The powers conferred under this chapter shall not be used to discourage the production of supplies of foods and fibers sufficient to maintain normal domestic human consumption as determined by the Secretary from the records of domestic human consumption in the years 1920 to 1929, inclusive, taking into consideration increased population quantities of any commodity that were forced into domestic consumption by decline in exports during such period, current trends in domestic consumption and exports of particular commodities, and the quantities of substitutes available for domestic consumption within any general class of food commodities. In carrying out the purposes of this chapter it shall be the duty of the Secretary to give due regard to the maintenance of a continuous and stable supply of agricultural commodities from domestic production adequate to meet consumer demand at prices fair to both producers and consumers.