If I went in to an auto parts store and came out with only a steering wheel and announced that I was going to drive somewhere you'd think that I had gone insane. Obviously, you need much more than just a steering wheel in order to go places.
Yet that is exactly how scientists treat food. They see that folks who eat food X have less disease Y. Upon examining the food and discovering it has high levels of nutrient Z, they'll announce that taking a supplement of nutrient Z will lower a person's risk of disease Y.
But they are making two grave errors. First they are ignoring the fact that food X is not just nutrient Z, it is a whole host of other things that work together to make food X what it is. Secondly, they are assuming that they know everything there is to know about what makes up food in the first place. And if you take a look at the history of nutrition science you'll see how often scientists find new players in the game of food.
Scientific reductionism, as the practice is known, may make sense in certain circumstances. But it leads to sloppy decisions regarding a subject much too important to be playing chemistry with ... our food.
For more information see Michael Pollan's book "In Defense of Food: An Eater's Manifesto."