In a nutshell, the idea is this: if one gains weight, then by definition one has taken in more calories than one has expended -- one has "overeaten". That's what thermodynamics tells us. What it doesn't tell us is why one has overeaten. Most people believe -- and it seems obvious by observation -- that overeating causes weight gain. But here is the other possibility: weight gain causes overeating. This might seem silly and perplexing at first, but I propose a thought experiment below to explain the profound difference.
Let's imagine I have been affected with an intestinal parasite, like a tapeworm. This worm lives for itself, and steals some of the food I eat. It is capable of growing to an enormous size, capable of weighing as much as its host, and it has food preferences.
Now suppose this parasite steals ten percent of all the calories I ingest. Over time, I find that I am less satisfied with my portions, because the amounts I am used to eating now leave me hungry. So I have an extra snack or meal. If I had been living happily on an average of 2,000 calories a day, I find that I must now eat 2,200 calories, because the tapeworm has eaten and stored 200 of them to foster its own growth. After 150 days of this, my tapeworm now weighs a pound. The extra 200 calories a day is hardly noticed by me, because it is so easily obtained, and after ten years of living with my tapeworm, it has grown to weigh 20 lbs.
Now poor me, ten years older and 20 lbs. heavier, wonders how I gained this weight, in spite of watching my portions and exercising regularly! "Eat less!" "Exercise more!" say all of my friends, doctor, and the conventional wisdom.
So I cut my portions for a while, and guess what? I lose weight! Yes, my tapeworm slimmed down a bit, as have I, but now I am trying to live on reduced calories, and my worm still takes its (now reduced) tithe of my meals. My metabolism and cells are screaming for fuel by making me hungry. I feel tired and have no will to exercise. I still need my daily 2,000 to function well.
Eventually hunger gives in, and over time I must return to eating 2,200 calories a day, and my worm gets bigger than ever. And again I need even more energy to carry around the extra weight.
Now, knowing that my weight gain is due completely to the pirated and stored calories in my parasitic worm, and inaccessible to my metabolism, would any reasonable person blame my weight gain on my "overeating"? Is my overeating causing my weight gain?
No, the root cause is that the fuel I consume is not all getting to my metabolic engine. It is the weight gain of my parasite that is driving me to overeat. And if I don't overeat, I will become sedentary, or lose lean mass as my body attempts to compensate for the semi-starvation imposed upon it.
Now let's imagine two other conditions: my tapeworm loves, just loves sugars and starches! It will actually eat not 10% but 75% of the sugars I ingest, but it's not so fond of protein and fat. Furthermore, it is triggered to start eating by sensing high insulin levels in its host. Now what kind of diet would be the worst I could eat? What kind of diet would go a long way to fattening my worm, and leave me with the least share of the calories I eat? What kind of diet would ensure that I became really, really hungry not long after eating? I think you know the answer.
If you can now transfer these parasitic qualities to fat tissue itself, you begin to see the idea. Imagine that Metabolic Syndrome is an imbalance in the regulation of the fat tissue itself, causing it to hold on to stored calories more than release them. The end result is the same. The fat tissue has become like a parasite, robbing the rest of the body of the fuel it needs, and the body responds the way it always does, signaling the host to eat by inducing hunger. If that hunger is not met, it will slow down metabolism to compensate for the lack of energy. Fat storage is causing overeating, not the reverse!
If this is indeed the cause, or one possible cause of obesity, it explains several problems that the caloric-balance theory does not. For instance, why caloric restriction and exercise are so ineffective as a long term solution: it does not correct the underlying imbalance.
As far as I know, there has not been enough research done that can actually pin down the cause of the metabolic syndrome. Once you get it though, it may be incurable, and if I go out on a limb here, the best treatment may be a low carbohydrate diet.
I hope the above has made the understanding of the idea of causality in the conservation of energy a little easier -- and thank goodness we know how to treat tapeworm infections!