They call bread the staff of life, using staff in the sense of “a long stick used as a support when walking or climbing or as a weapon”, which is to say, metaphorically, since even the most warlike among us seldom take up baguettes and plunge into battle. What we mean when we talk about bread this way is that it supports us and keeps us alive.
This was true all through the historical period in which I interest myself – the seventeenth to nineteenth centuries. Bread provided most of the calories of the average person’s diet. Maybe 60%. (This was in the days when most folks were trying to scrape together enough calories to keep themselves alive, not trying to avoid them.) Beer – bread’s funtime cousin – contributed another 20% of calories. That’s 80 % of what folks lived on. Bread and beer were fueling the European working man.

click on this for a closer look

They didn’t necessarily know they were getting their protein from bread, because getting protein in the diet does not seem to have been a high priority, as per this handy table which may be taken as more or less representative.
From this you will see that your average bloke in 1750 Strasbourg (this was a table easy to find if not totally relevant to 1800 London, but I’m talking Big Picture here) was spending 20% of his income on beer and getting only a teensy bit of his yearly protein. Put another way, the fellow was spending as much on beer as on soap, linen, candles, lamp oil, and fuel combined. He doubtless found this worthwhile.
Bread was almost sacred. The custom I’ve seen of making a cross on a loaf of bread before slicing it would have been widespread a century or two back. In church, bread was the body of Christ and a sacrament. You didn’t mess around with bread.
Beer didn’t have quite that cachet, but it was still pretty cool.

click on this for a closer look

Bread was cheap protein too. Lookit this nifty comparison of the cost of protein in silver. Bread and beans were king. Half the price of meat when it came to providing protein.

I admit I’m surprised to see the relative expense of eggs as a source of protein. We think of eggs as cheap protein nowadays.
Cheese which is so expensive on that chart I keep thinking it must be some kinda typo. When we think of a farm wife as in charge of the eggs and cheese economy of the house, this certainly implies she was running a profitable little business of her own.

typical bread chez jo

But there it is, laid out in very general terms. Up to modern times European folks were bread and bean eaters. We’ve left this behind in a lot of ways. Bread is no longer the center of people’s diets. (Though I remember my father always wanted to have bread on the table, even if it was cornbread, often as not.)