The stereotypical picture of the Scottish diet is oatmeal and haggis -- the latter being served with neeps and tatties.
We can find support for the image of the Scots as oat-eaters dating back 250 years. Dr. Johnson's Dictionary of 1755 tells us that in England, oats are "generally given to horses, but in Scotland support the people." And in 1787 Robert Burns wrote his humorous address to the haggis, the "great chieftain of the pudding race," pudding being a corruption of the French boudin.
But this carbohydrate-based diet goes back only a couple of hundred years before that. In the sixteenth and seventeenth centuries, the food of the poor declined in nutritional value all across northern Europe. The sixteenth century was a time of tremendous inflation. Prices rose faster than incomes. John Knox remarks that the result in Scotland was dire poverty, the likes of which had never been seen. The medieval diet of meat and dairy gave way to the modern diet of grains and root vegetables. [1]
This was especially true in the lowlands, where the land was flat enough to grow crops. The typical diet of lowlanders by early modern times consisted of oatmeal, along with dairy products and kale. Meat had become a rare and expensive luxury.
In the highlands, the change took place more slowly. As late as the year 1600, the diet of the highlanders still consisted of cheese, meat, and milk. When Scottish soldiers were sent to Denmark in 1564, they wanted a little bread, but mainly they wanted their traditional diet of milk and meat. And around the year 1400, Jean Froissart described the typical diet of the Scottish soldier. The Scots carried a small amount of oats, which they would mix with water then toast over a fire, to make a small oatcake or unleavened bannock. But this was just to settle the stomach. The main body of the Scots' diet consisted of meat, meat, and meat.
[1] The main source for the information in this note is A. Gibson and T.C. Smout, "Scottish Food and Scottish History, 1500-1800," in Scottish Society, 1500-1800 (Cambridge University Press, 1989).