Looking at the current trends in worldwide health, especially as it relates to obesity, it doesn't take a lot of mental muscle to realize that something is wrong with the current system and dietary guidelines. Realizing that a problem exists is the first step in attempting to solve it. As mentioned by other posters, the self experimentation yielded the most dramatic results that myself and others have had.
To take it a step further: I had an anecdotal discussion with my father about the current state of health and nutrition compared to the food choices he had growing up. A little background: This was a fairly isolated mining town in rural america that was self sufficient. Their food consisted of:
- Their own grassfed meat
- fresh fruits and vegetables from their gardens
- raw dairy (used to make their own butter) they even still have the butter churn as a decorative piece in their home.
- Organ meat typically liver and onions once a week.
- Their own chickens
- Deer and other wild game
Also add in the fact that they worked outside (Vit D.), followed their circadian rhythms (got up with the sun, went to bed at night). The only thing lacking from the diet was a multitude of fish as they didn't have access (landlocked state, fairly isolated location).
From talking, he couldn't remember any incidence of obesity or health problems arising from it. Even though everyone worked underground without respirators and smoked fairly heavily, except for black lung from the coal dust and smoking there weren't any health problems. I would even suggest that their diet helped them to survive longer than typical people would today in those conditions.
This conversation coupled with my own progress, observations, research, and common sense made me a believer.