I was on a blog recently discussing this very advice and the idea that "your appetite will adjust, at least to some extent, to your intake."
Here is an abridged post of what I wrote. The bolded headings are the 'arguments' used to explain obesity.
"Eat food. Not too much. Mainly plants" - sounds simple doesn't it. Along with "eat a balanced diet" it is actually misses the point. Let me explain...
I have said before that few people if any, think "You know what, I have eaten 'enough food', but I think I will now eat some more!" People eat in accordance to their appetite. So why can't we trust our appetite to dictate how much we eat (or indeed to ensure we eat 'not too much')? Is it because our appetite can be misled by certain foods or is there some other mechanism at work?You would think that when in calorific deficit - particularly after exercise, our appetite would spur us to eat a bit more to rebuild our reserves. If we have had a particularly easy day, and energy expenditure is low, you'd think our appetites would diminish accordingly.
This seems to be the way it works in much of the animal kingdom (I have yet to see a wildlife program featuring a fat lion in its native habitat). What is undoubted is that nobody can persist in a chronic state of hunger - eventually you will give in to your hunger. Thus, the advice to eat 'not too much' seems to miss the point.
It has been suggested that we are fat because portions are bigger. I don't buy this idea! This might seem like a contrived analogy, but think about your breathing. Breathing adapts to your body's oxygen requirements - but if you force yourself to breath harder, faster and deeper several times a day, over time will you develop a NEED to breath in more air? I think not.
Hunger is controlled by appetite which one presumes evolved over millenia to regulate energy consumption and respond to energy requirements. If people eat to the point they become obese, we have to entertain the possibility that our appetite has somehow been blunted/desensitised/misled by *something*. Even considering affluence (which may mean we can buy more food), it does not follow that you will simply eat more food (again we are back to the breathing analogy). Affluence might however affect the TYPE of food you eat. The type of food you can now buy may offer poorer satiety or maladapt the hunger response.
Clean Your Plate/Social Conditioning/Learned Behaviour
The idea of a learned behaviour is interesting - as children, most of us will have had our parents demand we eat all of our dinner with a 'reward' of desert, but if I over-eat (like at Christmas), and push myself to eat more, I like most people feel sick. Furthermore, I eat less at/delay my next meal.
In addition, if I expend more energy, I eat more. This is why I don't but the 'eat more do less' advice. Exercise makes me hungry - something most of us experience, particularly after something like swimming. If I cut back on food, I feel weak and less able to exercise (or at least limited to exercising at a lesser intensity).
If I cut back on food intake for a few days, I have to binge. The binge is my body telling me that it has been running on fat stores and needs to replace the fat.
US food outlets offer massive portions. They are also an affluent nation. They are less physically active than just about any other nation and car ownership is high. Ownership of labour saving devices is also high (such as dishwashers). But the nutritional profile of the American diet is markedly different to that of the Japanese for example, who are generally thin. Look at the consumption for HFCS in America. This is the kind of thing that I would suspect of 'blunting' our appetite response.
Eating To Regulate A Large Body-Mass
I am not sure that upkeep of a large body mass is credible. Fat guys have to carry more weight around, they have to work harder simply to move and so I suspect will have a higher basal metabolic rate. So there is no advantage for the body to adapt this way.
For me, it is more likely that our physiology dictates consumption/appetite rather than psychology. I still hold the idea that our appetite is being misled somehow.