What I find amusing is that there is a grain of truth in most of those fad diets. In many cases it is a matter of taking a good, or maybe not so good idea to an extreme. Like smoking. One of the things the medical science does not like to tell us is that smokers are, on average, thinner than nonsmokers. Not there aren't a helluva lot of other negative consequenses associated with smoking that make it an unacceptable diet program, but, in truth, smoking is a well known appetite suppressant.
The carbohydrate vs protein debate falls into the same category from a scientist perspective. Environmentally, a high animal meat diet isn't such a good idea, but if you are only eating 1500 calories a day of which 30% to 40% is protein that really isn't such a big deal. The problem arises when we are eating 3500 calories a day and 40% of it is protein and the rest processed carbohydrates - now we have more of an environmental problem.
Wheat, corn, potatoes and other grains have been historical staples of human culture because they are 100% digestable, especially following grinding and/or processing. However, in our culture where we have more calories available to us than we need, these foods are more of a problem, therefore subsituting less digestable foods decreases fat accumulation.
In my opinion, many of the diet guru's today are taking a fundamentally good idea to an lousy extreme. Protein is great and for many of us increasing it while reducing carbs really helps eliminate pounds. Taken to extreme you get kidney failure, mental sluggishness and muscle paralysis.
Anyway, you get the idea... Some is good, more.... not so good.