The Brittany Spaniel is a breed that originated in the northwestern side of France during the Middle Ages. However, the Brittany Spaniel has ties going back to when the Norman nobles conquered England in 1066. Since the northwestern side of France was a very popular hunting ground, they were developed as a hunting dog mainly to retrieve birds. The first images of the Spaniel breed was first seen in the 17th century, then the first written record came in 1850. Around the early 1900's the first Brittany Spaniel that we know of was breed using English Setters and then later official named a breed.
The environment the breed would of been brought up in would of been a swamp. The native foods would of been poultry, fish, beets, potatoes and some grains. Some people believe that the grains was the factor that made the difference between the Brittany Spaniel and English Springer Spaniel nutritional differences. Brittany Spaniel can utilize a higher starch and carbohydrate to protein ratio, but unlike the English Springer it doesn't do well with corn, barley and wheat.
With Brittany Spaniel you should look for all-breed commercial dog foods with poultry, lamb and beet pulp. I would avoid beef / horse meat, by-products as well as any corn, barley, wheat or soy products.