Ernst Engel quotes:

He who attempts to draw any conclusion whatever as to the nation's wealth or poverty from the mere fact of a favorable or unfavorable Balance of Trade, has not grasped the first fundamental principle of Political Economy.

With rising incomes, the share of expenditures for food products declines. The resulting shift in expenditures affects demand patterns and employment structures.

The poorer is a family, the greater is the proportion of the total outgo which must be used for food... The proportion of the outgo used for food, other things being equal, is the best measure of the material standard of living of a population.