And what will it tell you? Let's say you have one guy who is morbidly obese, another who is visibly in top shape, a seriously underweight fashion model, and an average guy. Who are you going to trust the most?

The obese guy obviously has some problem with nutrition, but he may also the one who is most interested the subject and the most knowledgeable. Obesity is also a mental disorder and while he may know a lot about nutrition, he has trouble applying this knowledge.

The guy who is in top shape most likely has good nutrition, but he may also simply have good genetics, do a lot of physical activity, and not care much about what he eats.

The fashion model probably knows how to lose weight, which may be as simple as not eating enough. But how much she knows about health is completely unknown.

As for the average guy, that's the most "no idea" you can get.