Why are Dietary Guidelines So Often Wrong?

Bradley Fikes had an interesting column in the San Diego Union Tribune that complained about the current problems with the US Federal Dietary Guidelines for Americans. The last guideline was published in 2010 Apparently the guidelines are being revised again, and are due out by the end of 2015.   There is concern that the Guideline is wrong on the amount of fat, salt and cholesterol recommended in our diet.  Each time the guidelines have been rewritten, there have been major changes, which implies that the previous guidelines were wrong, and maybe even harmful to those who followed them.  How can we, and our expert scientists be so very wrong so often?
I've often wondered why we really need to have such a guideline.  It often takes years of "negotiation" to establish a guideline, because so many vested special interests are involved in the process.  The producers of the food we eat all have a major financial interest in even subtle changes made in the guideline.  A slight change in recommended fat in our diet could impact the meat and cooking oil industry.  Because the guidelines are developed by a committee, and involve such negotiations, the results are not necessarily "scientific."  --they may include some science but also involve politics and popularity.
One of the problems is that it is difficult to create one guideline for all people.  Another problem is the difficulty of determining long-term effects of various dietary changes.  Scientists continue to learn more and more about how our bodies work and the effect of various foods on it.  We are also learning that our genes have something to do with how we handle our food, and possibly even more so, the bacteria and other organisms living in our body help determine how our body uses the food we eat.  This problem is extremely complicated, and I suspect that we will continue to learn more about what we should be eating in the future.