Nutrition beliefs are often based on ideology.
The large gaps in our scientific knowledge of human nutrition have left many an opening for ideologists of various stripes to peddle dietary philosophies based as much on politics and emotion as on fact. The immense cultural importance of food makes these philosophies very seductive to their peddlers and consumers alike. There are primitivists who contend that one should eat only raw plant foods, conspiracy theorists who view all processed foods as poisonous, food industry toadies who insist that the best foods are engineered, contrarians who are all too ready to subvert the conventional nutritional wisdom, classic conservatives who will go to any lengths to defend the majority opinion against all evidence, and other types. There will come a day when our growing scientific knowledge of human nutrition squeezes out most of the ideologists, but we’re not there yet.