Nutrition, in particular, seems to be in constant flux.
I think it's less because the science isn't good and more because of the following:
1. Various food industries fund and propagate studies promoting their products and publish tons of propaganda, much as the tobacco industry did. Corn is the industry where I have noticed it most in the recent years because of the attacks on high fructose corn syrup, but it occurs across the board.
2. Big industry doesn't just distribute propaganda - they actively work to suppress studies, intimidate researchers, and sue individuals that paint their product as bad. The most obvious related example is the cattle industry going after Oprah over mad cow comments - and that wasn't even challenging the nutrition side.
3. A particular sort of diet has become the norm in the US - highly processed, dominated by a couple of mono-cultured grains and heavy on meat & dairy. Diet and nutrition information that plays into supporting that norm tends to make it into the mainstream consciousness more readily.
4. Bias even tends to infect doctors and nutritionists fairly significantly. In one of the food documentaries I've watched recently, one doctor discusses how hard it is to give up the conventional wisdom that milk is good for you, even in the face of scientific data that says it's not.
5. Industry drives the information provided by the government on nutrition, so there's a fair amount of bullshit published by the US government. For instance, the recent argument about what can be called a vegetable to meet nutrition guidelines for children.
6. Moving deeper into nutritional information requires a certain amount of critical thinking ability. You have to be able to discern the validity of a study or article on your own. Many people lack that ability (as evidenced by people who believe vaccines cause autism and creationism is science) or the drive to read multiple studies from multiple sources to build baseline knowledge.