And let's not forget genetics. Some people have to work harder to stay fit.
Which is why all these anecdotes on this thread are stupid. They prove nothing other than the fact that the human body is extremely adaptable.
No, the human body is SPECIFICALLY made to eat a wide range of foods. To do otherwise, while certain an individual right, is silly.
You are of course correct. But I do tend to believe the statistics that suggest that we are getting fatter as a nation, and I can't help but think it is tied far more to activity level than nutrition.
While expecting our first child we read an interesting book about feeding children. The author advocated a division of labor whereby parents decide what and when to eat, and children decide how much to eat. Her thesis was that people are quite capable from birth of regulating their food intake but that we tend to screw things up by interrupting that self-regulation ("you can't leave the table until you clean your plate", "my you look skinny", etc.).
But her thesis depends on moderation in nutrition and at least some physical activity. One of the studies she cited was conducted in a factory in India, where they studied the relationship between activity level and food consumption. There was a very strong correlation between food intake and the physical demands of the jobs---the more activity, the more food consumed. Only the most sedentary folks deviated---and tended to be overweight. But even the next activity level up did just fine.
I think that technological advancement and social evolution have combined to give us all quite sedentary lives and many of us are suffering the consequences.