I ran across an article recently that points out the dangers of using relative language in regards to diet advice. This is a topic I spoke about at a recent nutrition lecture here at the gym. Here is a link to a widely publicized article on the advantages of one currently-popular diet over another.
Most of you can spot the problems already: "low-fat" and "low-carb" are the two terms used to describe these diets, yet the low-fat diet itself is far higher in fat than most people consider "low". And, the "low-carb" diet is lower in carbs than most people consider low.
There are several other problems with this article (such as the suggestion that you only have to stay on the diet until you lose some arbitrary amount of weight), but just the issue of relative language alone will leave most people thinking that low-fat is good and, by extension, the lower the fat the better the diet. We of course know nothing could be further from the truth. Likewise, those same people will conclude that low-carb diets are less healthy. Again, highly misleading.
All of these problems stem from the use of relative terms to define the diets. The low-fat diet of which they speak is actually very close to what we recommend for optimal performance and health, though even spelling out the ratios turns out to be a a rather crude way to sum up a diet. For example, there is no distinction made for the quality of the ingredients other than their caloric content.
The lesson here is to never entertain notions of "good" or "healthy" in regards to your diet in the absence of hard data and the use of absolute terms in defining the recommendations.