Although technically true, the in-box analogy fails in the real world for two reasons.
First, if you eat less your body will become more energy-efficient and burn less fat. Thus, if you put less into your in-box, your body will start putting less into the out-box. Although it is not a one to one correspondence, it does make weight-loss more difficult.
The second problem relates to self control. First let me debunk a general myth that says fat people have less self-control. My observation is that skinny people in general have NO MORE self control than fat people. Skinny people seem to either enjoy food less, or have a body that converts the food into something besides fat.
I look at self-control as scarce resource. We all have only so much self-control. For example, if your boss gives you a hard time at work, you "use up" a ton of self control to keep from ringing his or her annoying little neck. Then when you get home and your child is shouting because he wants you to buy him the latest fad for tomorrow's show-and-tell, your self-control is severely drained further.
Lets put numbers on this to clarify the concept. Suppose Bob has 100 units of self-control (SC). His boss then blames him for something that is really the bosses own fault. Bob then spends 50 units of SC to keep himself from telling his boss how he feels. He simply apologizes to his boss and says it won't happen again.
Then on the drive home Bob is cut off by a shiny red convertible with a smiling teenager inside. Bob feels like passing him, cutting back in, and giving him the bird. However, Bob resists and consumes another 30 units of SC. Bob now has 20 units left.
Now Bob gets home and his bratty kid demands a trip to the store to buy Mutant Wart Man. Bob uses the last 20 units and calmly thinks for a moment and reminds his son that he can take Barfy Joe instead. Barfy Joe is still in style. The son says, "Well, I guess that will be okay," and walks away.
Bob is now ready for a good night's sleep.
Suppose, however, that Bob was on a diet. All day he craved his favorite foods, but had to eat rabbit food instead. He is sick of rabbit food, but wants to look good. This battle with his appetite consumes 50 units of Bob's self-control. If his boss blamed him, he would still have enough SC to bite his tongue.
After this, though, Bob has spent all his SC. A rude driver or a spoiled son would now put him over the top and result in great fire-works display.
So, you think 50 units of SC is too high for a diet? Have you ever been on a diet for months or years? (Note: the author is not obese. The author could stand to loose some weight, but he is not obese. However, the author's wife does nag him about his weight and has found weight-loss to not be a simple task.)
Most animals spend about 95% of their waking time looking for food. Even 100 years ago, humans spent roughly 80% of their effort on getting enough food. The very core of our brains gives food a high priority because for at least 99.9999% of our evolutionary history only those who could get enough food could survive and reproduce.
During periods of plenty the body is designed to store energy (fat) to prepare for rough times ahead. (Remember, welfare is a very recent invention.)
Humans almost never had to deal with problems of plentitude. If things went well for several years, couples would simply make more babies, which would consume any excess energy. (Remember, condoms and birth-control are also recent inventions.) In some poor nations in Africa couples still are having children even though they are starving. (Image how horny they would be if they were full!)
Thus, a diet is forcing one to go against 3 billion years of evolutionary conditioning. I am not saying we are pre-programmed robots, but you better simplify other portions of your life before you do battle with your reptilian brain stem (which is exactly what the human brain stem is--a left-over reptile part).
In summary, the effort needed for dieting is not free. One must either live with more stress, find a simpler job for less pay, spend what little free time you have left on a daily exercise regiment, or give your children up for adoption.
(Note: If you hold a religious view of creation, then you may have a tough time explaining why God did not design the human body for the modern age of more food and less movement. Evolution has the excuse that it cannot predict the future, but God has no such excuse because the Bible says he knows the past and the future.)
Since I am a bear when I am hungry, my wife recommends that I exercise more as an alternative to eating less. After our kid was born I now have less free time for hobbies. But I enjoy my hobbies. Enough exercise to lose 10 pounds would probably take about 40 minutes out of each day. This would cut about 35% of my daily free time. Thus, I am given a choice of doing what I enjoy, or loosing weight. (Part of the 40 minutes includes the overhead of more showering, changing clothes, etc.)
What are the benefits of being a bit less heavy? Looking better and living longer are the two that come to mind. Why should I worry about looking better? I am married! Looking better will simply give me more temptation as beautify women jump all over my great trim body. Married men who exercise too much are probably having or planning an affair! FAT EQUALS LOYALTY! Do you hear that honey?
So what about the second advantage of a longer life? If you do the math it does not foot. For example, if I loose 10 pounds I might live roughly 10 years longer. But each day I only get about 2 hours of free time without exercise and only about 1 hour, 20 minutes with. The extra ten years is cancelled out because I have less free time per day. More exercise might help me live longer, but it will not give me more total free time to do the things I like.
