Ignoring that medieval peasants were likely highly active and regardless had a short, brutish life, the flours at the time shared little in common with modern flours.
Medieval flour would have had the entire grain -- the bran, germ and endosperm -- ground into the resulting product. The product would be high in fibre and nutrients, and with a ratio of good fats as well.
Modern flour separates out just the starch endosperm and discards the rest, then refining it to an ultra digestible product.
Nutritionally the former is healthy. The latter is not great, especially for people with a caloric surplus and a sedentary lifestyle, where it's just a massive glucose blast as that flour is 75% almost immediately turned into blood glucose.
Medieval flour would have had the entire grain -- the bran, germ and endosperm -- ground into the resulting product. The product would be high in fibre and nutrients, and with a ratio of good fats as well.
Modern flour separates out just the starch endosperm and discards the rest, then refining it to an ultra digestible product.
Nutritionally the former is healthy. The latter is not great, especially for people with a caloric surplus and a sedentary lifestyle, where it's just a massive glucose blast as that flour is 75% almost immediately turned into blood glucose.