled with simple carbohydrates (sugars, white flour, etc), saturated fats and trans fats, and most of it is processed, far from what nature intended.
For example, we go to Burger King and get a BK Quad Stacker (930 calories, 28 grams of saturated fat), a large fries, and a chocolate milkshake. That's lunch or dinner. That meal is easily over 1500 calories, and unless you plan to run a marathon in a couple of hours, that is about 1100 calories more than you need, not to mention all of the saturated fat, simple carbohydrates, sugar, and salt.
The body does its best to do its job. The stomach and large intestine digest the food and send the "nutrients" into the bloodstream to be used for energy. The pancreas gets the message that there is a serious load of energy in the blood, so it produces more insulin to handle the increased calorie load. Meanwhile, the liver is doing its job in converting the extra glucose (sugar) and fats (mostly unhealthy fats in this case) into triglycerides. Some of the saturated fat is also being converted into LDL cholesterol.
At this point, our blood stream is filled with glucose, triglycerides, lipids (fats), cholesterol, and now some insulin is shooting onto the seen. It's job is to kick ass and take names.
The insulin stores glucose in the muscles and liver until they are full (and unless you just worked out, they are already probably full), then it stuffs triglycerides into fat cells, as well as sending extra glucose and fat back to the liver to be converted into triglycerides to be stored as fat.
The fact that the body does this is essential. Too much glucose in the blood, as we know from diabetics, can cause blindness, neuropathy, and other serious health issues. To much fat in the blood clogs the arteries and we have a heart attack or a stroke.
If the body did not store all of this extra "energy" as fat, we would die young, but thinner.
What this explains, in part, is why overweight people can have normal cholesterol, triglyceride, and glucose levels. On the other hand, take these measurements following a meal at their favorite fast food joint and their scores will be off the charts - the heavier we get, the less well our bodies handle unhealthy foods, until we get to a point where the pancreas cannot generate enough insulin anymore.
I don't want to create the wrong idea here - being fat is not healthy. Fat cells are less sensitive to insulin the more full they are, until the body has to make more of them. In addition, fat cells produce estrogen, and the majority of major cancers are estrogen related (including breast and prostate). Finally, when fats cells are unresponsive, the body will store fat in muscle cells, and this has been linked to diabetes.
Over at the PLoS ONE blog, Obesity Panacea,