Nutrition is a soft science because it is in fact very difficult to prove anything when it comes to food. The reason is that everyone’s diet consists of a wide range of different foods, so that it is virtually impossible to show that for any specific food, here are the consequences, as all the other foods in one’s diet will have played a role too.
This is the reason why there is so much controversy in nutrition on virtually every point or aspect of different diets. It almost seems as if for any given issue, there will inevitably be “authorities” arguing for both sides of the coin. For instance, there is a huge debate in nutrition over how unhealthy, as in heart disease, saturated fat is — those who argue that it should be avoided at all costs and those who argue just as vehemently that it is harmless.
And also there is the camp that points out that correlation doesn’t prove causality, i.e., that two things happened to be very correlated could just be random chance and not causal at all. This is the argument that attempts to debunk many of conclusions drawn in the famous China Study that had such an impact on the course of nutrition as science (I don’t buy the argument here; I think the conclusions in the China Study are indeed causal). But in the absence of the type of concrete and irrefutable proofs that you can arrive at in other sciences, the argument that correlation isn’t proof has some weight.
And if this confusion of conflicting opinions isn’t bad enough to begin with, you must consider this: that not all the so-called “experts” on nutrition out there are speaking from a purely disinterested point of view where truth is the objective, but in fact are putting out ideas that support an agenda of a particular food industry. So you have pundits from the meat lobby throwing verbal grenades against the use of soy as a protein alternative because, according to these shills, it promotes estrogen in men! In fact, a huge percentage of the nutrition literature is pure propaganda from writers paid for and in the pockets of particular food industries. They are not telling you THE truth, but THEIR truth.
So what is the layperson to do with such a welter of contradictory and even perverse points of view in the “science” of nutrition? First, don’t give up. Second, keep listening to various experts and soon enough, you will find ones that are more convincing in their arguments. Third, when you have enough experts that you have come to trust, if they have common views about specific foods and specific diets, then that majority opinion among these experts that you have come to trust is what you ultimately have to go with. Not proof certainly, in the scientific sense, but definitely an educated guess.