It is not a group of people in a room together evaluating, and reevaluating foods over and over.
Changes in dietary recommendations come from studies done in various areas. Sometimes there is something specific being looked for. Sometimes a thing is discovered by accident. For instance, doctors may notice a percentage of patients are not getting good results from a medication which has been dependable for a long time. They will have areas of those people’s lives researched. Someone gathers information about age, and family history. Nutritionists will examine their diet, past and present, and so on.
Perhaps a group of immigrants to a new environment will suffer poor health not common in their home of origin.
As new information comes known, a food once considered bad, because of one element, will be later seen as having another element which cancels the bad one.
So no, they actually get excited as more becomes known about the positives and negatives of food options.