7-course land rotation system, increased production which allowed export of cheese and butter. Use of cast iron in mass-produced farm equipment reduced price and dependence on local blacksmith. Agricultural development slower in France than in England and Low Countries, delayed until French Revolution. Vitamin D moderates mobilization of calcium from bone, increases intestinal absorption of calcium. Vitamin D also controls expression of certain genes.
Bones fail to calcify properly, become weak, to the extent that leg bones become bowed.
Egg yolk, liver, fatty fish. Major sources in modern US are fortified milk and margarine. And endogenous biosynthesis in skin. Note weak sunlight in temperate climates, exacerbated by smog of industrialized cities. Enabled transport of live cattle and dressed meat, fresh milk from farms to urban markets.