What is the theme in Shaking the Nickel Bush by Ralph Moody?

Illness is a major theme in the book. The novel begins with Ralph being diagnosed with diabetes. In 1919, diabetes did not have a treatment. The specialists at the Boston hospital where Ralph was diagnosed tell him he will die within six months. The family doctor, Dr. Gaghan, however, tells him that if he gets lots of sunshine and follows a special diet he has a good chance of living much longer. Dr. Gaghan gives the diet to Ralph's mom who immediately goes out and buys a large number of the items Ralph will need to eat, including gluten flour. Ralph's mother makes several loafs of bread and then puts Ralph on a train to Arizona with a basket full of food.