In Michael Polan's In Defense of Food he argues that eating food has become obscured and changed by marketing that has refocused us onto nutrients rather than food itself. I totally agree. Polan states that it is not only food marketers that have driven us to eating what he calls "foodlike substances" but nutritionists and scientific journalists are also to blame for our current obsession with nutritionism.

Polan's point in the first few chapters that we no longer view food in terms of a whole, as had been passed down through generations, but rather in terms of nutrients is perfectly apt and can be clearly seen by a visit to any local supermarket or grocery store. "Low fat", "good source of omega 3", "high fiber" are just a few terms thrown around on a majority of packaging to convince the consumer that the product is indeed a healthy choice.

One of the main reasons nutritionism took hold in the minds of the general populace is a corporate agenda. For example, government nutritionists can't just say "eat less meat and dairy" because the dairy and meat industry lobbyists would freak out. Instead they have to say "eat less saturated fat" and hope that people understand that meat and animal products are the main sources of saturated fat. They also can't just give positive recommendations like "Eating more fish could be beneficial to your health" instead they have to something along the lines of "Consuming more Omega 3 fatty acids is beneficial to your health.". Thus we have a ton of products claiming to be a good source of certain nutrients of marketing themselves on a lack of certain nutrients.

A more bulleted argument can be found here.