Here is my Catholic point of view on fad diets --
God made animals to be our meat (he specifically gave them to us for meat AFTER the Flood, interestingly). And from these animals, we also get dairy products (milk, butter, cheese, eggs).
He also made fruits/vegetables/nuts/grains and men have been eating these things since ancient times.
He also made the bacteria that ferment sugars and produce alcohol. And good guys have been drinking wine (and other drinks) for millennia.
And sweets have existed too -- sugar cane, honey, and other substances (plants like Stevia?) to sweeten a dessert once in a while.
I think when it comes to eating, it's a classic case of "all things in moderation". But since the world is so messed up compared to the Ancient World, we also have to strive to be as natural as possible. For example, although sugar is natural, people didn't used to coat everything with it, or include it in every recipe (which processed food does today). Ditto for grains.
But vegetarianism? People always ate meat. Not at every meal, but they ate meat.
Atkins diet? People have always eaten "bread", have they not?
And so forth.