Why do so many health studies contradict one another?

It seems to happen so often that there will be some study saying something is bad for you then a few years later another study saying it is good for you followed by a third study saying that it does not make much difference one way or the other. Here is an article about a recent salt study. It reminds me of the Woody Allen movie Sleeper, where he wakes up in a future world where smoking is good for you and coffee is a health food.

It is good that science is self-correcting, but why are there so many flawed results? It gives credence to those who say that science is just a human construct. I guess that part of it is that work done on humans is inherently difficult. That being the case, people who do such research should take extra care to make sure that sample sizes are sufficient and that they are properly controlled.