The myth of nutrition is shown up by rapid changes in food fashions.
Availability is of course important.
As waves of different foods hit Europe, eating habits changed. At first these foreign foods, particularly spices, like foreign fashions were a privilege of the rich, but they soon percolated down. But once foods become plentiful and varied, fashion takes over, and the lure of novelty the trendy is often disguised as concern for nutrition. Thus vegetarian diets and nouvelle cuisine, high fiber diets and cuisine minceur, all masquerade as healthy. Just as clothes indicate our trendiness, so does food.