I found this excellent concise summary by Todd Caldecott:
The macrobiotic diet is perhaps the best known advocate of brown rice. This therapeutic diet was developed by George Oshawa as a detoxification program, based on the simple nutritious diet of
northern Japan. The only difference between Oshawa's diet and the traditional diet was the absence of animal products, including seafood, which of course figures very importantly in the traditional Japanese diet. Nonetheless, Oshawa never meant for people to eat this way all the time, but periodically, or as a treatment to correct the excesses of the modern diet, heavy in meat, sugar and processed foods.